ECNG2006 (EE25M) Introduction to microprocessors
Course Notes
Contributors:
K. Hall
F. Mohammed
C. Radix
A. Sinanan
A. Williams
K. Edwards
A. Joseph
O. Regalado
K. Narine
I. Mohammed
Y. Panchu
S. Patrick
A. Abdool
C. Arneaud
N. Harrichand
H. Lawrence
D. Caberrea
P. Pollucksingh
c Dept. of Electrical and Computer Engineering,
U.W.I. , St. Augustine
March 11, 2008
ECNG2006 (EE25M) March 11, 2008 2
Contents
Course Outline 8
Coursework Portfolio 20
Course Perceptions Questionnaire 21
Prerequisite Skills Questionnaire 23
I Microprocessor Overview 1
1 Microprocessor Basics 1
2 Microprocessor System Basics 11
3 Microprocessor support circuitry 21
4 Microprocessor architecture 30
5 Typical Microprocessor instructions 39
II Microprocessor-based system development 1
6 Software tool chain 1
7 Development support (hardware) 9
8 Microprocessor families and forms 17
III PIC16 Introduction 1
9 PIC16F877 Overview 2
10 MPLAB Overview 13
IV Numbers & Data 1
11 Real numbers 1
12 Integer arithmetic 13
13 Real number arithmetic 22
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ECNG2006 (EE25M) March 11, 2008 3
14 Data structures 27
V Algorithms on PIC16 1
15 Basic operations 1
16 Code comparison 41
17 Compiler limitations 57
VI Interfacing Peripherals 1
18 Interfacing Idioms 1
19 Interrupt Basics 14
20 Typical peripherals 28
21 Digital troubleshooting 37
22 PIC16 peripherals 45
VII System Issues 1
23 Interrupt issues 1
24  P interface/timing issues 24
25 Power/Signal issues 35
26  P choice/comparison 42
27 Case study 50
28 Design guidelines 57
VIII Communication 1
29 Communication protocols 1
30 Bit-banging vs. hardware 12
31 I/O on the PC 17
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ECNG2006 (EE25M) March 11, 2008 4
32 PC to  P serial link 20
33  P Parallel Interface 46
IX Appendices 1
A Data Sheets 1
B Glossary 3
C Study tips 15
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ECNG2006 (EE25M) March 11, 2008 5
References
Dictionary.com. Internet. Http://dictionary.reference.com/.
Mathworld. Internet. Http://mathworld.wolfram.com/.
1991. Can speci cation version 2.0. Http://www.interfacebus.com/CAN 2 Spec.pdf.
1993. The free on-line dictionary of computing. Internet. Http://www.foldoc.org/.
1994-2000. PCW Compiler Help  les.
1995. 80C51 family architecture. http://www.semiconductors.philips.com/acrobat/various/80C51 FAM ARCH 1.pdf;
http://www.grifo.com/press/DOC/Philips/FAMARCH.PDF.
1997. PICmicro Mid-Range MCU Family Reference Manual. Technical Reference Document 33023a,
MicroChip Technology Inc.
1999. HD44780U (LCD-II) (Dot Matrix Liquid Crystal Display Controller/Driver). Datasheet,
Hitachi.
1999. MPASM USERS GUIDE with MPLINK and MPLIB. Manual DS33014G, MicroChip Tech-
nology Inc.
2000. MPLAB ICD USERS GUIDE. Manual DS51184D, Microchip Technology Inc.
2000. MPLAB IDE, SIMULATOR, EDITOR USERS GUIDE. Manual DS51025D, Microchip
Technology Inc.
2000. The I2C-bus speci cation.
http://www.semiconductors.philips.com/acrobat/various/I2C BUS SPECIFICATION 3.pdf.
2001. At90s8535 summary. Available at www.atmel.com. Rev. 1041HS - 11/01.
2001. Atis telecom glossary 2000. Internet. Http://www.atis.org/tg2k/t1g2k.html.
2001. PIC16F87X Data Sheet: 28/40-Pin 8-Bit CMOS FLASH Microcontrollers. Technical Refer-
ence Document 30292c, MicroChip Technology Inc.
Arnold, Ken. 2001. Embedded controller hardware design. LLH Technology Publishing.
Awtrey, Dan. 1997. Transmitting data and power over a one-wire bus. Sensors Reprint downloaded
from www.dalsemi.com.
Ball, Stuart R. 2000. Embedded microprocessor systems. Butterworth-Heinemann.
Barr, Michael. 1999. Choosing a compiler: The little things. WebPage:
http://www.netrino.com/Articles/CrossCompilers/.
|||. 2000. Language lessons. WebPage: http://www.netrino.com/Connecting/2000-
03/index.html.
Buchanan, William, and Austin Wilson. 2001. Advanced pc architecture. Addison-Wesley.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 6
Cady, Fredrick M. 1997. Microcontrollers and microcomputers: Principles of software and hardware
engineering. Oxford University Press, Inc.
Fisher, Matt. 2000. Protecting binary executables. Webpage:
http://embedded.com/2000/0002/0002feat1.htm.
Hartman, Hope J. 2001. Developing students’ meta-cognitive knowledge and skills, chap. 3, 33{68.
Kluwer Academic Publishers.
Horowitz, Paul, and Win eld Hill. 1989. The art of electronics. 2nd ed. Cambridge University
Press.
Katzen, Sid. 2003. The quintessential pic microcontroller. Springer. 2nd printing.
Kernighan, Brian W., and Dennis M. Ritchie. 1988. The C Programming Language Second Edition.
Prentice-Hall, Inc.
Mazidi, Muhammed Ali, and Janice Gillespie Mazidi. 1995. The 80x86 ibm pc & compatible com-
puters volumes i & ii: Assembly language design and interfacing.
Mitra, Sumit. 1997. Power-up considerations. Application Note AN522, DS00522E, Microchip
Technology Inc.
Noergaard, Tammy. 2005. Embedded systems architecture. Newnes.
Palmer, Mark. 1997a. An520: A comparison of 8-bit microcontrollers. Application Note DS00520D,
Microchip Technology Inc.
|||. 1997b. Power-up trouble shooting. Application Note AN607, DS00607B, Microchip Tech-
nology Inc. Contributions: Richard Hull & Randy Yach.
Peri, Vamsi, and Dan Simon. 2005. Fuzzy logic control for an autonomous robot. In Annual meeting
of the north american fuzzy information processing society (na ps 2005) 26-28 june 2005, 337{
342. Downloaded from http://academic.csuohio.edu/simond/pubs/Vamsi.pdf on March 7th 2007.
Predko, Myke. 2000a. Debugging your applications, chap. 12, 561{569. In Predko (2000d).
|||. 2000b. Designing your own picmicro R mcu application, chap. 11, 547{560. In Predko
(2000d).
|||. 2000c. Picmicro R mcu application design and hardware interfacing, chap. 6, 253{316. In
Predko (2000d).
|||. 2000d. Programming and customizing picmicro R microcontrollers. McGraw-Hill.
|||. 2001. Picmicro R microcontroller pocket reference. McGraw-Hill.
Stallings, William. 1996. Computer architecture and organization. 4th ed. Prentice-Hall, Inc.
|||. 2000. Computer architecture and organization. 5th ed. Prentice-Hall, Inc.
Vahid, Frank, and Tony Givargis. 2002. Embedded system design. John Wiley & Sons, Inc.
Wakerly, J. F. Digital design principles and practices. 3rd ed. Prentice-Hall.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 7
Wilmhurst, Tim. 2001. An introduction to the design of small-scale embedded systems. Palgrave.
Wolf, Wayne. 2000. Computers as components:principles of embedded computing system design.
Morgan Kaufmann.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006(EE25M) –                     
Introduction to Microprocessors 
The University of the West Indies 
Department of Electrical and Computer Engineering 
Last Review:  December 2, 2005 
 
 
Course Instructor: 
Name: Ajay Joshi 
Email: ajoshi@eng.uwi.tt 
Phone: ext. 3144 
Office: Room 325 
Office Hours: as posted on door 
 
Course Support 
Name: Aaron Joseph 
Email: Given at Later Date 
Phone: ext. 3156 
Office: Room 329 
Office Hours: Tuesday and  
                      Thursday 1pm-3pm 
 
Aims 
To cultivate the student’s ability to utilize, comprehend the operation, and criticize the 
efficacy of, a microprocessor in an applied context. 
 
Objectives 
At the end of this course the student will be able to: 
• in general, or (given relevant diagrams and specifications) for a particular system, 
identify, and describe the role of, the components of  
(a) a microprocessor,  
(b) a microprocessor-based system, 
(c) a development suite (hardware, software) for a microprocessor-based system. 
• illustrate how data can be represented in (and accessed from) memory, implement 
arithmetic and data manipulation operations in assembly language, and assess how 
well code will perform in a given context, given the architecture and instruction set of 
a microprocessor 
• design and implement an appropriate interface between a microprocessor-based 
system and a peripheral device (or another microprocessor-based system), given 
relevant datasheets and suitable parts 
• select (and critique the selection of) a microprocessor-based system for an 
application, given relevant datasheets and application requirements 
 
Course Overview 
Microprocessors have been one of the most widely used methods of incorporating 
flexibility and intelligence into automated devices.  Their general-purpose nature, speed 
and size have made them one of the most common components in Electrical Engineering. 
It is therefore necessary to develop a good understanding of their operation and how they 
can be used as building blocks for automated systems and control applications. 
This course explores the inner workings of a microprocessor from the programmer's 
perspective, as well as treating with external hardware issues such as interfacing, and 
ECNG2006 (EE25M) March 11, 2008 8
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selection criteria for microprocessors. Exercises and examples are based on the PIC 
16F877 microcontroller. The syllabus follows:  
Microprocessor architecture (PIC16F877); Microprocessor development and 
support systems(MPLAB); Binary, integer and floating point arithmetic 
operations; (PIC16Cxxx) assembly language programming; Interfacing 
(PIC16F877): I/O ports, Timers, Interrupts, A/D conversion, PWM; System 
Issues; Serial/Parallel Communication.
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Prerequisites 
There are no departmental pre-requisites, however, it is presumed that the student has 
acquired the necessary pre-requisite skills in the preceding electronics and computer 
systems courses. It is presumed that the student is able to: 
• correctly interpret syntax of simple C programs, representation of different numeric 
bases in C, and utilize an ANSI C compiler/IDE. 
• treat with the concept of numeric bases, and perform translation between binary, 
decimal and hexadecimal bases. 
• predict the operation of simple digital/analog circuit components, in particular 
resistors, capacitors, latches and operational amplifiers. 
• interpret pin-out diagrams, functional block diagrams, and timing diagrams as found 
on datasheets. 
• effectively use basic lab equipment: meters, oscilloscopes, power supplies, 
breadboards. 
• troubleshoot basic signal problems in a circuit (i.e. no/low voltage, intermittent 
signals, different grounds, noise). 
 
Weighting 
3 credits 
requires 12 teaching weeks; week 13 activities/mock exam not compulsory 
each teaching week: 3 hrs lecture; 3 - 6 hrs self-study; 1 hr tutorial 
additional time for group project 6-9 hours. 
associated labs in ECNG2005 (EE24D):  
6 labs @ 3hrs/lab; individual project @ 6 hours 
 
Evaluation 
Type of Evaluation Description % of 
Course 
Grade
End of Semester Exam: 
3 hours – compulsory pass  
Short answer/design (cover major learning objectives) 
60% 
In-semester Exam: 
Mid-term exams in Wk 6 and 12 @ 6% 
Mock Exam in Wk 13; not assessed 
12% 
Laboratory: 
Lab exercises & individual project contribute to grade in 
ECNG2005 (EE24D)  
0% 
Mini-Project: 
Group lab exercise (5-6 people per group) 6% 
Research  Paper: 
Reading Assignment in Wk 12 2% 
Other: 
Online Tutorials (10 % )  
5 Assignments (2% each) 
Optional participatory activities (1% each contribution;  
total not exceeding 4%; category total not exceeding 20%)  
20% 
TOTAL: 
100% 
 
ECNG2006 (EE25M) March 11, 2008 10
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2007/08 Semester II Calendar 
 
 Mon Tue Wed Thu Fri Sat Sun 
Wk 1 14   Jan 15 16 17 18 19 20 
Wk 2 21 22 23 24 25 26 27 
Wk 3 28 29 30 31       1   Feb 2 3 
Wk 4 4-Carnival 5- Carnival 6 7 8 9 10 
Wk 5 11 12 13 14 15 16 17 
Wk 6 18 19 20 21 22 23 24 
 25 26 27 28 29 1 Mar 2 
Wk 7 3 4 5 6 7 8 9 
Wk 8 10 11 12 13 14 15 16 
Wk 9 17 18 19 20 21-Good 
Friday
22 23 
Wk 10 24-Easter 
Monday 
25 26 27 28 29 30-Baptist 
Liberation
Wk 11 31 Bap Lib 
Holiday 
1 Apr 2 3 4 5 6 
Wk 12 7 8 9 10 11 12 13 
Wk 13 14 15 16 17 18 19 20 
Exams 21 22 23 24 25 26 27 
Exams 28 29 30 1 May 2 3 4 
Exams 5 6 7 8 9 10 11 
Exams 12 13 14     
 
Coursework 
Laboratory Exercises:  Marks towards ECNG2005 (EE24D) 
 
Mini Project:   Group must choose one of the assigned projects 
4-6 people per group w/up to 5 groups per topic 
Marks: Design 2%; Implementation 2%;  
Report 1%; Demo/Presentation 1% 
Weighted by individual contribution to group 
 
Assignments 
Research Paper/s:          Submit an informative abstract about an article 
chosen/assigned from IEEE Magazines 2002-7. Item 
should be a 5-7 page article on a topic of relevance to 
microprocessor-based systems. 
Presentation/s:                Associated with the Group Project – 1 minute  
Problem Set/s:   5 Assignments; written, 2 weeks to submit 
Other:   tutorial questions online; 30 minute completion time
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Resources 
Textbooks:             Category A: Essential Texts  
1) An introduction to the design of small-scale embedded 
systems, T. Wilmhurst. 
 
Category B: Highly Recommended Texts  
1) The Quintessential PIC Microcontroller, S. Katzen. 
2) Embedded Systems Architecture, T. Noergard 
 
Category C: Recommended Texts  
1) Programming and customising PICmicro microcontrollers, M. 
Predko (2nd Edition).  
2) Microcontrollers and microcomputers, F. Cady. 
 
General Reference:  
1) Computer Organisation & Architecture, W. Stallings, (5th 
Edition or later) 
2) Embedded System Design: A Unified Hardware/Software 
Introduction, F. Vahid, & T. Givargis  
3) Logic and Computer Design Fundamentals, M. Mano & C. 
Kime 
4) Digital Design: Principles & Practice by J. Wakerly, (3rd 
Edition or later) 
 
Course Notes:   will be distributed in class and/or made available online 
On-Line Resources:  Topical Videos: http://murl.microsoft.com/default.asp 
 Course site: http://www.eng.uwi.tt/depts/elec/staff/cradix/ee25m/ 
 Study Tips: http://www.iss.stthomas.edu/studyguides 
PIC Micro WebRing: 
 http://www.webring.org/cgi-bin/webring?ring=picmicro;list 
Simulation Tools:  (in microprocessor and shared labs) MPLAB/CCS compiler 
Laboratory Tools: 20 seat lab.  
Per seat: LED's, resistors, potentiometers, ICD header + RJ12 
cable + ICD debug module + serial cable, Breadboard, wire and 
5V power supply, 7 segment LCD display, transistors, LCD text 
display, DS1821 temperature sensor, additional serial cable, DC 
motor, stepper motor, encoder disc/IR led/detector, keypad. 
 Additional components for group projects.  
Other:  --none-- 
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Lecture/Tutorial Schedule  
Course has: 33 learning units; each unit 1 hr lecture + coursework 
In addition to learning units there is: 
 1 hr overview + 1 hr mid-term + 1 hr mid-term 
Tutorial activities include 
4 hours topic review 
2 hours guest lecture – informative abstract; 
2 hours midterm review 
4 hours group project work 
 
No. Unit #: Topic Notes/Delivery/Comments
1
  Text
2
 Associated C'work Deadline  
1.    Course Overview Survey   
2. 1: ?P basics Tutorial 1:a  Ch. 1 
3. 2: ?P system basics Tutorial 1:b  Ch. 1/2/4 
4. 3: ?P support circuitry Tutorial 2:a  Ch. 2/4 
5. 4: ?P architecture Tutorial 2:b  Ch 1 
6. 5: Typical instructions Assignment 1  Ch. 3 
7. 6: Software tool chain Tutorial 3:a  Ch 3/7 
8. 7: Development support Tutorial 3:b  Ch 3/7 
9. 8: Families & forms Tutorial 4:a  Ch. 1 
10. 9: PIC16F877 overview Tutorial 4:b  Ch. 1/2/3 
11. 10: MPLAB overview  Lab 1  Ch. 3 
12. 11: Real numbers Assignment 2  Ch. 11 
13. 12: Integer arithmetic Tutorial 5:a  Ch. 11 
14. 13: Real number arithmetic Tutorial 5:b  Ch. 11 
15. 14: Data structures Tutorial 6:a  Ch 3 
16. 15: Basic operations Lab 2  Ch 3 
17. 16: Code comparison Assignment 3   
18. 17: Compiler limitations Tutorial 6:b  Ch 7 
19. 18: Interfacing Idioms Tutorial 7:a  Ch. 2/9 
20. 19: Interrupt Basics Tutorial 7:b  Ch. 3 
                                                           
1
 Whichever applicable 
2
 Specify 
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No. Unit #: Topic Notes/Delivery/Comments
1
 Text
2
 
21. Mid-term  
22. Midterm review+ Stud't Feedback Grp. Project  
23. Guest Lecture – H. Lawrence   
24. Guest Lecture – H. Lawrence Reading Assignment
25. Topic Review   
26. Topic Review   
27. 20: Typical peripherals Tutorial 8:a  Ch. 5/9 
28. 21: Digital troubleshooting Tutorial 8:b  Ch. 3 
29. 22: PIC16 peripherals Lab 3  Ch. 2/4/5 
30. Group Project   
31. 23: Interrupt issues Assignment 4  Ch. 8 
32. 24: ?P interface/timing issues Tutorial 9:a  Ch. 9 
33. 25: Power/Signal issues Tutorial 9:b  Ch. 10 
34. Group Project   
35. 26: ?P choice/comparison Tutorial 10:a  Ch. 12 
36. 27: Case study Tutorial 10:b  Ch. 12 
37. 28: Design guidelines Assignment 5  Ch. 12 
38. Group Project   
39. 29: Communication protocols Tutorial 11:a  Ch. 6 
40. 30: Bit-banging vs. hardware Tutorial 11:b  Ch. 6 
41. 31: I/O on the PC Lab 4  Ch. 2 
42. Group Project   
43. 32: PC to ?P serial link Lab 5  Ch. 6 
44. 33: ?P Parallel Interface Lab 6  Ch. 2 
45. Topic Review   
46. Topic Review   
47. Mid-Term  
48. Stud't F’back;+ Midterm Review   
49. Group Demos/Presentations   
50. Mock Exam (3 hours; proctored)   
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Unit objectives 
At the end of this course the student will be able to: 
Section I: ?P Overview 
1. identify the components of a typical microprocessor (ALU, registers, stack), describe 
the operation of the components of a typical microprocessor, and describe the 
representation of instructions in the operation of the instruction cycle. 
2. identify the components of a microprocessor-based system (CPU, bus, 
memory/layout, peripherals), describe the operation of typical components of a 
microprocessor-based system, and discuss issues involved in the design/choice of the 
various components of the microprocessor(-based system)  
3. describe (and/or illustrate) how bus addressing, conflict resolution and memory maps 
may be implemented, and explain the function of basic support circuitry for 
microprocessors such as clocks, reset circuitry, watch dog timer, capacitors to ground 
etc. . 
4. differentiate between organization and architecture, and relate instruction set design 
to the programmer’s model of the microprocessor,. 
5. explain the operation of typical machine instructions, addressing modes, status bits, 
and infer the consequences of sequences of generic instructions. 
Section II: ?P development 
6. explain the roles of the compiler, linker, assembler, simulator, emulator, debugger in 
the software development tool chain, and the role of scripting languages, 
make/configure-like utilities and IDE’s in the software development process. 
7. explain the role of firmware, operating systems, oscilloscopes, logic probes/analyzers, 
terminals, disk drives, external memory, debug protocols e.g. JTAG and host 
computers in the development and support of microprocessor based systems. 
8. recognize and differentiate between the different commercially available 
families/forms of microprocessor based systems (Motorola, AVR, PIC – 
chip/component packages -- SOC,  microcontroller, PC, back-plane bus, PC104, 
standalone, embedded) based on their pictures/properties/descriptions and supported 
development tools. 
Section III: PIC16 Introduction 
9. explain the memory layout, operation of the instruction cycle, and the basic 
instructions (move, add, subtract, shifts) for the PIC16F877 microcontroller. 
10. utilize the MPLAB IDE , CCS compiler, and other software tools, to develop 
software, for the MicroChip PIC16xxx series of microcontroller, in C, C++ and 
assembly language. 
Section IV: Numbers & Data  
11. represent real numbers using fixed and floating point binary representations. 
12. perform integer arithmetic operations using binary representations of the operands. 
13. perform fixed/floating point arithmetic operations using binary representations of the 
operands. 
14. describe how data (and data structures) can be represented and manipulated, with 
reference to alignment and byte/bit ordering issues. 
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Section V: Algorithms on PIC16 
15. implement basic programming operations (loops, swaps, lookups), integer arithmetic, 
and other computation/numerical methods, in assembly language on the PIC16Cxxx 
series of microcontrollers. 
16. contrast alternate programming techniques in terms of size/speed of the code 
produced for the PIC16Cxxx series of microcontrollers. 
17. give examples of the limitations placed on compiler-generated code, for the 
PIC16Cxxx series of microcontrollers, by the need for generality. 
Section VI: Interfacing Peripherals 
18. identify alternate ways in which components may be interfaced with a microprocessor 
(i.e. common idioms for interacting with hardware): single bit (with debounce), 
matrix, map into memory space, etc. . 
19. explain the operation of interrupts, the different types of interrupts (h/w, s/w), how 
interrupts may be prioritized (peripheral interrupt controller), and identify common 
applications of interrupts. 
20. explain the operation of commonly used peripherals: PPI/PIA, A/D, Timers/Counters, 
PWM, D/A. 
21. select and apply techniques for troubleshooting digital circuitry using standard 
laboratory equipment. 
22. utilize built-in peripherals of the PIC16F877 microcontroller in simple applications. 
Section VII: System Issues 
23. understand the implications of (nested/multiple) interrupts for execution times, and 
the need for context saving. 
24. interpret and recognize the implications of, microprocessor timing and interfacing 
requirements. 
25. determine the noise, voltage, current and power considerations due to 
microprocessors and peripheral fan-in/out and how different logic families (as well as 
inverted voltage logic) may be used in conjunction in a design. 
26. compare alternate microprocessors in terms of the instruction set, and features(such as 
cache pipelining etc.) and the best performance, which may be facilitated by the 
instruction set and features. 
27. recognize the issues and tradeoffs involved in the design of a microprocessor based 
(embedded) system, and justify the role of a microprocessor(-based system) in an 
applied context. 
28. remember and follow a checklist of design guidelines, in order to perform a simple 
design for a microprocessor based system. 
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Section VIII: Communication 
29. understand commonly used parallel and serial communication protocols (I2C, CAN, 
RS232, USB, Firewire) and interpret descriptions of proprietary protocols (Dallas 1-
wire). 
30. differentiate between bit-banging and dedicated hardware interfaces for serial/parallel 
communication (PPI/PIA vs ACIA/UART, PPC)  
31. perform I/O programming on the PC in C/Assembly using the Visual C++ compiler 
32. produce assembly language programs for communication between a microprocessor 
and a PC using the parallel and/or serial port, given UART and PPC 
datasheets/details, and respective instruction sets. 
33. interface external peripherals/devices to the PIC16F877 microcontroller using a 
parallel bus, and interface the PIC16F877 microcontroller as an external device on a 
parallel bus (memory mapped). 
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Course Rules: your rights and responsibilities 
You have the right to receive a fair mark for any assessed coursework which you 
produce.  
• Weighting of individual assignments will be issued at the beginning of the 
semester.  
• Marking schemes will be issued with each piece of assessed coursework.  
• You may request clarification of marking schemes at any time prior to 
submission. 
• You should receive feedback (individual, general or both) on coursework 
within 1 week. 
• You may query the mark assigned to your coursework, if it has been totaled 
incorrectly, or is not consistent with the published marking scheme. 
You have the right to know the examination format, as well as the requirements for 
passing the final examination, and the course.  
You are entitled to lectures, which start and finish on time. You are entitled to 
appropriate notice of lecture/lab cancellation.  
You are entitled to have any concerns about the course heard. Individual queries 
should be addressed to the lecturer (or TA) in the first instance and the HoD in the 
next instance. Anonymous or group queries should be directed to the student 
representative or your tutor in the first instance, who is obliged to bring it to the 
attention of the lecturer and/or HoD.  
You are responsible for checking posted examination schedules and arriving on time 
for exams with your examination and/or ID cards, and writing implements.  
You are responsible for arriving to lectures/labs on time, and participating in all 
course activities.  
You are responsible for submitting coursework by the posted deadline, and for 
securing a receipt (or signing a submission sheet) for all submitted coursework.  
You are responsible for seeking assistance from or notifying staff if you any 
difficulties (academic, personal or health-related).  
You will be held at fault if you do not comply with all university, departmental and 
faculty regulations, as well as with the course policy on collaboration. Ignorance is 
not a valid excuse.  
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Policies on plagiarism, collaboration and late submission 
 
Late submissions may fall into one of two categories, late submissions for online 
coursework and late submission for written coursework. All online coursework will be 
due at 11:59PM on the submission date and all written coursework will be due at 4PM on 
the submission date. 
For online coursework, once the submission time has passed, the coursework will no 
longer be accepted; and you will receive a grade of zero for the piece of coursework. 
For written coursework, will be assessed in accordance with the published marking 
scheme, but 0.5% of your course mark will be deducted for each 24 hour period (or part 
thereof) a piece of coursework is late. The minimum mark awarded will be 0%. e.g. an 
assignment graded at 8 marks out of 10, which translates to 1.6% towards the course 
mark. If the tutorial was handed in at 8am (16 hours late), 0.5% would be deducted, 
leaving 1.1% towards the course mark. 
Written coursework must be in the possession of the Course Lecturer or Support Team 
Leader to have been submitted. 
The ONLY exception to this rule is if you have a valid medical/sick leave for the days in 
question, which you have submitted to the Electrical office/Registry; or if an 
unforeseeable national/personal emergency prevented you from getting here. In either 
case, I need to get formal notice from the Head.  
 
"Plagiarism is using others' ideas and words without clearly acknowledging the source of 
that information" -- from Plagiarism: What It is and How to Recognize and Avoid It, 
http://www.indiana.edu/~wts/wts/plagiarism.html 
Plagiarism can occur in many different ways:  
• using someone else's words/work without quotes or acknowledgement  
• using someone else's ideas without acknowledgement  
• using information gathered or collected by someone else without 
acknowledgement  
-- from Plagiarism: What It is and How to Recognize and Avoid It, 
http://www.indiana.edu/~wts/wts/plagiarism.html  
``Plagiarism is the presentation by a student of an assignment which has in fact been 
copied in whole or in part from another student's work, or from any other source (e.g. 
[solutions,] published books or periodicals), without due acknowledgement in the text.'' -- 
EE26A policy on plagiarism  
 
Collaboration on assignments is often helpful, and has been shown to increase 
understanding of all parties involved in the collaboration. It is OK to ask someone else if 
they think your work is correct, and if not, they can point out where you made a mistake, 
and give an explanation of the error. If you give (or are given) a worked example to 
improve understanding, the example should be as unrelated to the assignment as possible. 
Inappropriate collaboration (for the purposes of this course) includes:  
• for individual or group assignments allowing your assignment to be copied, or 
copying someone else's assignment, either electronically, manually or by 
photocopy. Ignorance of the copying is no excuse -- you are advised to secure all 
coursework material.  
ECNG2006 (EE25M) March 11, 2008 19
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Coursework Portfolio
AIM: to promote re ective learning of both students and lecturer
O ine Version1
Students are asked to keep a manila folder with all coursework and feedback forms. Items should
be fastened into the folder using a single hole and a treasury tag. Your ID# should be placed on
the folder tab. Your name is not required.
You should use this folder to review your progress throughout the semester. The folder should be
submitted to the lecturer after the  nal exam, who will use the information to determine appropriate
course adjustments.
Please feel free to make frank observations, constructive criticism, and realistic suggestions in the
feedback sections.
Feedback will not adversely a ect your course grade. Any students who have reservations about
how such feedback is being used may withhold feedback until after the coursework is marked, or
omit feedback entirely.
Please start your folder with the Course Perceptions and Skills Questionnaires within the  rst 2
weeks of the semester.
Online Version
Students are asked to keep a Wiki containing scans of/re ections on all coursework and feedback
forms. You should use this Wiki to review your progress throughout the semester. The Wiki will be
viewed by the lecturer, who will use the information to determine appropriate course adjustments,
and o er correction or advice.
Please feel free to make frank observations, constructive criticism, and realistic suggestions. These
will not adversely a ect your course grade. Any students who have reservations about how such
information is being used may withhold comments until after the coursework is marked, or until
the end of term.
Please start by executing the online Pre-Test and making a Wiki entry reviewing your course
preparedness within the  rst 2 weeks of the semester.
1Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
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ECNG2006 (EE25M) March 11, 2008 21
Course Perceptions Questionnaire O ine Version2
Please read the course outline and/or the course web page before attempting this questionnaire.
The questionnaire should require about 30 minutes to complete. It is not a test, and will not
contribute to your course grade.
 What do you think this course is about?
 What do you want to get from this course?
 What do you think you will have to do in this course?
 What skills do you think you need in order to do well in this course?
 Fill in the blanks in the following statements:
1. Each week, I expect to do hours of self-study for this course.
2. The coursework for this course is worth % of my  nal grade.
3. There are learning units in this course.
4. Open Hours will be held on from to .
 Label each of the following statements as either True(checked)/False:
{ Week 13 classroom activities are all voluntary.
{ I need to pass the exam in order to pass this course.
{ If I cannot make the tutorial, then the deadline doesn’t apply to me.
{ Ignorance of departmental, faculty or university regulations is a valid excuse for violating
them.
2Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
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ECNG2006 (EE25M) March 11, 2008 22
 Label each of the following statements, concerning student behaviour in the course, as either
Acceptable (checked) or Unacceptable:
{ Student A copies Student B’s program onto a diskette with Student B’s permission.
{ Student A copies Student B’s program onto a diskette without Student B’s permis-
sion.
{ Student A does Student B’s coursework.
{ Student A does a tutorial set individually and compares answers with Student B.
{ Student A has a problem with an assignment, and reads Student B’s answer.
{ Student A has a problem with an assignment, and uses "draft" for assistance.
{ Student A has a question about a lab, which Student B answers.
{ Student B reads, and points out an error in, Student A’s code.
{ Student B steals Student A’s printout o the printer.
{ Student B submits a program that contains code taken from the Internet, and does not
acknowledge his source.
{ Students A and B submit identical assignments.
{ Students A and B work together to understand an assignment, and then complete it
individually.
{ Students A and B work together, and submit identical assignments.
 For each of the following questions, select the most appropriate answer.
1. The penalty for late submission of coursework is:
(a) 1.25 marks per hour.
(b) 0.125% per 3 hours.
(c) 0.125% per day.
(d) there is no penalty for late submission.
2. The purpose of all these feedback forms, and tutorials is to:
(a) stress me out.
(b) catch cheaters.
(c) make sure that I am doing the work.
(d) provide continuous feedback about my performance in the course.
3. In order to learn how to program microprocessors in assembly, which of the following is
MOST e ective:
(a) read books about assembly.
(b) practice writing assembly programs on paper.
(c) test assembly programs in simulation.
(d) test assembly programs on microprocessors.
4. Which of the following is NOT a prerequisite skill for this course:
(a) I must be able to program in C++.
(b) I must understand hexadecimal and binary number representations.
(c) I must be able to predict how simple digital/analog circuits will work.
(d) I must be able to read/understand diagrams typically found on datasheets.
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Prerequisite Skills Questionnaire O ine Version3
The questionnaire should require no more than 60 minutes to complete. If you are unable to
answer a question please write ‘Unable to answer’ in the space provided. It is not a test, and will
not contribute to your  nal grade.
1. Indicate which of the following courses you have already registered for, and the last grade
you obtained in each course
 analog electronics
 digital electronics
 introduction to computers
 computer systems / C++
2. Name any C/C++ compilers/IDE’s which you are familiar with/have used.
3. Write down the screen output of the following program:
int main()
{
int i;
for (i=0; i<5;i++)
{
cout<<"The number is "<<i<<"\n";
}
return 0;
}
4. A group of 12 marbles may be grouped as:
 1 group of 10, and 2 groups of 1, OR
 1 group of 7, and 5 groups of 1
Use this example to explain the concept of numeric bases.
3Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
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5. Convert the number 22710 to binary (base 2) and hexadecimal (base 16).
6. How would you write the number A16 in a C program so that the compiler understood that
it was a hexadecimal number?
7. What is the relation between current, voltage, and power dissipated for a resistor of value R?
8. What is the relation between current, charge held, and voltage for a capacitor of value C?
9. Draw the symbol for and describe the operation of a D-type latch.
10. Draw the electrical symbol for and describe the operation of a simple NPN transistor.
11. Draw the electrical symbol for and describe the operation of a basic operational ampli ers.
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12. Describe in your own words, the function of each of the following pieces of lab equipment:
 meter
 oscilloscope
 power supply
 breadboard
13. Which piece of lab equipment would you use to determine if there is no/low voltage on the
power line.
14. Which piece of lab equipment would you use to determine if there are intermittent signals on
a signal line.
15. Identify one possible source of EM noise in the lab.
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16. A pin-out diagram, functional diagram and timing diagram for an DIP packaged IC are shown.
(a) Which pin is the Vo signal connected to?
(b) What is the relation of the OUT signal to the signals at pins 1 and 2?
(c) What time will elapse between a change at pin 1 and a corresponding change in Vo?
Diagrams taken from
http://www.datasheetarchive.com/semiconductors/download.php?Datasheet=1696386
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) Introduction to microprocessors
c DECE, UWI, St. Augustine, Trinidad
original author: Feisal Mohammed; updated: March 11, 2008{ CLR
Part I
Microprocessor Overview
Central processing units (CPU s) for early computers, were originally constructed from discrete
electronic components. The microprocessor as we know it today has all the CPU functions contained
in a single silicon chip. Because of the component densities involved, such chips are designed using
hardware de nition languages to facilitate component layout; however they are still made up from
the same basic digital components: logic gates, adders, data latches, counters etc.
The intent of this course is to bridge the gap between high-level programming (which requires
no knowledge of microprocessor internals) and digital circuit theory (which do not explain how a
microprocessor can be constructed therefrom) as covered by other courses; and further to examine
the ways in which microprocessors can be used to control external devices.
In this  rst part, a quick review of microprocessor operations is made to familiarise the reader with
terminology and issues.
1 Microprocessor Basics
At the end of this unit the student will be able to:
 identify the components of a typical microprocessor (ALU, registers, CU, bus),
 describe the operation of the components of a typical microprocessor,
 describe the representation of instructions in the operation of the instruction cycle.
The central processing unit consists of four major parts:
 the registers, are a form of internal memory, which may be implemented using edge triggered
 ip- ops (latches). Registers within the CPU are used to specify the address of the next
program instruction (program counter PC), to hold the retrieved instruction (instruction
register IR), the instruction arguments (general purpose registers), the instruction result
(accumulator or working register), and the validity ( ags or status register) of the instruction
operation.
 the Arithmetic and Logic Unit (ALU ) consists of various combinatorial and/or sequential
logic circuits which perform addition, shift, complement, bit-wise AND, bit-wise OR etc. on
the data inputs, producing an output. The function to be performed is speci ed by the control
inputs.
 the Control Unit (CU ) exerts the control signals for all the other CPU components; it co-
ordinates the activities of the CPU components in order to retrieve and perform the desired
machine instruction through a sequence of micro-operations.
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Figure 1: Figures from (Stallings 2000)
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 the internal bus, which transports information between the di erent components of the CPU.
The bus contains several electrical \lines", whose values are changed/read simultaneously. All
other CPU components are connected to the bus, and place data on/read data o it, when
instructed by the CU.
As in sequential logic circuits, activities within the CPU are coordinated by an externally provided
clock signal. The time between successive clock edges is often referred to as the clock cycle (period).
The machine cycle is the smallest quantum of time in which the CU carries out batches of micro-
operations; the machine cycle must be a multiple of the clock cycle.
The machine instruction is represented as a sequence of 0’s and 1’s known as bits. It is often
written as as binary or hexadecimal number. Typical machine instructions include data transfer and
arithmetic operations. A program which executes on the CPU is a sequence of machine instructions,
which are carried out one at a time. The instruction cycle as it is called can be broken down into
actions which must be repeated for each instruction, such as fetch, indirect (decode), execute, and
write. The CU is responsible for co-ordinating the fetch of an instruction, decoding the instruction,
co-ordinating the retrieval of any additional operands, instructing the other parts of the CPU to
carry out the instruction, and co-ordinating the storage (write) of the  nal result. It does so by
using a sequence of micro-operations in consecutive machine cycles. Each micro-operation in e ect
translates to the assertion of particular control signals by the CU.
For example, to execute an addition, the CU must:
 signal for values to be placed on the bus,
 signal the ALU adder inputs to latch it’s inputs from the bus,
 assert the ALU control signal for addition
 trigger the ALU
 signal the ALU adder output to be placed on the bus,
 signal for values to be latched into storage from the bus
Each step is a micro-operation. Micro-operations can be carried out in parallel as long as they
are not dependent on the result of a previous micro-operation. The number of sequential micro-
operations required for the various stages of each instruction in the instruction set, determines the
duration of the instruction cycle.
The indirect and execute parts of the instruction cycle will use di erent micro-operations depending
on the machine instruction. The amount of logic circuits (and hence time for the indirect part of
the instruction cycle) for an instruction will depend on the instruction bit-width and the complexity
of the instruction (i.e. number and type of operands etc). The execution time will also depend on
the sequence of control signals the CU must perform, and the time taken for the various parts of
the CPU to respond.
Note: Where there is great disparity, certain instructions in the instruction set may be speci ed as
taking more than one instruction cycle.
The sequence of micro-operations, required to implement a particular instruction, will also depend
on the organisation/layout of the CPU. The sequence/time can be reduced by using additional
special-purpose hardware (for example the inclusion of a multiplier unit in the ALU), or duplication
of certain heavily used functions (for example multiple addition units).
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The micro-operations for a particular instruction may be hard-wired (i.e. the control signals are
 xed w.r.t the instruction code), or can in turn be implemented as micro-code (i.e. the micro-
operations for a particular instruction are in turn looked up in some internal memory of the CU;
e ectively another (albeit primitive) programming level). Examples of CU implementation may be
found in (Noergaard 2005; Section 4.2.1 Central Processing Unit).
Micro-code is most useful in Complex Instruction Set Computer (CISC ) architectures, where large
multi-operand instruction sets require make it impractical to hard-wire control signal logic. CISC
architectures arose when the time for external memory fetch vastly exceeded the processing time
within the CPU. It was thus presumed to be advantageous to reduce the number of instruction
fetches by making more complex, wider, instructions. For example an addition may be performed
in two ways: load 5, add 1 or add 5,1. The former requires two memory fetches, the latter a
single memory fetch.
In contrast, once external memory access speeds improved, a di erent philosophy was proposed:
Reduced Instruction Set Computer (RISC ) Architecture. The main principle behind Reduced In-
struction Set Computing (RISC) is that it is better to get a processor to do a few simple instructions
very quickly, than to get it to do many di erent instructions at the expense of speed and e ciency.
RISC architectures have so few instructions that there is no advantage (and a time disadvantage)
to adding another level of decoding, therefore micro-code is rarely used for such architectures.
Pipelining is a technique developed to improve performance by overlapping operations which can be
performed concurrently. Pipelining can be applied to the four parts of the instruction cycle listed
previously, i.e. Fetch, Decode, Execute, and Write. Non-pipelined processors do all four actions for
a single instruction before starting the process on a new instruction. However, if each stage could
be simpli ed, and the hardware designed such that each operation can occur independently, then
a pipeline can be implemented such that the outputs of each stage provide the input to the next.
Although each instruction will take the same time for complete execution, consecutive instructions
will be executed more quickly. Pipelining works best if all instructions execute within the same
time. RISC architectures more readily facilitate pipelining, because of the small  xed instruction
size, and the emphasis on the speed of instruction execution.
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Review Exercises
1. Write a de nition for all the italicised terms in this unit.
2. Label the following statements as True/False:
(a) The CU co-ordinates the operation of the various parts of the CPU.
(b) Registers are located in a special section of RAM external to the CPU.
(c) The PC contains the address of the last instruction fetched.
(d) The CPU co-ordinates all operations within the CU.
(e) The Program Counter (PC) contains the address of the instruction being executed.
(f) The ALU is responsible for decoding instructions.
(g) The ALU is the part which executes the program.
(h) The internal bus conveys information between the CPU and the microprocessor.
(i) The ALU is separate from the CPU.
(j) Registers are storage locations external to the CPU.
(k) A CPU must have a stack, as well as special function registers.
(l) The internal CPU bus transports information between the CU and the CPU.
(m) The clock signal is used by the CPU to time the phases of the instruction cycle.
(n) CISC architecture processors have larger, more  exible instruction sets than RISC ar-
chitectures.
(o) RISC instructions require fewer micro-operations per instruction than CISC instructions.
3. For each major part of the CPU
(a) state the primary function of the part
(b) describe the role of the part in a typical CPU instruction cycle
(c) draw a diagram to illustrate how the part could be implemented using digital logic.
4. Which ONE of the following stages is NOT part of the microprocessor instruction cycle:
(a) instruction fetch stage
(b) execute stage
(c) instruction decode stage
(d) micro-operation stage
5. The overall purpose of the CU is (choose ONE answer):
(a) to co-ordinate the operation of the various parts of the CPU.
(b) to place information on the internal CPU bus.
(c) to communicate with memory and peripherals.
(d) to pipeline instructions, by performing di erent micro-operations simultaneously.
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6. Choose the word combination which best completes the following sentence:
Within the microprocessor, the 6I perform(s) arithmetic operations on information
delivered from the 6II via the 6III .
(a) 6I: ALU 6II: registers 6III: bus
(b) 6I: ALU 6II: registers 6III: CU
(c) 6I: CU 6II: ALU 6III: bus
(d) 6I: CU 6II: ALU 6III: registers
(e) 6I: registers 6II: CU 6III: ALU
7. Di erentiate between the following items:
(a) CPU and microprocessor
(b) machine instruction and a micro-operation
(c) ALU and CU
(d) machine cycle and instruction cycle
8. For a simple CPU, list the micro-operations the control unit must do to execute a sub-
traction; presuming that the ALU can perform subtraction, and that the arguments are in
the accumulator and instruction respectively.
9. RISC, CISC and pipelining are all approaches to improving CPU performance, yet they are
based on very di erent concepts. What would you consider if you had to decide which was
best for a particular application?
10. Role Play: Assign people to be the parts of the CPU: CU, ALU, bus, and the registers (PC,
IR, Accumulator, and a general purpose register). Work your way through the instruction
cycle for each instruction of a simple program, involving load, store, jump, and arithmetic
operations.
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Tutorial Exercise 1A O ine Version4 ID#
1. In your own words, describe what a register is, and give examples of how it interacts with, or
is used by, each of the other three parts of the CPU. 4 marks
2. In your own words, di erentiate between the following items:
(a) machine instruction and machine cycle 3 marks
(b) clock signals and micro-operation control signals 3 marks
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
4Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
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T
U
T
1
AUnit 1 Your lecturer will put up a diagram with several parts labeled with letters.
Write the letter you have been assigned here .
Answer the following questions for the part whose letter you were assigned.
1. The part is called the .
2. Circle the correct answer. The primary function of the part is:
(a) to perform arithmetic and logic functions
(b) to co-ordinate operations within the CPU
(c) to store data within the CPU
(d) to transfer data between di erent parts of the CPU
3. Describe the role of the part, using a typical CPU instruction cycle.
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ identify the components of a typical microprocessor (ALU, registers, CU, bus),
{ describe the operation of the components of a typical microprocessor,
{ describe the representation of instructions in the operation of the instruction cycle.
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
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2 Microprocessor System Basics
At the end of this unit the student will be able to:
 identify the components of a microprocessor-based system
 describe the operation of typical components of a microprocessor-based system
 discuss issues involved in the design/choice of various components for a microprocessor-
based system
Ball (2000) de nes a microprocessor as
\An integrated circuit containing, at minimum, a central processing unit and a means
to access external memory."
The central processing unit (CPU) accesses external memory to obtain machine instructions (what
to do) and stored data (what to do it to).
There are several types of memory; all memory may be read from, but only certain types may be
written to. RAM (Random Access Memory) is the most  exible, allowing the user to read and
write without any special considerations. However this is volatile (i.e. it disappears when power
is removed). Further, dynamic RAM must be periodically refreshed even while power is supplied,
making access times relatively slow. The advantage of dynamic RAM is that it is denser (more
memory per unit area) and cheaper than static RAM due to the simpler circuitry required.
ROM (Read Only Memory) and PROM (Programmable Read Only Memory) may only be con g-
ured once, and are then permanent. The di erence between these two is the means by which they
are written. ROM’s are con gured in the factory before packaging. PROM’s o er  exibility, in
that they are con gured after the chip has been packaged. EPROM ’s are erased under UV light,
and can then be re-programmed in a similar manner to PROM’s. Flash EEPROM memory has the
advantage that it may be re-programmed in-circuit. While access times for ROM’s are comparable
to or better than those for RAM, the write time for Flash EEPROM’s are signi cantly longer.
Each of these types of memory can be visualised as an array of memory cells. The ability to
access external (or internal) memory which consists of more than one item, requires some method
of addressing. i.e. a means of specifying which location within the memory is to be accessed. A
single memory cell may be represented stylistically as a black box which stores a binary value (0/1)
with 3 connections: a Write Line, a Select Line and a Data Line. When the Select line is not active,
no action is taken. When the Select Line is active, and the Write Line is active, information from
the Data line is stored. When the Select Line is active and the Write Line is not active, the stored
data is output. The actual implementation of the memory cell may be latch based (static RAM),
capacitive based (dynamic RAM), or hard wired connections (ROM/PROM).
When choosing memory, there are three major considerations:
 whether it is Read-Only or Read-Write
 the timing requirements of the CPU/memory for read/write access, and
 the bit-width of the stored data
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Address decoding logic is required to connect cells in the array to the addressing mechanism. This
can be made up of a sequence of logic gates. Memory is assigned addresses in sequential order
according to the line of the decoding logic it’s select line is connected to. Multiple memory cells
may be connected to the same address line, with their outputs connected in parallel to separate
output lines. In this way a data word may be retrieved from a single address. The data word size
is typically a power of 2. The most frequently used sizes are the nibble { 4 bits, byte { 8 bits, 16,
32, 64 bits. The number of bits which make up the address are determined by a) the o set address
and b) the memory size (power of 2). It is often convenient to make the address and data bit width
identical to facilitate the sharing of the bus lines.
To retrieve data from (or send data to) external memory, the CPU must use some means of
communication with the memory. This interface is known as a bus. A bus consists of one or
more lines which are connected to multiple components. Data may be sent in one/both (uni/bi-
directional) directions across the bus. The bus may be synchronous (information is read/written
at particular instances as determined by the clock line on the bus). If more than one component
may place data on the bus, it must be arbitrated (components must seek permission to write to the
bus) either centrally, or in a distributed manner. Information sent across a bus may use a single
(one address - one data word) or block transfer(one address - multiple data words over successive
bus cycles).
The bus lines used for providing the memory address may be shared with (time multiplexed), or be
independent of the bus lines used to return the data to the CPU (i.e. separate data and address bus
lines). Bus operation is often described using timing diagrams. These diagrams are drawn using
horizontal traces each representing the signals on an individual or group of bus lines. A trace moves
between two levels representing binary 0 and 1. Where lines in a group may be at either value,
a box is used. The relative positions of signal transitions, indicate the sequence in which events
occur while cause-and-e ect relationships between signal transitions are indicated using arrows.
For synchronous buses, all transitions occur on the edge of the clock signal.
The type of bus used for a particular application depends on:
 the size of the memory/device map (i.e. the address space accessed by the bus)
 the width of the data transferred across the bus
 the timing requirements of the components connected to the bus
The CPU as reviewed above interacts only with memory, across one or more busses. If the CPU
is to interact with peripheral devices (e.g. Keyboards, LCD displays, UART, DAC), some means
of doing so must be established. Because such devices employ physical phenomena, transducers
must be used to convert to/from electrical energy. The transducers cannot be directly connected
to the system bus; I/O modules are used to interface the transducer to the system bus. An I/O
module consists of: circuitry for reading/controlling the transducer(s), a set of registers (to bu er
the transducer reading/control commands, control  ags etc), and address decoding logic. Each
I/O module responds to requests for information in a manner similar to memory. The range of
addresses to which the I/O module will respond depends on the number of registers which need to
be accessed.
The simplest means of interacting with I/O modules is to use a separate bus (or bus lines), and
include separate instructions for retrieving data from the devices. The addresses for I/O are
completely independent of the memory addresses, and can even be duplicated (as for code and
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data in the Harvard architecture). This is referred to as isolated I/O. More commonly, the memory
and I/O share the bus and address space. This is referred to as memory mapped I/O. It’s primary
advantage is that separate instructions for accessing I/O do not have to be added to the instruction
set.
In practice, the information read from certain devices needs to be sequentially stored in memory
over time (e.g key-presses). The CPU may spend a signi cant portion of time, polling/waiting for
the (slower) I/O device. Direct memory access (DMA) is the practice of delivering data from the
I/O module to a speci c area of memory without the involvement of the CPU; thereby removing
the need to poll, bu er (or lose) data in the I/O module. When the information becomes available,
the DMA controller sends it out over the bus one byte at a time. To gain control of the bus, the
DMA controller waits until the processor is about to use the bus, and puts the processor to sleep
for a bus cycle (cycle stealing). Once all data has been transferred, the DMA controller triggers an
interrupt to let the CPU know.
Interrupt processing (if the microprocessor supports it) is part of the instruction cycle. After the
write phase, the control unit checks to see if the interrupt line has been raised. If it has, it executes
call-like jump to the interrupt handler. The di erence between a call and an interrupt is in the
level of protection given to the registers. For a call, only the PC is stored and restored on return.
An interrupt should store/restore all registers, so that the program can proceed as though nothing
occurred. The starting location of the interrupt handler is generally  xed (in the same way as the
reset location is  xed). Interrupts can be used, not only by the DMA controller, but by any I/O
module which has access to the interrupt line. In doing so, the CPU can recieve/process information
right away. Interrupt-driven I/O is a viable alternative to polling, or DMA, when the interrupt
routine is small.
Because the I/O module possesses registers, and often has it s own clock signals, and requires
timing/complex algorithms to sample/control the transducer at regular intervals, the I/O modules
for certain standard peripherals are often implemented using microprocessors (albeit smaller than
the main CPU), and packaged separately as Peripheral Interface Controllers (PIC s).
The use of DMA, bus arbiters, PIC’s and/or interrupts are dependent on the properties of the
individual peripheral,
 access time,
 frequency of input/output,
 bit-width, and density of the data to transfer
as well as whether the CPU supports the use of these particular \aids". A microprocessor-based
system may contain several of each of these \aids".
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ECNG2006 (EE25M) March 11, 2008 I 17
Review Exercises
1. Write a de nition for all the italicised terms in this unit.
2. Non-volatile RAM (NVRAM) is a combination of RAM and EEPROM memory. Data is read
from/written to RAM, and backed up to the EEPROM at appropriate times. Can you think
of an application where this would be useful?
3. (a) Outline how I/O modules could use the interrupt line to move data to memory as soon
as it is available. What are the relative advantages/disadvantages compared to DMA.
(b) Most microprocessors have a single interrupt line; multiple interrupts are handled by a
special I/O module known as an interrupt controller, which has access to the micropro-
cessor interrupt line. Suggest ways in which interrupts from multiple devices could each
be handled (with their own handler) in such a scheme. Should an interrupt be allowed
to interrupt another interrupt?
(c) \A DMA module is transferring characters to memory using cycle stealing, from a device
transmitting at 9600 bps. The processor is fetching instructions at the rate of 1 million
instructions per second (1 MIPS). By how much will the processor be slowed down by
DMA activity?" { question taken from (Stallings 2000; 6.5)
4. List the issues to be considered in the design/choice of the following items for a particular
application:
(a) CPU
(b) bus
(c) memory
(d) peripherals
5. Role Play/Challenge question taken from (Stallings 2000; 6.9):
\
Two boys are playing on either side of a high fence. One of the boys named Apple-
server, has a beautiful apple tree loaded with delicious apples growing on his side
of the fence; he is happy to supply apples to the other boy whenever needed. The
other boy named Apple-eater, loves to eat apples but has none. In fact, he must
eat his apples at a  xed rate . . . If he eats them faster than that rate he will get
sick. If he eats them slower, he will su er malnutrition. Neither boy can talk, so
the problem is to get apples from Apple-server to Apple-eater at the correct rate
(a) Assume that there is an alarm clock sitting on top of the fence and that the
clock can have multiple alarm settings. How can the clock be used to solve the
problem? Draw a timing diagram to illustrate the solution.
(b) Now assume that there is no alarm clock. Instead Apple-eater has a  ag that
he can wave whenever he needs an apple. Suggest a new solution. Would it
be helpful for Apple-server also to have a  ag? If so, incorporate this into the
solution. Discuss the drawbacks of this approach.
(c) Now take away the  ag and assume the existence of a long piece of string.
Suggest a solution that is superior to that of (5b) using the string.
"
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ECNG2006 (EE25M) March 11, 2008 I 18
6. One di erence between Dynamic RAM (DRAM) and Static RAM (SRAM) is:
(a) DRAM loses information when power is switched o , but SRAM retains information
when power is switched o .
(b) DRAM tends to be more expensive than SRAM.
(c) DRAM tends to be less dense than SRAM.
(d) DRAM is faster than SRAM.
(e) DRAM loses information unless it is periodically refreshed, SRAM will retain information
inde nitely.
7. Which of the following statements about the components of a computer are correct(if any):
(a) The ALU is the part which executes the program.
(b) The bus conveys information between the CPU and the microprocessor.
(c) The clock signal is used by the CPU to time the phases of the instruction cycle.
(d) A microprocessor is a CPU plus peripherals on a single chip.
(e) Memory may be read-only (RAM) or read-write (ROM).
8. Which ONE of the following statements about interrupts is correct:
(a) Interrupts occur when peripherals send bit-parallel signals to the CPU across the bus.
(b) Interrupts occur when memory devices send bit-parallel signals to the CPU across the
bus.
(c) Interrupts occur when the CPU exerts a special signal line connected to a peripheral.
(d) Interrupts occur when peripherals exert a special signal line connected to the memory.
(e) Interrupts occur when peripherals exert a special signal line connected to the CPU.
9. Choose the best answer. The memory map size a ects the nature of the microprocessor-based
system bus, because:
(a) the number of data lines must match the memory bit-width.
(b) the number of address lines must allow the entire address range to be addressed.
(c) if the number of data and address lines are the same, the lines can be multiplexed.
(d) 9a and 9b are true but 9c is false.
(e) 9a, 9b and 9c are all true.
10. Role Play: Investigate bus issues such as, contention arising from two devices with the same
address, or no device responding to the address, on both a synchronous and asynchronous
bus, and where data and address lines are shared or separate.
Suggest lining up people (devices) with transparencies representing the data they will place
on the bus. Person at back (the bus state) writing on a pad of slightly translucent paper,
from back to front, so we can see the transient state of the bus. Other persons to represent
the clock or signalling lines, and the address lines(transparency if shared bus lines).
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ECNG2006 (EE25M) March 11, 2008 I 19
Tutorial Exercise 1B O ine Version5 ID#
1. Draw a diagram showing the  ve phases of the instruction cycle. 3 marks
2. Identify two advantages/disadvantages of memory-mapped I/O when compared to isolated
I/O. 4 marks
3. Di erentiate between bus arbitration and DMA. Your answer should make reference to the
CPU’s ability to use the bus. 3 marks
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
5Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
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ECNG2006 (EE25M) March 11, 2008 I 20
T
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BUnit 2 Your lecturer will put up a diagram with several parts labeled with letters.
Write the letter you have been assigned here .
Answer the following questions for the part whose letter you were assigned.
1. The part is called the .
2. Circle the correct answer. The primary function of the part is:
(a) to transport data between the CPU and external devices
(b) to store data outside of the CPU
(c) to interface the CPU to external devices e.g. actuators, sensors
(d) to allow devices use of the system bus independently of the CPU
3. List 2 things you need to consider when choosing this component.
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ identify the components of a microprocessor-based system
{ describe the operation of typical components of a microprocessor-based system
{ discuss issues involved in the design/choice of various components for a microprocessor-
based system
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
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ECNG2006 (EE25M) March 11, 2008 I 21
3 Microprocessor support circuitry
At the end of this unit the student will be able to:
 explain the function of basic support circuitry for microprocessors such as clocks,
reset circuitry, watch dog timer, capacitors to ground, etc.
 describe (and/or illustrate) how bus addressing, con ict resolution, and memory
maps may be implemented
A microprocessor requires power and ground inputs. There may be more than one pin assigned for
each of these, and if so, the group of pins for each should be tied together. The power line(s) are
typically labelled Vdd or Vcc for the positive supply input and Vee or Vss for the negative supply
input. In most cases, we will be using the positive supply input as +5V, and the negative supply
input as ground; however this is not always the case. You are advised to check the datasheet(s) for
individual IC’s.
Typically, datasheets recommend that a capacitor be placed between the power and ground lines.
This is to decouple the supply lines. Sourcing additional current, can cause the power supply voltage
to  uctuate; similarly, a change in current being sunk can result in ground bounce. The capacitor
helps to mitigate these e ects. In an ideal world, these e ects should be minimised through proper
circuit speci cation/design.
The pins on a microprocessor may be grouped6 as control and data pins. A signal is said to be
asserted when the microprocessor is actively placing a value onto the pin (as opposed to letting
the line  oat). The actual voltage on the pin when it is asserted, depends on two things:
 the voltages placed on the pins for true/false; 0/1; logic high and logic low
 the polarity of the logic (true { logic high or logic low)
 whether the signal is asserted with logical true, or logical false; active high or active low.
For example the CPU may wish to put the number 5 out on 4 data pins. In binary, 5 is 01012.
The actual voltages on the 4 data pins will be 5V, 0V, 5V, 0V or 0V, 5V, 0V, 5V depending on
the above items. By convention, the names of signals which are active low, are written with a bar
e.g. D1: When you are wiring up a microprocessor (whether physical wires or traces on a circuit
board) all connections should be kept short, and isolated from strong sources of electro-magnetic
interference. This will reduce signal corruption.
It is presumed that all devices to be connected to the microprocessor, operate at same voltage levels
for logic high and logic low. In addition to voltage considerations, there will be a maximum amount
of current which can be detected/sunk or sourced by an individual pin. This will either be speci ed
in mA, or for certain logic families, as \fan- in/out". Further there is generally a maximum current
that the entire microprocessor can source/sink.
Where voltage and/or current speci cations of the microprocessor will be violated, one of the
following strategies may be used:
 change signal from active low to active high or vice versa (in order to reduce the total current
sunk by microprocessor)
6This division is arti cial, there is nothing to stop you using a data pin for control, or vice-versa. Typically control
pins are associated with particular SFR’s or speci c hardware within the CPU
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 pull-up/pull-down resistors (provide alternate current source/sink);
 bu ers (reduce loading on source);
 opto{isolators(electrically disconnect source/sink);
 relays and/or transistors (trigger higher powered circuitry)
The preceding discussion presumes that the outputs of the microprocessor are push-pull (totem-
pole, active pull-up) i.e. logic high and logic low both translate to a virtual connection to either
Vcc or GND. Outputs can also be open-collector (open-drain), where there is a virtual connection
only to GND; in the alternate logic state, the output \ oats". A third output structure is the
tri-state; almost the same as the push-pull output except that it also has a third \ oating" state.
The clock signal plays a major role in the operation of the CPU. Clock signals are the basis on
which CPU instruction cycles occur, synchronous bus transactions occur, and I/O devices sample
transducers. It is important to recognise that:
 all these signals need not necessarily be of the same frequency, or come from the same source.
 instructions are NOT performed at the same rate as the CPU clock frequency.
 the physical circuitry, imposes restrictions on the nature of the clock signal (minimum rise
time, minimum fall time, maximum frequency, noise tolerance, load capacitance)
 di erent types of oscillators have di erent characteristics : Typical sources of clock signals
include relaxation (RC) oscillators, and quartz-crystal oscillators. The choice depends on the
frequency, accuracy, and consistency required in the clock signal, and the tolerance of the
program/application for changes in temperature/pressure and resulting frequency drift.
 The oscillator may also be (partially) integrated into an IC along with the CPU core.
For example, let us consider a CPU with a 4MHz clock signal, connected to a 2MHz synchronous
bus. If we presume 1 machine cycle for each of the phases of the instruction cycle (fetch, indirect,
execute, write) the instruction cycle will be 4 times as slow as the machine cycle. i.e. the maximum
instruction execution frequency will be 1MHz. If instructions must be fetched across the bus,
the minimum time in which an instruction fetch can occur (ignoring memory access timing) is
1MHz. This will not a ect the instruction execution frequency if the processor is pipelined, but
a non-pipelined processor will be slowed further to approximately 500kHz instruction execution
frequency.
When power is  rst applied to a microprocessor, it is in an indeterminate state. For instance, the
clock signal (if source powered at same time) may not yet be stable, or initial power transients may
result in spurious values in registers/on buses. Typically, the microprocessor contains an input,
known as the \reset" or \master clear". Internally, this line forces all components of the CPU to a
pre-de ned initial state. In order to make certain of the CPU state, the reset line must be exerted
for a certain minimum length of time. An RC series circuit may be used to ensure the pin is active
for the required time. In many cases, a switch is connected to the reset line, so that the CPU can
be reset without cycling power.
A transient in the power supply may also cause the processor to behave in a strange way, by
corrupting register contents/bus states. The transient may not be long enough to properly trigger
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ECNG2006 (EE25M) March 11, 2008 I 24
the reset. To prevent such occurrences, either condition the power supply, or use an external device
which will generate a proper reset when a transient is detected. Watchdog timers (either internal to
the microprocessor or external) are devices which will trigger an output if they are not periodically
reset. They are useful devices for determining if the CPU has been subject to a transient, or has
\locked-up" or has broken.
The bus is utilised by multiple devices, so voltage/current issues need to be examined with care. If
items connected to the bus simultaneously assert signals, the bus will be in an indeterminate state.
This is handled by
 using tri-state connections between device and bus; the device keeps connections in the high
impedance state until bus access is granted/required
 actively detecting/avoiding bus con icts7
Transactions carried out across the bus require that the line voltages are stable before an attempt
is made to read the information; and further that the voltages on the bus remain stable for a
certain length of time. These times are referred to as the setup and hold times respectively, and are
speci ed on the device data sheet. These times are generally speci ed with reference to a timing
diagram, which shows the sequence in which the bus lines must be asserted for \correct" device
operation.
The bus accommodates Read and Write Requests by the requestor. The device which is the target
of the request, must respond to a pre-set address, and either accept the information, or respond
with information. The number of address locations is determined by the number of address lines
on the bus (e.g. 8 lines; 28 addresses from 0 to 255 ).
The entire address space may be diagrammatically represented as a memory map. The memory
map may, or may not, be contiguous.
The location of the device within the addressing space, depends on
 built-in device support for addressing { lines are connected to a sub-set of the bus address
lines
 external address decoding logic, whose inputs are a sub-set of bus address lines, and whose
output is connected to the device \enable" line
Finally, it is entirely possible that the address space accessible using the bus address lines is too
small. There are two possible ways to extend the address space:
 The memory space can be \re-used" by using additional \non-bus" lines to select a particular
device sub-set. This is the principle used to share I/O and memory space with a single bus,
and may also be used to \bank" or \page" memory.
 The address width can be increased, by using multi-stage addresses, which are transmitted
across the bus in successive words, and stored before device access is triggered.
7In the absence of an arbiter, bus con ict may be avoided by monitoring the bus while using it. If what is read is
not what was written, there is a con ict, and the device backs o for a random time before retrying the transaction.
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Review Exercises
1. Write a de nition for all the italicised terms in this unit.
2. The minimum support circuitry required to make a microprocessor operate includes:
I Address decoders for memory and memory-mapped perpherals.
II Oscillator (or other external) clock signal
III Power and Ground connections, with decoupling capacitor
IV Reset Circuit and/or Button
V Watchdog Timer
(a) I, III and V
(b) I, IV, and V
(c) II, III, and IV
(d) II, IV, and V
3. The function of the watchdog timer in a microprocessor-based system is BEST described as:
(a) to alert/reset the CPU when it has been inactive for a certain period of time
(b) to hold the reset line low for a minimum time on power-up
(c) to alert/reset peripherals when there has been no communication for a certain period of
time
(d) to count the number of times the incoming signal goes low
4. You are working on a prototype microprocessor-based system, which contains a CPU and
a synchronous bus with independent clocks. The only device on the bus apart from the
CPU is the ROM containing the program. Suggest possible reasons for each of the following
observations.
(a) You change the bus frequency, and do not get an improvement in overall performance.
(b) You change the CPU clock frequency and the system stops working.
(c) The circuitry works correctly when connected to the 5V desk supply, but fails when
connected to a 5V battery.
(d) The circuitry works for the test cases: all bit inputs o , and each bit input tested singly;
but fails when more than 3 bit inputs are tested together.
5. In your own words, comment on the reasons why:
(a) Unacceptable levels of noise in the clock signal will result in the CPU performing badly
(incorrectly).
(b) "Most processor control lines are active low." (Arnold 2001; pp 56)
(c) The maximum current sunk/sourced by an IC is generally less than the sum of the
maximum currents sunk/sourced by individual pins.
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ECNG2006 (EE25M) March 11, 2008 I 27
6. A device has support for addressing 8 locations (i.e. it has 3 address lines). It needs to be
connected to an 8 bit bus. What addresses does this device occupy in the memory map if:
(a) the lower 3 address lines are tied to the device address lines, and the upper  ve lines go
through a decoder which asserts the device select if the  ve lines read 101012
(b) the upper 3 address lines are tied to the device address lines, and the lower  ve lines go
through a decoder which asserts the device select if the  ve lines read 101012
(c) the upper 4 lines go through a decoder which asserts the device select if the four lines
read 11112, the next line is unconnected, and the lower 3 lines are tied to the device
address lines.
7. A microprocessor comes in a 24 pin package; if the data-bus width is 8 bits, what is the
maximum size of the memory map (i.e. 2n). Hint: remember the standard pins.
8. (a) Question from Wilmhurst (2001; 2.8):
\A small embedded system is powered from a 5V supply. The supply itself
cannot source more than 75mA. The total system decoupling capacitance is
220 F. Estimate the time required on power-up for the system voltage to reach
5V. Assume the supply is otherwise ideal, and that the current drawn by the
system is small."
(b) Using the preceding calculation as an example, in your own words, explain why the reset
line must be asserted for a certain minimum time after power is applied to the circuitry.
9. Question from Wilmhurst (2001; 2.9):
\ A micro-controller, with a clock frequency of 1MHz, is to be used for a time mea-
surement application, which must be accurate to within  0:1% over an operating
temperature range of 12 C to 30 C. The designer can choose between a crystal
clock source of stability 100 ppm (parts-per-million)/ C, a ceramic resonator of
stability 800 ppm/ C, or an R-C network of stability 0.4%/ C. Assuming each
source is set initially to give the correct frequency at the mid-range temperature,
what frequency range will each clock source give for this temperature range? Which
sources, if any, are acceptable for this application? "
10. Role Play/Challenge question:
(a) \ Suggest ways in which:
i. broken bus/signal wires can be detected by a self-test program in a micro-
processor based system.
ii. 16-bit data can be passed to a device over a bus with 8 data lines.
"
(b) Most of the devices used in this course are from the CMOS digital logic family. Identify
one other digital-logic family and determine what must be done to \correctly" interface
devices from the two families.
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ECNG2006 (EE25M) March 11, 2008 I 28
Tutorial Exercise 2A O ine Version8 ID#
1. Explain why devices should use either tri-state or open-drain outputs (rather than push-pull
outputs) to connect to the bus. 2 marks
2. Is it possible to have \holes" in the memory map (i.e. addresses to which neither memory
nor I/O devices respond)? Explain your answer. 2 marks
3. Di erentiate between signals which are logically high, active high and asserted. You may use
diagrams/examples to supplement your written answer. 6 marks
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
8Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
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ECNG2006 (EE25M) March 11, 2008 I 29
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AUnit 3 Your lecturer will put up a diagram with several parts labeled with letters.
Write the letter you have been assigned here .
Answer the following questions for the part whose letter you were assigned.
1. The part is called the .
2. Circle the correct answer. The primary function of the part is:
(a) to provide an independent trigger signal if it is not periodically reset
(b) to provide a  xed frequency signal
(c) to counteract transient voltage/current e ects in the supply/ground lines
(d) to ensure that the reset line is asserted for an appropriate time
3. Identify 2 problems which would arise if the part failed to perform as required, and explain
how each of these problems could occur.
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ explain the function of basic support circuitry for microprocessors such as clocks, reset
circuitry, watch dog timer, capacitors to ground, etc.
{ describe (and/or illustrate) how bus addressing, con ict resolution, and memory maps
may be implemented
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
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4 Microprocessor architecture
At the end of this unit the student will be able to:
 di erentiate between organization and architecture;
 relate instruction set design to the programmers model of the microprocessor;
\Computer architecture refers to those attributes of the system visible to the program-
mer, or put another way, those attributes that have a direct impact on the logical
execution of a program. Computer organization refers to the operational units and
their interconnections that realize the architectural speci cations." { Stallings (2000)
When designing a CPU the architectural decisions come  rst. The CPU architecture determines
whether the machine instructions and data can be stored in the same memory space, and whether
they can be accessed using the same mechanism (von Neumann (same) vs. Harvard (di erent)).
The CPU architecture also determines whether the CPU can manipulate or operate on the data
directly from memory, or must load it into an internal location (i.e. working register or accumulator)
before manipulations can take place (memory-based vs. load-and-store).
The means by which data may be manipulated by the CPU, can be based on one of several di erent
paradigms. Accumulator based architectures use a single register in conjunction with data from
memory (or other registers) returning the result to the accumulator. Register based architectures,
provide a small set of registers general purpose registers to the programmer, within which any reg-
ister may be speci ed as an instruction argument/destination. Stack based architectures, perform
operations on the items on the stack, returning the result to the stack. The stack size therefore
limits the number of instruction arguments.
CPU organization concerns the way in which the machine instructions are carried out within the
CPU i.e. the sequence of micro-operations which actually occur. For example, a multiply instruction
can be implemented using shifting and addition in a dedicated multiplier unit; 16 bit addition can
be carried out in a single 8 bit adder, in two stages, or in a single 16 bit adder. CPU organization
could also involve consideration of the logic family used to implement the design, and related layout
issues such as power consumption/heat dissipation and track lengths.
The manufacturing/fabrication process involves alternately placing insulating/metal layers on sili-
con wafers, and exposing them (using photo-resist coatings) to light projected through a "negative".
Areas which are not exposed to light are etched away. The process is often identi ed by the width
of the smallest circuit wires; the most recent processes for commercially available micro-processors
operate at 65nm. The advantages of using smaller manufacturing processes include:
 more transistors (processors) per wafer
 operation at lower voltages
 increased clock speeds (due to less power consumption/heat dissipation)
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The set of architectural features which the programmer can manipulate via the machine instructions
provide the \programmer’s view" of the microprocessor; hence in order to e ectively program any
microprocessor at the level of machine instructions, it is necessary to understand it’s particular
architecture, but not necessarily the CPU organization.
In a similar manner, higher level languages (e.g. C, Basic) tend to mask the programmer from
the computer architecture, making it unnecessary to appreciate the characteristics of an individual
microprocessor. Whichever language is used by the programmer, it must be converted into machine
language, and then somehow placed in memory for the microprocessor to run.
If a machine language program is to be launched each time the microprocessor is reset, then the
system must be con gured so that the reset memory location (speci c to the processor, generally
memory location 0000) contains a branch instruction to the start of the program. Alternatively, if
the microprocessor is running a program which supports the load/start of a new program (bootloader
or operating system), a reset may not be necessary.
Review Exercises
1. Label the following statements True or False.
(a) Higher level languages mask the CPU architecture and organisation.
(b) Harvard architecture CPU’s must have a stack.
(c) Choosing between RISC and CISC is an organisational decision.
(d) Accumulator-based architectures do not work without a stack.
(e) The reset vector is not always located at address 0x0000.
(f) Von Neumann architectures have separate data and program memories
(g) Stack-based architectures assume the instruction arguments are in the instruction.
(h) The reset vector is always located at 0x0000
(i) Assembly/Machine language masks the programmer from the CPU organisation.
2. A programmer knows that there is a multiply instruction in the instruction set of a micro-
processor, which takes 4 cycles to multiply two 4 bit numbers, and an add instruction which
takes 1 cycle to add two 4 bit numbers. Based on this information he can deduce which of
the following (if any):
(a) the microprocessor ALU contains a 4 bit adder
(b) the microprocessor ALU contains a 4 bit multiplication unit
(c) the microprocessor ALU contains a 4 bit multiplication unit that performs partial sums
to arrive at it’s answer
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3. A CPU designer wishes to include a multiply instruction in the instruction set of a micro-
processor which takes not more then 4 machine cycles to multiply two 4 bit numbers, and
an add instruction which takes 1 machine cycle to add two 4 bit numbers. He MUST obey
which of the following constraints (if any):
(a) the ALU needs to have a 4 bit adder
(b) the ALU needs to have an 8 bit adder
(c) the ALU needs to have a 4 bit multiplier
4. You are told that a microcontroller has a RISC based instruction set and uses Harvard
Architecture. Which of the following can be deduced? 3 marks
4-I: the organisation of the processor architecture is not complicated
4-II: data and instructions are stored in the same address space
4-III: peripheral registers are memory-mapped
5. The PRIMARY di erence between the Harvard and Von Neumann architectures is that Har-
vard architectures:
(a) always have CISC-type instruction sets.
(b) can support pipelining.
(c) generally support isolated peripheral input-output.
(d) have separate program and data memories.
(e) support faster clock speeds.
6. We have three CPU’s, which all use the same instruction format. Processor A supports an
add instruction. Processors B and C support both add and multiply instructions. Processor
B has a 4-bit multiplier in the ALU. Processor C has an 8-bit multiplication unit in the ALU.
All processors are accumulator-based and support load and store operations. Processors A
and B are microprocessors manufactured using a 180nm process. Processor C was assembled
from discrete components. Indicate which of the following statements are TRUE (there may
may be more than one):
(a) Processors A and B support the same instruction set.
(b) C compiler generated code for Processor A, will also run on Processor C.
(c) C compiler generated code for Processor B, will also run on Processor C.
(d) To the assembly language programmer, Processors B and C are the same.
(e) Processors A and B have the same architecture.
(f) Processors B and C have the same architecture.
(g) Processors A and B have the same organisation.
(h) Processors B and C have the same organisation.
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7. Answer the following questions in your own words:
(a) What is the di erence between CPU organisation and CPU architecture?
(b) Why is the distinction between CPU organisation and CPU architecture necessary?
(c) When choosing between RISC and CISC are we making a decision about organisation,
architecture, or neither? Explain your answer.
(d) When choosing an IC manufacturing process, are we making a decision about organisa-
tion, architecture, or neither? Explain your answer.
(e) What is the advantage of using the Von Neumann memory structure instead of the
Harvard memory structure?
(f) Is it possible for two devices to have the same address in a microprocessor based system?
Explain your answer making reference to the architecture and organisation.
8. A microprocessor comes in a 40 pin package.
(a) name the standard pins that MUST be on this IC
(b) if there are equal numbers of address and data lines, what is the maximum size of the
memory map
(c) what does this tell us about the microprocessor architecture and/or organisation.
9. Role Play: You work for a company which is designing a door lock which can be triggered
from a cell phone. One of the managers has just heard a sales pitch about RISC and Harvard
from a microprocessor company; he wants them used in this product. Form teams which
support each of the following combinations for this application:
 RISC Von Neumann
 RISC Harvard
 CISC Von Neumann
 CISC Harvard
Each team leader should make a 5-minute presentation stating their case.
10. Challenge:
(a) Investigate, and then draw/construct a poster to illustrate, the semiconductor manufac-
turing process.
(b) View the video "Alpha architecture" by Dick Sites & Dirk Meyer. Identify the architec-
tural, organisational and manufacturing process features described.
(c) Observe a PC booting, and then starting an application. Explain the sequence of events
you observe, and relate it to what you have learnt about the boot and program load
processes in a microprocessor.
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Tutorial Exercise 2B O ine Version9 ID#
1. You are a system designer who has been asked to compare the PIC16F877 and ONE other
controller/embedded board. Name the processor you are comparing the PIC16F877 with,
and identify at least one di erence in the areas shown. 4 marks
Core/Platform Name
Architecture Support Circuitry
Peripherals/Outputs Memory Map
2. Are the manufacturing process, organisation and architecture of a processor core independent
of each other? Explain your answer. You should use an example to make your points. 6 marks
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
9Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
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T
U
T
2
BUnit 4 Write the letter & number you have been assigned here .
Answer the following questions for the items whose letter & number you were assigned.
1. The item you have been asked to look at for the PIC16F877 is .
2. This should be determined primarily by the
(a) (instruction-set) architecture
(b) (processor-core) organisation
(c) (integrated-circuit) manufacturing process
3. Where and what did you  nd about your assigned item?
4. Which additional processor were you asked to look at? .
5. This can be described as
(a) RISC
(b) CISC
(c) Harvard
(d) Von Neumann
6. Where and what did you  nd out about your assigned item?
7. Give an example of ONE issue relating to this item which can a ect the programmer of an
embedded system, when using either of these platforms.
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T
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2
BUnit 4
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ di erentiate between organization and architecture;
{ relate instruction set design to the programmer’s model of the microprocessor;
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
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5 Typical Microprocessor instructions
At the end of this unit the student will be able to:
 explain the operation of typical machine instructions, addressing modes, status bits;
 infer the consequences of sequences of generic instructions;
The machine instruction is a unit task which the CPU knows how to perform. The instruction set
is the collection of instructions which the CPU understands. Machine instructions may be roughly
classi ed as:
data transfer (move, store, load, exchange, swap, clear, set, push, pop)
arithmetic (add, subtract, multiply, divide, absolute, negate, increment, decrement)
logical (and, or, not, xor, complement, bit-shift, bit-rotate, bit-set, bit-clear, test, compare)
control transfer (skip, branch, conditional branch, subroutine, return, nop, halt, wait)
special purpose (i/o, data conversion, system operation)
There may also be combined instructions which perform one or more operations consecutively(e.g.
subtract and branch if not zero). The selection of instructions to be included in the instruction set,
and the way in which they may best be encoded will re ect the CPU architecture.
Each instruction retrieved from memory is a binary number of a particular width. This number
represents the required instruction, i.e. the instruction has been encoded in some manner. The
layout of the encoded instruction is known as the instruction format. Within the instruction format
are several  elds, the  rst of which (the op-code) indicates the operation to be carried out. In order
to facilitate easy reading, the opcode is often written in mnemonic form (e.g ADD { addition;
JMP { jump (branch) ). An assembly language program consists of a mnemonic listing of the
program’s machine instructions. The choice of op-code for a particular instruction is made to
facilitate decoding by the control unit. For example, all instructions which use the ALU may be
grouped together, so that the ALU can be triggered on the basis of certain instruction bits.
Within the instruction format, the opcode is followed by operands, which the particular instruction
uses to perform it’s task. Operands may be unused, binary data (e.g. ADD 5), or a reference to a
register (ADD R1). The binary data may be used directly, or used to access a memory location.
The di erent ways in which this data may be used are known as the addressing modes. The most
common addressing modes are:
immediate operand is the data e.g. load #12
direct operand is the address of the data e.g. load [12]
indirect operand is the address of a pointer to the data e.g. load [[12]]
register operand is a register; register contains the data e.g. load r1
register indirect operand is a register; register contains the address of the data e.g. load [r1]
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stack no operand is listed in the instruction e.g. load
Operands are located on the stack either
1. de ned by the stack pointer register
2. or located wholly in register memory
displacement two operands: one of which is a register, and the other a number. e.g. load r1,#5
Displacement addressing arguments are combined in one of several ways to form the e ective
address which is subsequently used to retrieve the data:
1. relative : e ective address = current PC + number
2. base-register : e ective address = current register value + number
3. indexing : e ective address = number (main memory address) + current register value
4. post-indexing : e ective address = current register value + current memory location
value (whose address is number)
5. pre-indexing : e ective address is the contents of the memory location whose address is
determined by adding the number to the register contents.
Most microprocessors do not include all of the above addressing modes in the instruction set; how-
ever the more complex addressing modes can be implemented using multiple direct and immediate
addressing instructions. Where a variety of addressing modes are required, there are two ways in
which this may be encoded. The  rst is to attach the addressing mode to the instruction, i.e. there
as many variants of the instruction as there are addressing modes. The second is to attach the
addressing mode to the operand. This is more  exible, but requires more room in the operands
section of the instruction format.
The number of operands in a particular instruction depends both on the nature of the instruction10,
and the space available within the instruction format11. Typically instructions in a set may have
di erent formats, however in RISC architectures (and to facilitate pipelining), it is often advanta-
geous to have a  xed instruction format. A  xed instruction format dictates that all instructions
in the set use the same boundaries between op-codes and operands.
The  nal instruction characteristics to be considered are  xed vs. variable width instructions. The
base instruction size is dictated by the bit-width of the CPU/program memory. However, as long as
the opcode is contained in the initial instruction word, subsequent words in program memory may
be used to provide supplemental data. One example would be for instructions which must refer to
memory addresses where the address width is greater than the remaining bits in the instruction
format. The advantage of a variable width instruction set, is the space saved for instructions which
require no arguments.
Assembly language is essentially the binary machine instructions (opcode and operands) in which
op-codes and the common operands (registers) are replaced with mnemonics/higher base numbers
for easy sight reading. As such it is the lowest level at which the microprocessor can be programmed,
and requires an understanding of the underlying architecture.
Typical instructions found in assembly language for an accumulator based architecture:
10for example the ADD instruction may be speci ed with no (stack based), one (accumulator based), two or three
(memory/register based) operands
11for example, in a 16 bit instruction format with a 4 bit op-code, there are 12 bits left to specify operands. These
may be used as three 4 bit operands, a 2 bit and a 10 bit operand etc. etc
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load { place the speci ed value in the accumulator (multiple addressing modes)
store { store the accumulator value at the speci ed position (multiple addressing modes)
add { add the speci ed value to the accumulator
jmp { change the program counter to the speci ed value
One architecture-dependent special function register, which the programmer needs to interact with,
is the status (or condition code) register. The bits in this register (status  ags) individually rep-
resent a particular condition. The conditions represented, and the order in which they appear
are architecture dependent. Typically they re ect the results of the ALU operations and/or any
internal CPU error conditions (e.g. stack over ow; ALU carry; ALU zero). Note:  ags tend to
remain on (set; 1) until another operation explicitly turn them o (reset; 0). In particular ALU
 ags will remain in the same state if the following instruction does not utilise the ALU.
The status  ags may be used in several di erent ways:
 the CU may use them to execute conditional machine instructions; the programmer merely
uses the conditional instruction.
load a
add 5
addnc 4
 the programmer may explicitly test their value using a special status testing instruction which
returns a result or sets alternate  ags.
load a
add 5
store a
tstc
jmpnz .....
load a
add 4
.....
 the programmer may explicitly test an individual  ag in the status register by applying a
bit-mask to, and/or shifting, the register.
A bit mask contains a 1 in the position of the bit we wish to test, and 0’s elsewhere. To determine
if a bit is set, bitwise-and the status register with the bit mask. The result will be non-zero only if
the  ag was originally set.
In some cases, bits in the status register must be explicitly manipulated by the programmer. To
set a bit, bitwise-or the status register with the bit mask. To toggle a bit (o  > on; on > o )
bitwise-xor the status register with the bit mask. To clear a bit, bitwise-and the status register
with the inverted bit mask (you can invert the mask using bitwise-xor  rst).
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Review Exercises
1. Label the following statements True/False:
(a) Immediate addressing requires no memory referencing, but the range of values is limited
by the operand space in the instruction.
(b) Indirect addressing potentially allows access to a large addressing space, but requires
multiple memory references.
(c) The branch (jump, goto) instruction changes the program counter value, only after
storing the previous value.
(d) The noop instruction has no e ect other than incrementing the program counter.
(e) Register addressing requires two memory references.
(f) The push and pop instructions are used to manipulate the accumulator.
(g) The skip instruction is the same as the call (subroutine) instruction.
(h) The range of addresses which are accessible using Direct Addressing, is limited by the
operand space in the instruction.
2. The following values are in the data memory of a Harvard machine with an accumulator.
Address 0x20 0x21 0x22 0x23 0x24 0x25 0x26 0x27 0x28 0x29 0x2A
Data 0x44 0x10 0x02 0x53 0x43 0x23 0x11 0x34 0x23 0x38 0x22
What value is in the accumulator after each of the following instructions are executed in
sequence:
LOAD IMMEDIATE 22
LOAD DIRECT 25
ADD IMMEDIATE 27
ADD INDIRECT 2A
Your answer should show your reasoning.
3. The question refers to the following program which runs on a machine with an accumulator:
Line 1 load 5
Line 2 begin: sub 1
Line 3 jumpnz Begin
Write down the line numbers, and accumulator values as the program executes.
4. (a) What is the di erence between a program branch, and a subroutine call?
(b) Branch instructions move to the speci ed instruction if the test condition is ful lled.
Skip instructions miss the next instruction if the test condition is ful lled. Why do we
have a skip instruction when we already have a branch?
(c) Add instructions add a value to the speci ed register. The increment instruction simply
adds one to a register. Why do we need an increment instruction when we have an add
instruction?
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(d) The nop(no operation) instruction has no e ect other than incrementing the program
counter. Suggest some uses of the nop instruction. (Stallings 2000; 9.5)
(e) How can CPU use a stack for any purpose, if there are no push and pop operations in
the instruction set? (Stallings 2000; 9.7)
(f) \The number of machine instructions in an instruction set is limited by the number of
bits used to encode the instruction, and the instruction format." Is this statement true
or false? Explain your answer.
(g) How would you perform an indirect addressing operation, using direct and immediate
addressing mode instructions?
5. Assuming a single address instruction, and a processor with an accumulator, use the "stan-
dard" instructions you have learned here to convert the following piece of C code into machine
instructions.
a=4;
do
{
a=a+5;
}while (a<25);
You should check your answer by writing down the output from the C code, and your assembly
language code.
6. Convert the following machine language instructions into C code.
load #0
store c
load a
loop:
inc c
sub b
jmpovf end
goto loop
end:
dec c
Your answer should explain what this piece of code does.
7. Assuming a single address instruction, and a processor with an accumulator, use the "stan-
dard" instructions you have learned here to convert the following piece of C code into machine
instructions.
a=0;
for (i=0;i<5;i++)
{
a=a+i;
}
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8. Convert the following machine language instructions into C code:
loop:
load a
add b
store a
jmpc end
goto loop
end: ....
9. Role Play:
(a) What are the relative advantages and disadvantages of each of the addressing modes?
(Hint see: (Stallings 2000; Table 10.1))
(b) Di erentiate between  xed length and  xed format instruction sets. What are their
respective implications for CPU organisation, architecture and operation?
(c) Assume an instruction set that uses a  xed 16 bit instruction length. Operand spec-
i ers are 6 bits in length. There are K two-operand instructions and L zero-operand
instructions. What is the maximum number of one-operand instructions that can be
supported? (Stallings 2000; 10.9)
(d) Is there any possible justi cation for an instruction with two opcodes? (Stallings 2000;
10.12)
10. Role Play/Challenge question:(Stallings 2000; 9.4)
\Consider a hypothetical computer with an instruction set of only two n-bit in-
structions. The  rst bit speci es the op-code, and the remaining bits specify one
of the 2n 1 n-bit words of main memory. The two instructions are as follows:
SUBS X Subtract the contents of location X from the accumulator, and store the
result in location X and the accumulator
JUMP X Place address X in the program counter
A word in main memory may contain either an instruction or a binary number in
twos complement notation. Demonstrate that this instruction repertoire is reason-
ably complete by specifying how the following operations can be programmed:
(a) Data transfer: Location X to accumulator, accumulator to location X
(b) Addition: Add contents of location X to accumulator
(c) Conditional branch
(d) Logical OR
"
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Assignment A 12 ID#
1. An 8-bit processor-status register is con gured so that bit-2 is the carry  ag, and bit-7 is the
zero  ag. Write down the bit mask for each  ag, and explain how they can be combined, and
then used, to simultaneously detect if both bits in the status register are clear. Use examples,
and counter-examples, to support your answer. 8 marks
2. The binary number 111000102 is subjected to the following operations on an 8-bit processor
core: 4 marks
arithmetic shift right
bit-wise xor $0011 1110_2$
rotate left
logical shift left
What is the resulting value? Show all working.
12Students are advised that there is no online version of this assignment, and that electronic submissions will not
be accepted without the explicit permission of the course lecturer.
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3. A CPU designer has decided that his new CPU core will use an 6-bit wide instruction memory,
and have a variable-width instruction format, with single operand instructions. The CPU core
will have a internal stack, and will be supplied in a 14-pin IC package.
(a) The support pins required to allow the IC to function include all of the following EX-
CEPT: 1 mark
i. 1 pin for clock signal
ii. 2 pins for power supply
iii. 1 pin for reset line
iv. 1 pin for watchdog timer
(b) The decision to have an 8 bit wide instruction memory, is a decision about: 1 mark
i. CPU architecture
ii. CPU manufacturing
iii. CPU organization
iv. none of the above
(c) The instruction format MUST: 1 mark
i. contain an operand which requires less than 6 bits.
ii. include push and pop instructions for the stack.
iii. specify a maximum of 26 unique instructions.
iv. support at least one form of addressing.
(d) Assuming that operands are at least 5 bits wide, and that instructions never require
more than 2 instruction memory words, calculate the maximum number of op-codes
that can be included in the instruction set. Show all working. 4 marks
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4. The following values are in the data memory of a CISC machine with an accumulator.
Address 0x20 0x21 0x22 0x23 0x24 0x25 0x26 0x27 0x28 0x29 0x2A
Data 0x44 0x10 0x02 0x53 0x43 0x23 0x11 0x34 0x23 0x38 0x22
What value is in the accumulator after each of the following instructions are executed in
sequence:
load direct 0x22
add immediate 0x27
store indirect 0x25
load immediate 0x20
add indirect 0x28
Your answer should show your reasoning. 5 marks
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5. The question refers to the following program which runs on a stack-based machine, that has
a carry- ag. Labels are used to refer to the address of the instruction.:
Address Label Instruction
0x00 push 11102
0x01 label begin: push 01002
0x02 add
0x03 push label end
0x04 jump-if-carry-set
0x05 push label begin
0x06 jump
0x07 label end: pop
As the program executes, write down the address of the instruction executed, line numbers,
the state of the carry  ag (if known) and the values stored on the stack, Your answer should
show your reasoning. 6 marks
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6. The instruction set for an accumulator-type Harvard-architecture microprocessor with 8-
bit instructions, 6-bit addresses, and 4-bit data is supplied. The C/C++ subroutine, to
calculate base-2 logarithms, must be compiled so that the  nal assembly language routine
starts executing from address 0x12.
Instruction set:
Description Format
add Add the contents of the accumulator
to the operand; Result in accumulator;
Sets/Clears zero and carry  ags
0010 nnnn
clf Clear all  ags 0001 0010
jnc Jump to the speci ed INSTRUCTION
address if the carry  ag is not set
01qq qqqq
lod Load the number from the speci ed
DATA address into the accumulator
10qq qqqq
nop No operation 0000 0000
rtn Return to the calling routine (return value
should already be in accumulator)
0001 1100
sbt Subtract the operand from the contents of
the accumulator; Result in accumulator;
Sets/Clears zero and carry  ags
0011 nnnn
shl Logical shift left contents of accumulator;
result in accumulator
0001 1011
shr Logical shift right contents of accumula-
tor; results in accumulator
0001 1010
str Store the number from the accumulator
to the speci ed DATA address
11qq qqqq
zer Zero the contents of the accumulator;
Clears the zero  ag
0001 1000
NIBBLE base2log_routine(NIBBLE c)
{
BYTE a,b;
b=0;
a=c;
while (a>=2)
{
a=a/2;
b=b+1;
}
return b;
}
(a) Manually translate the C/C++ code into assembly language: 5 marks
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(b) Encode your assembly language using the op-codes supplied. Indicate the memory loca-
tions used for variables and branch destinations using labels. Place the  nal values into
the data memory locations, presuming that routine was called with the value 0x9 as the
function argument. 5 marks
Data memory
0x10
0x11
0x12
0x13
0x14
Instruction memory
0x10
0x11
0x12
0x13
0x14
0x15
0x16
0x17
0x18
0x19
0x1A
0x1B
0x1C
0x1D
0x1E
0x1F
0x20
0x21
0x22
0x23
0x24
0x25
0x26
0x27
0x28
0x29
0x2A
This assignment is worth 2% of the 40% assigned for the ECNG2006 coursework mark.
It contains a total of 40 marks.
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
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ECNG2006 (EE25M) March 11, 2008 I 55
A
S
S
T
AUnit 5 Write the letter you have been assigned here .
Answer the following questions for ONE instruction whose letter you were assigned.
1. The instruction is .
2. The instruction has operands.
3. The format of this instruction is: .
4. Write a sentence which explains what the CPU does in response to this instruction.
5. Write a short program to demonstrate how the instruction can be used by the programmer
of an embedded system.
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ explain the operation of typical machine instructions, addressing modes, status bits;
{ predict the results of sequences of generic instructions;
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
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ECNG2006 (EE25M) March 11, 2008 I 56
Microprocessor Overview
This section presented a highly simpli ed view of microprocessors and microprocessor-based sys-
tems. The more astute student will recognise that the text contains several statements which were
never, or are no longer, strictly true.
Among the more obvious:
 The RISC{CISC border has blurred; in most cases features from both are merged. In fact
RISC processers are rapidly being outstripped by superscalar and parallel(e.g Dual core)
architectures.
 We have not mentioned SIMD (Single Instruction Multiple Data) or MIMD (Multiple In-
struction ...) processors
 Arbiters and DMA controllers have merged. In fact burst mode DMA, utilises the DMA
mechanism for sending data over more than one bus cycle.
 Memory is no longer directly accessed over the system bus: it may have a dedicated bus, (e.g.
RAM Bus) or Level I/II cache may be used (faster, smaller memory banks \closer" to the
microprocessor)
 Peripherals are becoming more intelligent, with a variety of peripheral buses (w/ controllers)
being used to transmit data independently of the system bus.
 Due to increasing performance demands, and shrinking transistor size, CPU design is now an
extremely complex  eld. The average chip designer is resorting to the use of prede ned CPU
parts in a custom SoC (system-on-chip), which includes other circuitry.
The purpose of this section was to address part of the  rst course objective; i.e. \identify, and de-
scribe the role of, the components of a microprocessor, a microprocessor-based system," and as such
it is su cient, if you the student possesses su cient appreciation of the parts/mechanisms
to facilitate assembly-language programming/debugging.
The following section will further address the  rst course objective.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) Introduction to microprocessors
c DECE, UWI, St. Augustine, Trinidad
original author: Feisal Mohammed; updated: March 11, 2008{ CLR
Part II
Microprocessor-based system development
The previous section talked about the hardware components which make up a microprocessor, and
a microprocessor based system, and the ways in which they interact. However, there are other
issues to choosing/designing a microprocessor; primarily, the issue of developmental support. This
support manifests itself in several ways: documentation, software development tools and hardware
development tools.
This part gives a brief introduction to these topics, to familiarise the reader with the terminology,
as well as common features and techniques.
6 Software tool chain
At the end of this unit the student will be able to:
 explain the roles of the compiler, linker, assembler, simulator, emulator, debugger
in the software development tool chain,
 explain the role of scripting languages, make/con gure-like utilities and IDEs in
the software development process.
When a program is written as text, it must  rst be compiled (high level language) or assembled
(assembly language). The compiler(or assembler) is a special program which converts the text into
relocatable machine instructions. Compilers and assemblers understand two types of commands:
programming statements, and pre-processor directives. Programming statements are code i.e. what
you want the microprocessor to do. Preprocessor directives are simple instructions which run
prior to the actual compiler(assembler); they make the programmer’s life easier. For example, the
"#de ne" in a C program is a preprocessor command, which replaces the speci ed text string
throughout the  le.
The microprocessor-based system developer must decide which language, assembly or one of the
higher-level languages to program in:
\The factors relevant to a language decision probably include at least:
 E ciency of compiled code
 Source code portability
 Program maintainability
 Typical bug rates (say, per thousand lines of code)
 The amount of time it will take to develop the solution
 The availability and cost of the compilers and other development tools
ECNG2006 (EE25M) March 11, 2008 II 2
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ECNG2006 (EE25M) March 11, 2008 II 3
 Your personal experience (or that of the developers on your team) with speci c languages
or tools
" (Barr 2000; { from Language Lessons by Michael Barr, Copyright 2000 by CMP Media, Inc.)
If the host and target are not the same type of microprocessor, a special cross-compiler or cross-
assembler must be used (because the host will not understand target instructions and vice versa).
The compiled (assembled)  le from must be linked before it can be used by the target. Linking is
used to provide several facilities, namely multi- le programming and/or use of libraries/pre-written
routines. The Run Time library is a collection of standard C functions e.g. printf compiled for
the particular tool-chain/target platform. The linker takes the various sets of relocatable code, and
generates the  nal executable which can be loaded into memory, or run by a simulator/emulator. To
do so, the linker decides where code and variables will be placed in memory, and resolves addresses
for items referenced in multiple modules. The linker must be aware of the target’s memory map,
in order to produce a valid executable or binary  le. Information about the memory map is often
speci ed in an additional text- le which is speci ed when the linker is invoked.
The  nal binary  le will contain machine instructions, and information about the speci c locations
in memory to which they are to be loaded. Because the  le contains code for the target processor, it
cannot be run on the host, unless we use another program to interpret the target code. Instruction
SetSimulators, and emulators are similar in that they may both run on the host processor, and
accept code produced for the target processor. However, simulators do not interact with hardware,
they run within a \virtual target" environment on the host. Whilst emulators, possess a hardware
interface, so that they may physically replace the target \in-circuit".
In both cases (simulator and emulator), the objective is to be able to \view" what is going on
\within" the target during program execution; this assists the programming in \debugging" i.e. lo-
cating portions of code which do not perform as expected/speci ed. Techniques typically employed
in debugging include:
 use of breakpoints to pause the program
 single stepping through program
 \watch"ing or setting speci c values in memory
An alternative to simulators, and emulators, is to actually build support for debugging into the
target software. When the program code is downloaded to memory, additional code (debug kernel)
is also downloaded. Using this additional code, the target will communicate with a program on the
host (debugger) via serial or network connections, and report the state/change of state in the target
as code executes. The advantage of using standard debug kernels, is that the same host debugger
software may be used for mutliple target microprocessors. Examples of debug kernels include: the
Angel debug monitor, and the gdb debug stub.
All three options have their relative advantages and drawbacks. Simulators (because they run
much more slowly), cannot cope with or be used to troubleshoot problems due to interaction
with external devices/timing. They can however be used to debug/predict execution times of
particular pieces of code which have no external interaction. Emulators also tend to run at lower
speeds than the physical target; emulators which can match target speeds are expensive compared
to other development tools. Both emulators and simulators are designed to match the target
speci cations and cannot identify problems which arise from targets which somehow violate their
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ECNG2006 (EE25M) March 11, 2008 II 4
speci cations (hardware or software). In-circuit debugging, will take CPU time and other resources
(e.g. interrupts), and cannot be performed without a ecting the performance of the code under
test. In practice, each method is applied (as appropriate) to deal with a sub-set of problems, and
any remaining problems must be dealt with using hardware-based troubleshooting methods.
Microprocessor software development tools are generally designed to work in conjunction with each
other. The term tool chain is often used to collectively refer to assemblers, compilers, linkers,
libraries, simulators, emulators and debuggers. These are all command-line tools, i.e. in order
to run the tool, you must type the name, and provide it with a list of options before it will run.
Repeatedly typing in command lines can become tedious, thus many tool chains also include other
programs to facilitate the development process.
These \helper" programs can be roughly subdivided into: scripts or batch  les, make les, window-
based IDE’s . Scripts or batch  les are simply sequences of commands, which could have been typed
in at an interpreter prompt. Often they support replacement of wildcards by parameters speci ed
when the script is invoked. They can be con gured to stop if any command gives an unexpected
result. In this context, the script  le would contain a sequence of tool-chain commands, with
wildcards used for the program name(s).
One popular tool coming from the UNIX development environments is the make le. Make les
operate on a similar principle to scripts, except that they are especially designed for software devel-
opment, and are executed by the make utility. Make les consist of a list of rules and dependencies
for individual make le-targets. If a make le-target does not exist, then make checks whether all
the dependencies exist. If a dependency does not exist, it looks for a rule/depedency list to make
the dependency. Once all dependencies exist, the rule is executed. Make also checks the date/time-
stamps on  les; if the dependencies are time-stamped later than the make le-target, then the rule is
executed. Make is invoked with a single make le-target By listing all the required make le-targets
as dependencies of a single make le-target, we can cause all  les to be built in the required order.
Integrated Development Environments(IDEs) are window-based software packages designed to ease
the use of the various software tools required to perform program development. They generally
provide text editing (tailored with highlighting/syntax checking), project  les to keep track of all
the contributing  les, and their relevant tool settings, and launch menus for the various items in
the tool chain. They may also present the results of these tools for easy correction/review of the
text program. For example, after compilation, a window with the errors comes up to prompt the
user for changes. Additional features may include a (make-like) build, version control, and code
generation from software design tools.
When choosing a compiler or tool-chain (tool-set), the following issues must be considered:
\
Target Platform The  rst step in selecting a cross compiler is  nding one that will produce
code for your target processor.
Host Platform The next step is to decide on a development platform. If there are several
platforms available, you may want to check some of the other items in this list before
making a decision about the host.
Integration with Other Tools Is the compiler compatible with any debugging environments?
Is a make utility included? If the compiler is shipped with an IDE, is it extensible so that
you can integrate your version control tool?
Standard Libraries Will you need functions from the standard C library, math library, or
C++ classes? If so, are they provided with the compiler? Are all of the functions in those
libraries reentrant?
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ECNG2006 (EE25M) March 11, 2008 II 5
Startup Code Is startup code for embedded systems provided? Are the code and its use well
documented? If you can’t  nd any mention of startup code in the user’s manuals for a
potential cross compiler, consider that a bad sign.
Execution Speed Optimizations If your program is too slow, you’ll want the compiler to try
to speed it up. Will the compiler do this? If so, what speci c optimizations are supported?
Can they be individually enabled or disabled?
Program Size Optimizations If your program is too big for your target memory, you’ll want
the compiler to attempt to reduce the amount of code space used. Will the compiler do
this? If so, what speci c optimizations are supported?
" { (Barr 1999; Extracted from Table 1. Checklist for Cross Compiler Selection Target Processor)
Review Exercises
1. Write a de nition for all italicised terms in this unit.
2. Di erentiate between:
(a) an emulator and a simulator.
(b) an interpreter and a compiler.
(c) a script and a make le.
(d) a library and a linker.
3. Label the following statements about the software tool-chain as True/False:
(a) The IDE provides provide a common editor/interface for other elements of the software
development tool chain.
(b) The assembler takes assembly language text and optionally produces object  les.
(c) The compiler only runs on the target platform, and generates code for the host platform
to run.
(d) The compiler takes output from the assembler, and produces the executable  le.
(e) The debugger runs on the host, and communicates with the target debug kernel code.
(f) The di erence between the simulator and the emulator is that the simulator runs on the
host, and the emulator runs on the target.
(g) The host and target platforms are always di erent.
(h) The linker is never used when the host and target platforms are di erent.
(i) The simulator runs on the host platform, and executes code compiled for the target
platform.
(j) The simulator takes assembly language text as input, and runs on the target.
(k) The tool-chain is a group of programs used to facilitate the development of programs for
a particular target(s) on a particular host.
4. From (Wilmhurst 2001; 12.3)
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ECNG2006 (EE25M) March 11, 2008 II 6
\ You are employed in a small company whose products incorporate PIC microcontrollers.
You are the company’s only development engineer, and you wish to persuade your manager
to agree to the purchase of an in-circuit emulator. Your manager, however, knows that
you have ...[a simulator] already, and believes (perhaps wrongly) that if you can simulate a
program then you don’t need the emulator. Write a justi cation for your proposed purchase
stating clearly the advantages that an emulator would give. "
5. In addition to pre-processor directives, C compilers often support special keywords, which
are not part the standard C. One example is the asm/endasm keyword pair used to delimit
blocks of assembly language code within a C program. What possible advantage could this
non-standard keyword o er?
6. \Decompiling is the process of generating source-level code from the executable object code
embedded in a product’s memory". (Fisher 2000) This can be a serious problem for developers
of proprietary systems. Investigate this issue, and identify one way in which code can be
protected from being decompiled.
7. In your own words, explain why the same C code, when compiled by di erent compilers may
generate di erent size/speed executables.
8. (a) Explain how scripts may be used to automate testing of the software, as dictated by the
client, both in the absence of the hardware, and after the hardware becomes available.
(b) Investigate and identify 2 scripting languages commonly used in software development.
9. Explain what a debug kernel is. Your answer should mention ways in which a debug kernel
can facilitate testing/ xing a micro-processor-based system.
10. Role play: Assembly language programming skills are often valued, because of the perception
that the assembly language programmer will always generate more e cient code. Form teams
which will debate this issue from the following viewpoints:
 In (Barr 2000), Michael Barr describes a case where the C program was faster than the
assembly language program: \The speedup was actually the result of a better design."
 \Assembly will always be at least as fast as C when executing the same algorithm."
(Barr 2000)
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Tutorial Exercise 3A O ine Version13 ID#
Congratulations! You have been assigned to the NOVATE company as a summer trainee in the software
development division. NOVATE has recently been granted a contract to develop an embedded system which
monitors oil  ow in pipelines. The system will provide an estimate of the oil transported via the pipeline, as
well as automatically detect changes in  ow (e.g. pipeline breaks). The sensor and microprocessor hardware
must be specially developed for solar power consumption, and adverse environmental conditions, and will not
be available for several months, although a functional/circuit diagram for the system is currently available.
You must develop the software and verify that it operates correctly in di erent (contract-speci ed) scenarios.
1. At this time, in this project, which of the following items could you use to DEVELOP your
software? 5 marks
(a) compiler
(b) IDE
(c) libraries
(d) linker
(e) loader
2. Choose the word combination which best completes the following sentence: 1 mark
Within the development tool chain, the simulator runs on the 2I and takes 2II
as input, while the online debugger runs on the host, and communicates with the 2III .
(a) 2I: host 2II: assembly language text 2III: target emulator
(b) 2I: host 2II: binary  le (hex  le or executable)2III: target debug kernel
(c) 2I: host 2II: binary  le (hex  le or executable)2III: target emulator
(d) 2I: target 2II: assembly language text 2III: target debug kernel
(e) 2I: target 2II: binary  le (hex  le or executable)2III: target emulator
3. For this project, how much development and testing can take place before the hardware be-
comes available? You may use drawings/diagrams to express your answer, if you prefer. 4 marks
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
13Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
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ECNG2006 (EE25M) March 11, 2008 II 8
T
U
T
3
AUnit 6 Your lecturer will put up a diagram with several parts of the software development tool-
chain labeled with letters. Answer the following questions for the part whose letter you were
assigned. Write the letter you have been assigned here .
1. The part is called the .
2. Circle the correct answer. The primary function of the part is:
(a) to convert assembly language text to relocatable machine code .
(b) to convert high level language text to relocatable machine code .
(c) to convert relocatable machine from many  les into a single binary/executable  le .
(d) to store relocatable machine code for commonly used functions, speci c to a tool-chain,
high-level language and/or target. .
3. Write a sentence which explains how this part of the software tool-chain operates, with ref-
erence to the host and target machines.
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ explain the roles of the compiler, linker, assembler, simulator, emulator, debugger in the
software development tool chain,
{ explain the role of scripting languages, make/con gure-like utilities and IDEs in the
software development process.
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
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ECNG2006 (EE25M) March 11, 2008 II 9
7 Development support (hardware)
At the end of this unit the student will be able to:
explain the role of
  rmware, operating systems, oscilloscopes,logic probes/analyzers,
 terminals, disk drives, external memory, host computers debug protocols e.g. JTAG
in the development and support of microprocessor based systems.
In the previous unit, we looked at the software needed to perform microprocessor-based system
development. Some of that software, (for example emulation) works in conjunction with specialist
hardware. In this unit we examine some of the more hardware-related items required to support
development.
The  rst two items  rmware and operating system kernels while software, are strongly tied to
the hardware upon which they run. Firmware is software stored in a non-volatile medium e.g.
EPROM, and is typically where the self-test, boot-loader and/or BIOS code is stored i.e. software
that needs to run when the system is  rst powered up. For EPROM/ ash media,  rmware may be
programmed using a dedicated universal or chip programmer, and then the media may be placed
in circuit; the newer  ash chips support serial programming while the chip is in circuit.
Operating system kernels may be stored as  rmware, or may be stored on volatile media, and
loaded into memory by the  rmware. Both  rmware and operating systems are written for spe-
ci c platforms, with known memory maps. They will not work on di erent platforms. A target
microprocessor-based system, need not necessarily have an operating system installed. In dedicated
single function applications, the program is compiled down to a an absolute binary which is placed
in memory starting at the reset vector. Where the operating systems on the host and target di er,
we must ensure that they both interpret  le(s) in the same manner. e.g. UNIX and Windows
operating systems interpret ASCII control characters di erently.
Development boards, are circuit boards  tted with a microprocessor, support circuitry for the
more commonly used peripherals, and expansion space for customisation. They are useful to the
microprocessor-based system software developer, as they give the programmer something to test
code on/with prior to obtaining the custom board being developed for the application. This means
that application hardware and software can be developed in parallel reducing overall development
time.
In order to troubleshoot timing and signal problems involving a microprocessor, it is necessary
to examine the signals at the pins (Note: we cannot get inside) using an oscilloscope or logic
analyser. Generally, an oscilloscope can show only 2 channels (2 pins) simultaneously. The logic
analyser however allows the user to capture either timing (continuous sampling at the logic analyser
frequency) or state information (sampling at the system clock frequency) on multiple pins. Because
of the quantity of information that the logic analyser captures, it needs to have large data bu ers,
and provide facilities to capture selective data (trigger conditions),  lter the captured data, and/or
download to a PC for analysis. Some logic analysers can even list the program begin executed,
simply by recording the data passing across the microprocessor bus lines, and disassembling the
instructions.
Logic analysers have two main drawbacks. The  rst is the physical di culty of making multiple
connections on dense pin packages. Special connectors are often delicate, and accidental shorts are
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ECNG2006 (EE25M) March 11, 2008 II 11
always possible. The second is the fact that signals within the IC cannot be examined. This may
not be an issue for microprocessor-only IC’s, but where ASIC’s or SoC’s utilise multiple parts (e.g.
memory, and CPU) on a single IC, hardware troubleshooting becomes di cult. Furthermore, the
disassembly technique will not work if a microprocessor has on-board cache memory.
To address these problems, the JTAG (Joint Test Action Group) protocol was developed. This
serial protocol is based on the concept of a simple sampling circuit connected to each "virtual pin"
within the IC. All the sampling circuits are connected in a loop, which in turn acts as a large shift
register. The state of all the virtual pins at any time, may be obtained by shifting out the value.
Similarly the value at any virtual pin may be set by shifting in a particular value.
In the previous unit, we mentioned that where host and target are separate machines, the debug
kernel on the target communicates with the debugger on the host. Alternatively, (or in conjunction
w/ the debug kernel) there may be built-in hardware support for debugging. This is referred to as
in-circuit debug, or background debug. Two examples are Microchip’s OCD, and Motorola’s BDM.
Specialty hardware within the IC can monitor/query the processor state, match a single breakpoint,
or stop execution when a signal is detected on a pin. The hardware is programmed/reports serially
across dedicated pins.
In order to support more sophisticated functions using the basic hardware debug functions, interface
circuitry is used between the target microprocessor and host circuitry. The interface circuitry
(often containing another microprocessor), generates the appropriate hardware commands, and
logs/bu ers information reported by the hardware.
Debugging with a debug kernel and debugger, has the disadvantage that the debug kernel code
may somehow interact with the application code. This is not a di culty for development. Both
JTAG and hardware debugging support methods have the advantage that each  nal product (with
optimized code) may be tested for hardware and/or software  aws before it is shipped.
The target system often has a rudimentary command line interface, which necessitates the use of
a keyboard and display. The terminal is a device with a serial connection, which is designed to
take keyed input, and relay it out as ASCII data, and display ASCII data transmitted back to it.
It is extremely rare to  nd text terminals e.g. VT100 in use these days, however most operating
systems on host computers will allow the PC to emulate a terminal (e.g. hyperterm).
The target system may also require a rudimentary  le storage system. Random Access  ling
systems, originated with disk-shaped magnetic media, and the terminology (tracks, sectors, blocks)
re ects this. The actual  le data is stored in a number of blocks. The FAT (File Allocation Table)
(stored at a known location on the medium) is a list of records representing the information stored
at each block. Each record speci es the following block. Filing systems for solid media (Flash,
RAM, ROM) follow the same format, so that they may be be used interchangeably.
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Review Exercises
1. Write a de nition for all italicised terms in this unit.
2. Label the following statements as True or False.
(a) A PC may be used to download software to the target (programming).
(b) A logic probe provides facilities to capture and display multiple signals simultaneously.
(c) Debug Kernels provide services which can be accessed by the programmer.
(d) Firmware is software which is stored in a volatile medium.
(e) JTAG allows the injection and extraction of values to/from an integrated circuit.
(f) Terminal emulation software allows a PC to be used as a dumb terminal.
(g) The  le storage system is a device with a serial connection, which is designed to take
keyed input, and relay it out as ASCII data.
(h) The host computer may be used for the display of information reported by the debug
kernel.
3. We would like to examine the supply DC voltage for a microprocessor based system, for
suspected noise. We should use:
(a) logic analyser
(b) logic probe
(c) memory chip
(d) oscilloscope
(e) volt-meter
4. You are developing software for a hardware platform which has been built, but the microcon-
troller is presently unavailable. Which ONE of the following items will you use to determine
whether your hardware platform functions as expected?
(a) debug kernel
(b) integrated development environment (IDE)
(c) emulator
(d)  rmware
(e) scripts
5. From (Wilmhurst 2001; 12.4)
\ You have been asked to set up a .... [microprocessor] development system. List
[two] items of hardware .... you would wish to purchase, stating brie y what each
would be used for."
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6. Di erentiate between the following pairs of items:
(a) an oscilloscope and a logic analyser
(b)  rmware and software
(c) JTAG and hardware debug support
(d) a microprocessor development board and a custom microprocessor-based board.
7. You are developing a microprocessor-based system which requires a display and a keypad.
You need to choose between:
 an LCD display and a keypad.
 a terminal.
Identify two issues that you would consider, and explain how you would make your decision
based on those issues.
8. A micro-processor based system fails to provide the required output in a limited number of
cases. You suspect that there may be a voltage problem which is causing certain bus lines to
be incorrectly interpreted as "low". What piece of equipment would you use to track down
this problem? Explain your answer.
9. Challenge Question: Investigate the following topics
 JTAG uses the concept of a large shift register. This can become unwieldy for large
numbers of virtual pins. How does the JTAG standard address this problem?
 How do developers of Flash Filing Systems cope with the fact that Flash memory can
be written in small blocks but must be erased in larger blocks?
10. Challenge question: ROM emulators, instead of emulating the entire microprocessor, simply
emulate the program/data memory. Outline how this could be used to implement debugging.
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Tutorial Exercise 3B O ine Version14 ID#
Congratulations! As a new graduate, NOVATE has hired you to complete an embedded system which
monitors oil  ow in pipelines. The system will provide an estimate of the oil transported via the pipeline, as
well as automatically detect changes in  ow (e.g. pipeline breaks). The contract also speci es that the system
must be demonstrated to operate correctly in a variety of di erent scenarios. The sensor and microprocessor
hardware have been specially developed for solar power consumption, and adverse environmental conditions,
and are now available for testing. The software was previously written and tested on a simulator.
1. Your boss says: \The software already works, so just download it onto the platform and send
it out. Don’t bother to test it again." What will you say to convince him that testing is
required? Your answer should identify potential system  aws, and the tools/methods you
will be using to test the software on the platform. 6 marks
2. The  nal system has passed all in-lab tests. The system fails in the  eld. The hardware
developers have determined that the hardware meets all electrical speci cations. Explain
(using diagrams if required) what you will do to  nd/ x the fault. 4 marks
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
14Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
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ECNG2006 (EE25M) March 11, 2008 II 16
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BUnit 7 Your lecturer will put up a diagram with several development support items labeled with
letters. Answer the following questions for the part whose letter you were assigned. Write the letter
you have been assigned here .
1. The part is called the .
2. Circle the correct answer. The primary function of the part is:
(a) provide random access storage for the target system .
(b) act as an initial hardware prototype for the designed target system .
(c) capture and view signals on the pins of target system .
(d) program  rmware for use in the target system .
3. If you did NOT have access to this part, what e ect would/could it have on your development
of a micro-processor based system? Suggest alternatives to this part.
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
Explain the role of
{  rmware, operating systems, oscilloscopes,logic probes/analyzers,
{ terminals, disk drives, external memory, host computers debug protocols e.g. JTAG
in the development and support of microprocessor based systems.
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
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ECNG2006 (EE25M) March 11, 2008 II 17
8 Microprocessor families and forms
At the end of this unit the student will be able to:
recognize and di erentiate between the di erent commercially available
 families of microprocessor based systems (e.g. Motorola, AVR, PIC )
 forms of microprocessor based systems i.e.chip/component packages { SOC, micro-
controller, PC, back-plane bus, PC104, standalone, embedded
based on their pictures/properties/descriptions and supported development tools.
Microprocessor technology is relatively young and constantly changing. Over the last 30{40 years,
most changes in CPU architectural design have been driven by the need for speed. Design tradeo s
made over time include increasing bit widths, RISC/CISC, use of DMA, peripheral support, cache
RAM, and dedicated/hierachical busses. Manufacturing improvements also improved clock speeds
by reducing heat/distance considerations. What does all this change mean for the developer of
a microprocessor based system? In manufacturing a system, we need to be concerned about two
things:
 the expected lifetime of an individual system, and whether we will be able to repair/support
it for it’s lifetime,
 the expected lifetime of the design, and whether we will be able to produce/sell the required
number of units to recoup design costs.
Clearly we can do neither if the processor around which we have designed our system becomes
obsolete soon after we have gone into production/sale.
To reconcile the reality of a rapidly changing marketplace, with the customers need for stability,
manufacturers of microprocessor have adopted several strategies including:
 clones { a classic case is the 8051; originally produced by Intel, many companies now o er
a pin compatible, architecturally equivalent processor. It is used as a replacement for the
original 8051, as well as in new designs which can take advantage of instruction set exten-
sions/enhancements.
 families { most people are familiar with the Intel x86 processors. These were designed to
be backward compatible i.e. code compiled for a 286 processor will run on a 486 processor.
Support for the base instruction sets is built into later members, even though the packaging
and overall architecture may be di erent.
 scaled function versions { where di erent variants of the CPU are made available, each with
a unique set of features, targeting a speci c market. For example, device A may require only
10 general I/O lines, but device B only 2 general I/O lines. The manufacturer makes two
versions of the CPU, one with more lines and one with less lines, and prices them accordingly.
The developer can then choose the lowest cost option that matches his needs.
The developer cannot decide on the CPU to be used for a particular application, without inves-
tigating the form in which the microprocessor is available. Apart from the myriad packages in
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ECNG2006 (EE25M) March 11, 2008 II 20
which they are singly supplied (DIP, PLCC etc.), they may also be supplied as either a part of
an IC containing other functional parts/peripherals (microcontroller, ASIC, SoC), or as part of a
bus-ready (PC104, back-plane) board containing other functional parts and peripherals, or as a
stand-alone computer system. In addition, any of these variants can come with di erent maximum
clock speeds, depending on the manufacturing processes used to produce the CPU. The choice of
form and clock speed will depend on the particular application.
Some additional items to consider include:
Environmental tolerance Will the processor have to work in dusty, chemical, radiation, haz-
ardous, vibrating, or zero gravity environments? If so special packaging will be required.
Business Is the CPU available? How long will it take to get the product to market? What will
the unit cost be? How many units can we sell? What will the net pro t be?
History What are we already familiar with? Do we already have the support tools? What legacy
code/features do we need to support?
Power What is our entire power budget? What power-saving options are available?
All other things being equal, we can choose a processor on the basis of code e ciency i.e. the time
it takes to execute an algorithm and the space required to store the algorithm in memory (Palmer
1997a).
Review exercises
1. In your own words, write out a list of guidelines for choosing a processor for a given application.
2. Using the Intel 8051 as an example, di erentiate between clones, families and scaled-function
versions of microcontrollers.
3. Manufacturer X supplies an IDE which is appropriate for all the microprocessors that he
supplies. We can presume that microprocessors supplied by Manufacturer X: (choose all the
appropriate answers)
(a) all have the same instruction format.
(b) are all supported by the same assembler/compiler.
(c) are all pin-compatible with each other.
(d) all operate at the same clock speed.
(e) all support on-line debugging.
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4. A microcontroller may be supplied in di erent variants. This means that: (choose all the
appropriate answers)
(a) The number, spacing and type of pins on the chip may be di erent for each package
variant.
(b) All variants will have the same operating voltages.
(c) Di erent variants may have di ering environmental tolerances.
(d) Di erent variants may have di ering maximum clock rates.
5. The microprocessor which was used in a particular design is no longer available. You have
been asked to locate a suitable replacement. In order to be able to use the existing code,
without re-assembly, the replacement MUST: (choose all the appropriate answers)
(a) come from the same manufacturer
(b) be pin compatible
(c) have the same instructions in it’s instruction-set
(d) have the same instructions & instruction-format
6. The microprocessor which was used in a particular design is no longer available. You have been
asked to locate a suitable replacement. In order to be able to use the existing circuitry,
without re-design, the replacement MUST: (choose all the appropriate answers)
(a) come from the same manufacturer
(b) be supplied in the same package
(c) have the same instructions in it’s instruction-set
(d) be a unit with the same power/current characteristics
7. AN520 (Palmer 1997a)compares the performance of several microprocessors using typical
algorithms. This is an example of benchmarking. Justify your answers to each of the
following questions. Is it appropriate to use benchmark data to choose a processor without:
(a) an understanding of the benchmark algorithm?
(b) an understanding of the application algorithm?
8. Based on (Vahid and Givargis 2002; 3.8): The PIC16F877 is one of a family of microcon-
trollers.
(a) Using the datasheet, identify
i. the basic instruction set features (# of instruction(s) variants, instruction width)
ii. the I/O facilities (# of ports, tri-state facilities)
(b) Go to the MicroChip web-site (http:\\www.microchip.com) and identify
i. a scaled back version of the same microcontroller
ii. another microcontroller belonging to the same family
iii. another family with a similar (but not the same) instruction set
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9. Role Play/Challenge Question: Imagine you are an engineer in a company which used an
Alpha processor in a design for a system. Your company would like to continue selling the
system for another 10 years. Investigate the current status of the Alpha family, and then
write a letter to your line manager making recommendations as to:
 how your company can continue to support the systems you have already sold,
 how your company can continue to produce the system.
10. Role Play: A microcontroller based product, for use in a hostile environment, was designed
using a microcontroller IC which has become obsolete. The company must decide between
a pin-compatible microcontroller which is not binary-compatible, or a binary-compatible mi-
crocontroller which is not pin-compatible. Form teams which will debate this issue from the
following viewpoints:
 The software developers want to redesign/modify the board and keep the software be-
cause the code has already been checked, and the modi ed system could be re-tested
with existing scripts.
 The hardware developers want to rewrite/recompile the software and keep the board
because it is already designed/checked for operation in a hostile environment.
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Tutorial Exercise 4A O ine Version15 ID#
Congratulations! Because of your excellent work, NOVATE has promoted you. You head the production
team for an embedded system which monitors oil  ow in pipelines. The system was successfully developed
with a socketed PIC16F877 microcontroller in a 40-pin DIP package, and a 4MHz clock signal. The system
utilises the Timer, PWM, and A/D features of the variant. NOVATE is committed to delivering and
installing 20,000 modules over 2 years. The procurement department has been ordering parts in batches for
1000 modules on a monthly basis. Ten months into production, procurement advises your team that the
PIC16F877 microcontroller is no longer available in the required variant. Furthermore, they cannot locate a
pin-compatible variant, with the required features, which will also be binary-compatible.
1. Identify and explain two choices you could have made back in the development stage which
would have made this transition easier. 6 marks
2. How will your team modify the system, so that the remaining modules can be delivered?
Your answer should identify changes in hardware, software, testing, tools and/or tool-chains.
4 marks
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
15Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
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ECNG2006 (EE25M) March 11, 2008 II 24
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AUnit 8 Write the letter and number you have been assigned here .
Answer the following questions for the Microprocessor Alternate whose letter you were assigned.
1. The CPU is made by ..
2. Circle the correct answer. What type of IC package is the CPU inside?:
(a) DIP
(b) PLCC
(c) QFP
(d) Unspeci ed or Other
3. In what form is the CPU presented for the micro-processor-based system developer?
(a) Microcontroller
(b) Microprocessor-only IC
(c) Motherboard, PC104 board, or Board for back-plane bus system
(d) Unspeci ed or other
4. Compare the PIC16F877 & this processor (assign one group member to each item)
(a) look at the block diagram(s)
 identify the internal features of the CPU core
 identify additional features of the microcontroller/board
 identify other family members/clones
(b) look at the memory map(s)
 is this Von Neumann/Harvard architecture?
 are devices memory-mapped?
 is there a separate I/O bus?
(c) locate the branch/jump instruction
 what is the mnemonic?
 what is the op-code?
 what is the branch address size?
(d) locate the clock signal/data speci cations
 what is the maximum clock frequency?
 what is the CPI? MIPS ?
 what is data/instruction width?
(e) locate the power/current requirements
 what is an appropriate operating frequency for both platforms?
 what is the minimum operating voltage for a given frequency?
 what is the average current (CPU Only) draw at a given voltage/frequency?
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Unit 8
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
Recognize and di erentiate between the di erent commercially available
{ families of microprocessor based systems (e.g. Motorola, AVR, PIC )
{ forms of microprocessor based systems i.e. chip/component packages; board forms
based on their pictures/properties/descriptions and supported development tools.
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
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Informative Abstract
You have been assigned an article on a topic of relevance to microprocessor-based systems.
Groups A1, B1, C1, D1 : \Fuzzy Logic Control for an Autonomous Robot" by V.M. Peri and D. Simon,
Annual Meeting of the North American Fuzzy Information Processing Society (NAFIPS), 2005, 26-28
June 2005, page(s): 337- 342.
Groups A2, B2, C2, D2 : \Sni ng Robot" by Silvio Tresoldi, Circuit Cellular, Issue 108, July 1999,
page(s): 12-16.
Groups A3, B3, C3, D3 : \Machine Chameleon" by D. Verkest, IEEE Spectrum, Volume 40, Issue 12,
Dec. 2003, page(s):41 - 46.
Groups A4, B4, C4, D4 : \The transistor laser" by Nick Holonyak Jr. and Milton Feng, IEEE Spectrum,
Volume 43, Issue 2, Feb. 2006, page(s):50 - 55.
1. Read your assigned article (or choose another article from an IEEE magazine, and have it
approved by your lecturer), and submit (at most) a one (1) page informative abstract of
the article. Your abstract should include the following areas: 12 marks
(a) The objectives of the article or research undertaken;
(b) The scope of the work;
(c) The methodology employed;
(d) The signi cant  ndings or results;
(e) The signi cant recommendations and;
(f) The conclusions drawn.
2. Identify, and explain in your own words, either
(a) a potential application of this research/work to
(b) a prediction about how this research/work may a ect
the design/production of microprocessor-based systems in the near-future. 2 marks
Your submission should be headed with the article reference and your name/ID number. You must
upload your submission to Moodle using the relevant assignment link. Your submission should be
named Fnnnnnnnn.xxx where nnnnnnnn is your ID number, and xxx is the  le extension re ecting
the  le type.
Files may be PDF, Microsoft Word (DOC) or plain ASCII  les (TXT).
Please remember to electronically submit AHEAD of the deadline in order to avoid possible prob-
lems with system overload.
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Microprocessor-based system development
There are many issues to be considered when developing a micro-processor based system. In this
section we have examined some of the technology available for software and hardware development
support, as well as the di erent ways in which CPU technology is made available to the designer
of a microprocessor-based system.
As such we have addressed part of the  rst learning objective \in general, identify, and de-
scribe the role of, the components of a development suite (hardware, software) for
a microprocessor-based system" and hinted at the  nal learning objective \select (and cri-
tique the selection of) a microprocessor-based system for an application, given relevant
datasheets and application requirements".
In the next section we will be looking in some detail at a particular CPU, it’s instruction set, and
memory map, and at the developmental support tools available for it. In doing so, we will complete
the  rst learning objective.
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ECNG2006 (EE25M) Introduction to microprocessors
c DECE, UWI, St. Augustine, Trinidad
original author: Feisal Mohammed; updated: March 11, 2008{ CLR
Part III
PIC16 Introduction
All microcomputer systems, irrespective of their complexity, are based on similar building blocks.
The CPU or microprocessor is the core component of any microcomputer and it requires the external
components such as the ROM, RAM, I/O etc. to accomplish its purpose. A microcontroller is a
stripped-down version of the very same architecture, with all the important features placed on one
chip. The same system using a microprocessor or a microcontroller looks like Figures 2,3. The
microcontroller based system requires no additional circuitry except a clock input and it can, in
many cases, directly drive peripheral outputs. The di erence between the microprocessor and the
microcontroller arises because of their di erent end-usage.
The microcontroller that will be investigated is the PIC16F877, which is at the upper end of the
mid-range series of the microcontrollers developed by MicroChip Inc. It is characterized by a RISC
architecture instead of the CISC architecture used, for example, by the Motorola 6809.
Figure 2: Basic building blocks of a computer
Figure 3: A microcontroller based system
ECNG2006 (EE25M) March 11, 2008 III 2
9 PIC16F877 Overview
At the end of this unit the student will be able to:
 explain the memory layout for the PIC16F877
 explain the operation of the instruction cycle for the PIC16F877
 explain the operation of the basic instructions (move, add, subtract, shifts) for the
PIC16F877
The history of the PIC series of microcontrollers started in in 1965, when General Instruments (GI)
formed a Microelectronics Division, and used this division to generate some of the earliest viable
EPROM and EEPROM memory architectures. GI also made a 16 bit microprocessor, called the
CP1600, in the early 1970s. While this was a reasonable microprocessor, it was not particularly
good at handling I/O. Therefore, around 1975, GI designed a Peripheral Interface Controller (or
PIC for short) for some very speci c applications where good I/O handling was needed. It was
designed to be very fast since it was I/O handling for a 16 bit machine, but it did not need a
large amount of functionality, so its microcoded instruction set was small. Its architecture was
substantially the PIC16C5x architecture of today. The market for the PIC remained small for
the next few years. During the early 1980s, GI restructured their business to concentrate more
on their core activity which was power semiconductors. As a result of the restructuring, the GI
Microelectronics Division became GI Microelectronics Inc (a wholly owned subsidiary) which, in
1985, was  nally sold to venture capital investors. The sale included the fabrication plant in
Chandler, Arizona. The new owners decided to concentrate on the PICs, the serial and parallel
EEPROMs and the parallel EPROMs. A decision was later taken to start a new company, named
Arizona MicroChip Technology, with embedded control as its di erentiator from the rest of the
industry. As part of this strategy, the PIC family was redesigned to use one of the other things
that the  edgling company was good at, i.e. EPROM. With the addition of CMOS technology and
erasable EPROM program memory the PIC family, as we know it, was born.
Architecture of the PIC microcontroller
The PIC series of microcontrollers are RISC-based processors with an accumulator(also called the
working register, W), which use the Harvard16 architecture; therefore the microcontroller has a
program memory data bus and a data memory data bus. Separate buses mean that simultaneous
access of program and data can be done, which gives a greater bandwidth over the traditional von
Neumann architecture. Separating the program and data memory, allows instructions to be sized
di erently than the 8-bit wide data word. This separation means that the instruction words can be
ideally sized for the speci c CPU/application. This is necessary since RISC architectures require
that instructions have the source and destination operands be encoded within the instruction. The
PIC opcodes for the mid-range processors are 14-bits wide, and the 14-bit wide program bus fetches
an instruction in a single cycle.
16This architecture had been a scienti c curiosity since its invention by Harvard University in a Defense Department
funded competition that pitted Princeton against Harvard. Princeton won the competition because the MTBF of
the simpler single memory architecture was much better, albeit slower, than the Harvard submission. MicroChip has
made a number of enhancements to the original architecture, and updated the functional blocks of the original design
with modern advancements made possible by the low cost of semiconductors.
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There are 35 single word instructions. A two-stage pipeline overlaps fetch and execution of in-
structions. As a result, all instructions execute in a single cycle except for program branches. The
pipeline uses static branch prediction and assumes that the branch is never taken, so that condi-
tional branch instructions can take either one or two cycles. One cycle if the branch is not taken,
since it would be in the pipeline and two cycles if the branch was taken. Non-conditional branch
instructions such as call or goto always take two cycles. All program memory is internal i.e. the
program bus is not accessible outside of the chip. The data path is 8 bits wide. Data memory may
be accessible from outside of the chip using the Parallel Slave Port; when enabled, the port register
is asynchronously readable and writable by the external world. The mid-range PIC processors can
directly or indirectly address their data memory17. All special function registers18, including the
program counter, are mapped in the data memory. The design of the instruction set is such that
it is possible to carry out any operation on any register using either addressing mode. This design
simpli es the programming of the device. The term  le register is, in PIC terminology, used to
denote the locations that an instruction can access via an address. The term register  le is used to
collectively refer to the group of registers.
The PIC ALU can perform arithmetic and Boolean functions between data in the working register
and any  le register. The unit is 8-bits wide and capable of addition, subtraction, shift and logical
operations. In two-operand instructions, typically one operand is the Working (W) register and
the other operand is a  le register or an immediate constant. In single operand instructions, the
operand is either the W register or a  le register. Depending on the instruction executed, the ALU
may a ect the values of the Carry (C), Digit Carry (DC) and Zero (Z) bits in the STATUS register.
The C and DC bits operate as Borrow and Digit Borrow bits respectively, in subtraction.
Memory Layout
The PIC16F877 has 8K  14-bit words of Flash program memory, 368 bytes of data RAM, 256
bytes of data EEPROM and an 8-level x 13 bit wide hardware stack.
The Program Counter (PC) is 13 bits wide, thus making it possible to access all 8K x 14 addresses.
The low byte is the PCL (Program Counter Low byte) register which is a readable and writable
register. The high byte of the PC (PC<12:8>) is not directly readable or writable and comes from
the PCLATH (Program Counter LATch High) register. The PCLATH register is a holding register
for the PC<12:8>. The contents of the PCLATH are transferred to the upper byte of the program
counter when the PC is loaded with a new value. Although the PC is capable of addressing the entire
program memory space, conceptually the program memory is represented by four banks of 2K  
14-bit words. Banking is necessary since there are only 11 bits for the address in the instruction
word for a call or goto. The other two bits are obtained from the top two bits of PCLATH (i.e.
PCLATH<4:3>). This means that the user must set those extra bits in PCLATH before branching
out of the 2K bank that contains the current instruction. Within the program memory space, the
reset vector (location to go to on reset) is at 0000h and the interrupt vector (location to go to on
interrupt) is at 0004h.
The data memory space e ectively has 9 bit addresses, and is also banked. There are 4 banks;
each bank holds 128 bytes of addressable memory. The banked arrangement is necessary because
there are only 7 bits are available in the instruction word for the addressing of a register, which
gives only 128 addresses. The selection of the banks is determined by control bits RP1, RP0 in
17Data memory may be EPROM based, or RAM based. General purpose registers are contained in the RAM
18Note: The Working register (W) is an 8-bit register used for all ALU operations; it is not addressable.
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the STATUS register (STATUS<6:5>). Together the RP1, RP0 and the speci ed 7 bits e ectively
form a 9 bit address. The  rst 32 locations of Banks 1 and 2, and the  rst 16 locations of Banks
2 and 3 are reserved for the mapping of the Special Function Registers (SFR’s). SFRs (e.g. PC,
STATUS) are used to control the \core" operation of the microcontroller as well as peripherals such
as the I/O ports. The SFRs are mapped into the data memory space to facilitate addressing.
The 368 bytes of static RAM which are used as general purpose registers (GPR), are arranged in
the data memory space so that each is accessed by a unique address, with the exception of the
last 16 bytes of each bank, which are shared. Each mapped SFR and GPR is 8-bits wide. The
entire data memory can be accessed either directly using the absolute address of each register  le,
or indirectly through the INDirect File (INDF) register and the File Select Register (FSR). The
INDF register is not a physical register. Any instruction using the INDF register actually accesses
the register pointed to by the FSR. Reading the INDF register itself, indirectly (i.e. FSR = 0)
will read 00h. Writing to the INDF register indirectly results in a no operation (although status
bits may be a ected). Because the FSR is 8 bits wide, indirect addressing can access two data
memory banks simultaneously. The e ective 9-bit address is obtained by concatenating the IRP
bit (STATUS<7>) and the 8-bit FSR register.
The 256 bytes (8 bit address) of EEPROM memory is indirectly mapped into the data memory
using the EEPROM ADRess (EEADR), EEPROM CONtrol (EECON1) and EEPROM DATA
(EEDATA) registers19. Reading the data EEPROM memory only requires that the desired address
to access be written to the EEADR register and the EEPGD bit of the EECON1 register be cleared
(to indicate the data memory is to be read). Then, once the RD bit of the EECON1 register is
set20, data will be available in the EEDATA register on the very next instruction cycle. EEDATA
will hold this value until another read operation is initiated or until it is explicitly written.
The stack is not part of the program or data space and the stack pointer is not readable or writable.
There are also no operations to place or remove data to or from the stack. Addresses are placed on
the stack by the CPU when a call instruction is executed or when an interrupt occurs. The various
return instructions then remove the previously stored address from the stack. The stack operates
as a circular bu er, meaning that after the stack is full, subsequent ’pushes’ start overwriting the
previously stored data from the top (the very  rst push). Similarly when the stack is ’popped’
nine times, the ninth value is the same as the  rst pop. The programmer never has to interact
directly with the stack, but should always remember the 8-level limit. The stack does not give any
indication if over ow or under ow occurs.
The 8-bit working register (W) is also not part of the program or data memory space. It can however
be read/written using particular instructions. Register-Register operations are not possible.
Instruction set for the PIC microcontroller
The format of the instruction words are shown in Figure 4(PIC 2001; Figure 13.1) . All instructions
can use either direct or indirect addressing to access a register  le. Since the addresses range from
00h to 7Fh in each bank, only 7 bits are required to identify the register  le address and this is
contained within the instruction. The eighth and ninth bits, which identify the bank, then comes
from the register bank select bits (RP1,RP0) from the STATUS register. Figure (5) illustrates how
the bits are composed to access the appropriate address. In indirect addressing, the 8-bit register
19The program memory could be read indirectly in a similar fashion, however this is a multi-cycle operation, and
is not normally required.
20The RD bit of EECON1 will automatically clear once the read is performed
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PICmicro MID-RANGE MCU FAMILY
DS31005A-page 5-4   1997 Microchip Technology Inc.
5.2  General Instruction Format
The Mid-Range MCU instructions can be broken down into four general formats as shown in
Figure 5-1. As can be seen the opcode for the instruction varies from 3-bits to 6-bits. This variable
opcode size is what allows 35 instructions to be implemented.
Figure 5-1:General Format for Instructions    
5.3  Central Processing Unit (CPU) 
The CPU can be thought of as the “brains” of the device. It is responsible for fetching the correct
instruction for execution, decoding that instruction, and then executing that instruction. 
The CPU sometimes works in conjunction with the ALU to complete the execution of the instruc-
tion (in arithmetic and logical operations).
The CPU controls the program memory address bus, the data memory address bus, and
accesses to the stack. 
5.4  Instruction Clock
Each instruction cycle (TCY) is comprised of four Q cycles (Q1-Q4). The Q cycle time is the same
as the device oscillator cycle time (TOSC). The Q cycles provide the timing/designation for the
Decode, Read, Process Data, Write, etc., of each instruction cycle. The following diagram shows
the relationship of the Q cycles to the instruction cycle.
The four Q cycles that make up an instruction cycle (TCY) can be generalized as:
Q1: Instruction Decode Cycle or forced No operation
Q2: Instruction Read Data Cycle or No operation
Q3: Process the Data
Q4: Instruction Write Data Cycle or No operation
Each instruction will show a detailed Q cycle operation for the instruction.
Figure 5-2:Q Cycle Activity        
Byte-oriented file register operations
13                          8     7    6                              0 d = 0 for destination W
OPCODE            d              f (FILE #) d = 1 for destination f
f = 7-bit file register address
Bit-oriented file register operations
13                         10  9        7   6                       0
OPCODE          b (BIT #)        f (FILE #) b = 3-bit bit addressf = 7-bit file register address
Literal and control operations
13                                  8    7                             0
OPCODE                              k (literal) k = 8-bit immediate value
13                 11    10                                          0
OPCODE                        k (literal) k = 11-bit immediate value
General
CALL and GOTO instructions only
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
TCY1TCY2TCY3
Tosc
Figure 4: Format of instructions
Mnemonic, Description Status
Operands A ected
Byte-oriented  le register operations
ADDWF f,d Add W and f C,DC,Z
ANDWF f,d AND W with f Z
CLRF f Clear f Z
CLRW - Clear W Z
COMF f,d Complement f Z
DECF f,d Decrement f Z
DECFSZ f,d Decrement f, Skip if 0
INCF f,d Increment f Z
INCFSZ f,d Increment f, Skip if 0
IORWF f,d Inclusive OR W with f Z
MOVF f,d Move f Z
MOVWF d Move W to f
NOP - No operation
RLF f,d Rotate Left f through Carry C
RRF f,d Rotate Right f through Carry C
SUBWF f,d Subtract W from f C,DC,Z
SWAPF f,d Swap nibbles in f
XORWF f,d Exclusive OR W with f Z
Bit-oriented  le register operations
BCF f,b Bit Clear f
BSF f,b Bit Set f
BTFSC f,b Bit Test f, Skip if Clear
BTFSS f,b Bit Test f, Skip if Set
Literal and Control operations
ADDLW k Add literal and W C,DC,Z
ANDLW k AND literal with W Z
CALL k Call subroutine
CLRWDT - Clear watchdog timer TO, PD
GOTO k Goto address
IORLW k Inclusive OR literal with W Z
MOVLW k Move literal to W
RETFIE - Return from interrupt
RETLW k Return with literal in W
RETURN - Return from subroutine
SLEEP - Clear watchdog timer TO, PD
SUBLW k Subtract W from literal C,DC,Z
XORLW k Exclusive OR literal with W Z
Field Description
f Register  le address (0x00 to 0x7F)
W Working register (accumulator)
b Bit address within an 8-bit  le register
k Literal  eld, constant data or label
d Destination select:
d = 0, Store result in W
d = 1, Store result in  le register f
default is d = 1
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ECNG2006 (EE25M) March 11, 2008 III 8
Instruction13 6 0
0 0 1 0 1 0 0
RP1 RP0 Bits 0 0
0 0 0 0 1 0 1 0 0 Address014h
Instruction13 6 0
0 0 0 0 0 0 0
IRP Bit 0
0 0 0 0 1 0 1 0 0
AddressofINDFsaystouseFSRasapointer
FSR
Address014h
Figure 5: Direct and indirect addressing modes
 le address is  rst written to the the FSR, which is a special purpose register that is used as an
address pointer to any address in the entire register  le. A subsequent direct access to the INDF
register will actually access the register  le whose address is contained in the FSR i.e. the FSR acts
as a pointer. Since the FSR is an 8-bit register, both banks can be addressed without needing to
switch between them. The INDF register is not a physical register like the other special function
registers. The FSR register is one of several Special Function registers that have been assigned
multiple addresses so that they can be accessed in all banks. Figure (5) shows how the address is
formed.
The sparseness of the instruction set means that more programming e ort is required in the pro-
gramming of the PIC. A few of the special considerations are listed below.
Default destination The instructions from the table of the instruction that have f,d as the
operands use the d as an indicator to tell where to place the result. The default is f ( le register),
but this can be confusing so it is better to always specify the destination directly as the following
code fragment demonstrates.
W EQU H’0000’ ; standard definitions
F EQU H’0001’ ; ...to avoid having to write numbers
incf var,W ; W = var + 1, var is unchanged
incf var,F ; var = var + 1, W is unchanged
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STATUS register The STATUS register contains the arithmetic status of the ALU, the RESET
status and the bank select bit for the data memory. As with any register, the STATUS register
can be the destination for any instruction. If the STATUS is the destination for an instruction
that a ects the Z, DC or C bits, then the write to these three bits is disabled. These bits are
set or cleared according to the device logic. Furthermore the TO and PD bits are not writable,
therefore the result of an instruction with the STATUS register as destination may be di erent
than intended.
For example: clrf STATUS will not set the STATUS register to all zeros as expected but will clear
the upper three bits and set the Z bit. This leaves the STATUS register as 000u u1uu (where u
= unchanged). Only the bcf, bsf and movwf instructions should be used to alter the STATUS
register because these instructions do not a ect any status bits.
Review Exercises
1. Label the following statements about the PIC16F877 as True or False:
(a) Both the data and program memory spaces require switching banks to access the full
space.
(b) Data memory is 8 bits wide with a 9 bit address.
(c) It has 8 bit data registers, and 16 bit instructions.
(d) PIC16F877 machine cycle consists of 4 clock cycles.
(e) Register to register operations are supported.
(f) The INDF register is used to specify the address which is indirectly addressed when FSR
is read.
(g) The PIC16F77 has a RISC-based Von Neumann architecture.
(h) The banking bits are located in the STATUS register.
(i) The data memory has 9 bit addresses and the program memory has 13 bit addresses.
(j) The stack used to store the PC during a subroutine call is only 8 levels deep.
2. Which ONE of the following is NOT a feature of the PIC16F877?
(a) 2 stage pipeline
(b) 4 bit multiplier
(c) Harvard architecture
(d) built-in timers
(e) indirect addressing
3. Certain general purpose and special function registers are mapped to multiple addresses in
the memory space of the PIC 16F877. What possible advantage could such a scheme o er?
4. Determine which of the following statements concerning the following code snippet is/are
NOT correct:
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movlw 0x21
movwf 0x04
movlw 0x09
movwf 0x00
incf 0x04,1
sleep
(a) After this code has been executed, the CARRY  ag will be clear.
(b) After this code has been executed, the ZERO  ag will be clear.
(c) After this code has been executed, the value stored in location 0x00 is 0x09.
(d) After this code has been executed, the value stored in location 0x04 is 0x21.
(e) The code performs direct addressing of location 0x04, indirect addressing of location 0x21, and
immediate addressing of literal 0x09.
5. (a) Explain (and illustrate using pieces of code) the direct and indirect mechanisms of ad-
dressing Special Function Registers.
(b) What happens when the assembly language statement clrf STATUS is executed?
(c) Explain how the INDF register behaves when it is indirectly addressed for read/write.
(d) Di erentiate between the indirect addressing mechanisms for the register  le and the
data EEPROM.
6. Each instruction requires one cycle for its fetch and one cycle for its execution, yet the
PIC16F877 executes a new instruction every cycle.
(a) Explain how this accomplished.
(b) Why does a branch instruction, which also requires the same two cycles for fetching and
execution, introduce an extra cycle in the CPU’s execution of instructions?
7. Explain what the following PIC16F877 assembly language instructions for the PIC16F877
will do (source; operation; destination):
(a) ADDWF 0x20,0x00
(b) XORLW 0x02
8. In the PIC16F877, how is the instruction cycle related to machine cycles and clock cycles?
9. Role play: For the PIC 16F877, "The programmer never has to interact directly with the stack,
but should always remember the 8-level limit". For a similar processor with a 2-level stack, il-
lustrate (using diagrams) the problems that may occur if the programmer overruns/underruns
the stack.
10. Challenge: Find a single instruction that will toggle bit 0 of a  le register in the PIC16F877
if:
(a) you do not care what happens to the other bits in the  le register,
(b) you wish to preserve the values of the other bits in the  le register,
(c) the  le register is the PC register
(d) the  le register is the STATUS register
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Tutorial Exercise 4B O ine Version21 ID#
1. Di erentiate between the operation of the following instructions: 2 marks
addwf Reg,F
addwf Reg,W
2. What types of applications are microcontrollers (as opposed to microprocessors) commonly
used for? Explain your answer. 3 marks
3. The PIC16F877 has both data EEPROM and static data RAM (general purpose registers)
included in the microcontroller. Suggest ONE reason why the data EEPROM is also included
in the microcontroller. 2 marks
4. BRIEFLY explain the concept of \banking" and give ONE reason why memory banks might
be used in a microcontroller. 3 marks
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
21Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
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T
U
T
4
BUnit 9 Write the letter you have been assigned here .
The registers are A: FSR; B: STATUS; C: PCL; D: INDF.
Answer the following questions for the PIC16F877 register whose letter you were assigned.
1. Locate all the mapped addresses in the data memory space
2. What happens when the assembly language instruction movwf is executed on this register?
Which  ags are a ected? Are there any strange/unusual e ects as compared to general  le
registers?
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ explain the memory layout for the PIC16F877
{ explain the operation of the instruction cycle for the PIC16F877
{ explain the operation of the basic instructions (move, add, subtract, shifts) for the
PIC16F877
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
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10 MPLAB Overview
At the end of this unit the student will be able to:
utilize the MPLAB IDE and CCS compiler, to develop software for the MicroChip
PIC16Cxxx series of microcontroller, in C, C++ and assembly language.
Assemblers and compilers tend to have speci c keywords or pre-processor directives, which are
used to: access non-standard hardware features, specify code generation options/placement, sup-
ply code \shortcuts" e.g. macros, de nes. In this unit, we will examine some of the relevant
keywords/directives of the MicroChip MPASM assembler and the CCS compilers.
\ Absolute code is the default output from MPASM. ... When a source  le is assembled in this manner, all values
used in the source  le must be de ned within that source  le, or in  les that have been explicitly included. If assembly
proceeds without errors, a HEX  le will be generated, containing the executable machine code for the target device.
This  le can then be used in conjunction with a device programmer to program the microcontroller. ...
[extracts from]Table 2.1: MPASM Default Extensions
Extension Purpose
.ASM Default source  le extension input to MPASM: <source name>.ASM
.LST Default output extension for listing  les generated by MPASM: <source name>.LST
.HEX Output extension from MPASM for hex  les (see Appendix A): <source name>.HEX
... The source code  le may be created using any ASCII text  le editor. It should conform to the following basic
guidelines. Each line of the source  le may contain up to four types of information: labels, mnemonics, operands,
comments. The order and position of these are important. Labels must start in column one. Mnemonics may start
in column two or beyond. Operands follow the mnemonic. Comments may follow the operands, mnemonics or labels,
and can start in any column. The maximum column width is 255 characters. Whitespace or a colon must separate
the label and the mnemonic, and the mnemonic and the operand(s). Multiple operands must be separated by a
comma. A label must start in column 1. It may be followed by a colon (:), space, tab or the end of line. Labels must
begin with an alpha character or an under bar ( ) and may contain alphanumeric characters, the under bar and the
question mark. Labels may be up to 32 characters long. By default they are case sensitive, ... If a colon is used when
de ning a label, it is treated as a label operator and not part of the label itself. ... If there is a label on the same
line, instructions must be separated from that label by a colon, or by one or more spaces or tabs. ... MPASM treats
anything after a semicolon as a comment. All characters following the semicolon are ignored through the end of the
line. String constants containing a semicolon are allowed and are not confused with comments.
The listing  le format produced by MPASM is straight forward: The product name and version, the assembly date
and time, and the page number appear at the top of every page. The  rst column of numbers contains the base
address in memory where the code will be placed. The second column displays the 32-bit value of any symbols created
with the ... EQU, ... directives. The third column is reserved for the machine instruction. This is the code that will
be executed by the PICmicro MCU. The fourth column lists the associated source  le line number for this line. The
remainder of the line is reserved for the source code line that generated the machine code. Errors, warnings, and
messages are embedded between the source lines, and pertain to the following source line. The symbol table lists all
symbols de ned in the program. The memory usage map gives a graphical representation of memory usage. X marks
a used location and - marks memory that is not used by this object. The memory map is not printed if an object  le
is generated.
Intel Hex Format: This format produces one 8-bit hex  le with a low byte, high byte combination. Since each address
can only contain 8 bits in this format, all addresses are doubled. Each data record begins with a 9 character pre x
and ends with a 2-character checksum. Each record has the following format: :BBAAAATTHHHH....HHHCC where:
BB - is a two digit hexadecimal byte count representing the number of data bytes that will appear on the line.
AAAA - is a four digit hexadecimal address representing the starting address of the data record.
TT - is a two digit record type record type that will always be 00 except for the end-of- le record, which will be 01.
HH - is a two digit hexadecimal data byte, presented in low-byte/high-byte combinations.
CC - is a two digit hexadecimal checksum that is the twos complement of the sum of all preceding bytes in the
record.
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... Directives are assembler commands that appear in the source code but are not translated directly into opcodes.
They are used to control the assembler: its input, output, and data allocation. ... ...
ORG Set Program Origin. Syntax: [<label>] ORG <expr>. Set the program origin for subsequent code at the
address de ned in <expr>. If <label> is speci ed, it will be given the value of the <expr>. If no ORG is
speci ed, code generation will begin at address zero.
LIST Listing Options. Syntax: LIST [<list_option>, ..., <list_option>]. Occurring on a line by itself, the
LIST directive has the e ect of turning listing output on, if it had been previously turned o . Otherwise, one
of the following list options can be supplied to control the assembly process or format the listing  le:
[Extracts from]Table 5.2: List Directive Options
Option Default Description
mm=ON|OFF On Print memory map in list  le.
p=<type> None Set processor type; for example, PIC16C54.
st=ON|OFF On Print symbol table in list  le.
INCLUDE Include Additional Source File. Syntax: include <<include_file>>. The speci ed  le is read in
as source code. The e ect is the same as if the entire text of the included  le were inserted into the  le at the
location of the include statement. Upon end-of- le, source code assembly will resume from the original source
 le. Up to six levels of nesting are permitted. <include_file> may be enclosed in quotes or angle brackets.
EQU De ne an Assembler Constant. Syntax: <label> EQU <expr>. The value of <expr> is assigned to <label>.
END End Program Block. Indicates the end of the program [ le].
"(MPA 1999)
For the CCS C/C++ compilers, \ Pre-processor directives all begin with a # and are followed by a speci c
command. Syntax is dependent on the command. Several of the pre-processor directives are extensions to standard
C. .... [e.g.The #DEFINE ] directive is used to provide a simple replacement of the ID with the given string.
... C provides a pre-processor directive that compilers will accept and ignore or act upon the following data. This
implementation will allow any pre-processor directives to begin with #PRAGMA. To be compatible with other compilers,
this may be used before non-standard features. ...
[The #ORG] directive will  x the following function or constant declaration into a specialized ROM area. ...
The lines between the #ASM and #ENDASM are treated as assembly code to be inserted. These may be used anywhere
an expression is allowed. The syntax is described on the following page. The prede ned variable _RETURN_ may be
used to assign a return value to a function from the assembly code. Be aware that any C code after the #ENDASM and
before the end of the function may corrupt the value. ...
[The #DEVICE] directive de nes to the compiler what the hardware architecture is. This will determine the RAM and
ROM map as well as the instruction set used. For chips with more than 256 bytes of RAM you may select either 8 bit
or 16 bit pointers. To use 16 bit pointers add a *=16 after the chip name or by itself after the chip is de ned. To use
the Microchip ICD add an ICD=TRUE after the chip name or by itself after the chip is de ned. ... [__DEVICE__]This
pre-processor identi er is de ned by the compiler with the base number of the current device (from a #DEVICE). The
base number is usually the number after the C in the part number. For example the PIC16C622 has a base number
of 622. ...
[The #INLINE ] directive tells the compiler that the procedure immediately following the directive is to be implemented
INLINE. This will cause a duplicate copy of the code to be placed everywhere the procedure is called. This is useful
to save stack space and to increase speed. Without this directive the compiler will decide when it is best to make
procedures INLINE. "(CCS 1994-2000)
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Review Exercises
1. The assembly listing  le generated by MPLAB PIC16F877 compilers/assemblers display all
of the following types of information except:
(a) original source code
(b) generated machine code values
(c) generated hex  le lines formatted for download
(d) address locations where machine code will be stored
(e) labels and their equivalent values
2. We wish to specify the target microprocessor to the CCS compiler. Which ONE of the
following pre-processor directives is most appropriate:
(a) #include
(b) #de ne
(c) #device
(d) #org
(e) #inline
3. Which ONE of the following statements about the MPASM assembler directives is correct:
(a) the END directive appears at the end of each subroutine
(b) the ORG directive indicates the data memory location that the variable is stored at.
(c) the DT directive is used for lookup tables
(d) the BANKSEL directive is used to set the tristate registers
(e) the INCLUDE directive is used to indicate the options to be used by the assembler
4. We wish to specify the data memory locations, for multiple multi-byte variables, to the
MPLAB assembler. Which ONE of the following pre-processor directives is most appropriate:
(a) CBLOCK
(b) EQU
(c) END
(d) LIST
(e) ORG
5. The instruction comf 0x1A4,W would be encoded by the assembler as:
(a) A40916
(b) 092416
(c) 09A416
(d) 240916
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6. You wish to assign an assembly language variable to a space in the memory map of a
PIC16F877 using the EQU directive. Which of the following is a suitable address:
(a) 0x003
(b) 0x000
(c) 0x020
(d) 0x300
(e) 0x004
7. In your own words,
(a) di erentiate between an instruction and a directive.
(b) explain what each of the following assembler directives does
i. ORG
ii. EQU
iii. LIST
iv. END
(c) explain what each of the following C pre-processor directives does
i. #ASM ... #ENDASM
ii. #ORG
iii. #DEVICE
iv. #INLINE
8. Identify the comments, operands, mnemonics, labels, and directives in the following PIC16F877
assembly code:
C EQU 0x20 ; store the variable C at location 0x020
movlw 0x04 ; initialise W to 0x04
loop movwf C
addwf C, W
goto loop
sleep
END
9. Challenge: In your own words, explain the need for
(a) the #PRAGMA C/C++ directive.
(b) a standard hex  le format
10. Role Play/Challenge: Write a program (or use role play) to illustrate how an Intel Hex File
generated by MPLAB can be \dis-assembled" into assembly language mnemonics.
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Lab 1
ID#
You should complete the pre-lab before coming to your lab session. You may not be allowed in the
lab if your pre-lab is incomplete. You will have 3 hours in the lab to complete the exercises. All
answers should be written on this lab-sheet in PEN. Please do not attach any extraneous pieces of
paper.
At the end of this unit the student will be able to:
utilize the MPLAB IDE, CCS compiler and other software tools, to develop software for
the MicroChip PIC16xxx series of microcontroller, in C/C++ and assembly language.
Pre-Lab
Please read through the entire lab script FIRST, so you know what is expected of you, and THEN
complete the following questions.
1. Please identify
 your lab group letter (E,F,G,H):
 your ECNG2006 uP group designator (e.g. A3):
2. In your own words, di erentiate between an integrated development environment (IDE) and
a simulator. Hint: what do they input/output? what features do they o er? You should 5 marks
use examples to support your answer.
3. Simulators come in three \ avors":
Groups A1,B1,C1,D1,C4 instruction-level,
Groups A2,B2,C2,D2,B4 cycle-level,
Groups A3,B3,C3,D3,A4,D4 gate-level.
In your own words, debate why it is advantageous to simulate at the level assigned to your
group, rather than at either of the other two levels. 5 marks
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4. For the Microsoft Windows 2000 operating system:
(a) How do you open a \command window"? 1 mark
(b) What is the meaning of the term \command prompt"? 1 mark
(c) What is the meaning of the term \command line parameter"? 1 mark
(d) How do you specify command parameters when you are using the \command prompt"? 1 mark
(e) How do you specify command parameters when you are using the Windows GUI? 1 mark
(f) What is the meaning of the term \environment variable"? 1 mark
(g) How do you set, clear OR examine the values of environment variables when you are at
the \command prompt"? 1 mark
(h) How do you set, clear OR examine the values of environment variables when you are
using the Windows GUI? 1 mark
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5. In this lab we will be looking at di erent tool-chains which can be used with the PIC16F877
under Windows 2000. For your assigned tool, identify and explain TWO features (apart from
price) which make it superior to the competing tool in it’s category. You should support
your arguments using user testimonials which are not supplied by the tool creators. Cite all
references. 7 marks
Group(s) Tool Category Tool Group(s)
A1 MPLAB IDE PICC C2, A2
B4 SDCC Compiler CCSC D1
A4, C4 GPLINK Linker MPLINK B3
B1 MPASM Assembler GPASM C3
B2 MPSIM Simulator GPSIM D4
C1, A3 Command Batch  les Script MinGW make les D3, D2
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6. Algorithms are descriptions of how a function can be achieved. in this lab, we will be using
a particular algorithm to calculate the logarithm of an integer. It is based on the principle of
comparing the number to an incrementally increasing exponent of the base. This is not the
only possible algorithm.
Groups A1,B1,C1,D1,C4 Borcharts algorithm
(e.g. http://www.dattalo.com/technical/theory/logs.html)
Groups A2,B2,C2,D2,B4 Exploits  oating point representation
(e.g http://wehner.org/fpoint/)
Groups A3,B3,C3,D3,A4,D4 Uses a lookup-table
(e.g. http://www.dattalo.com/technical/software/pic/piclog.html)
Read about the alternative algorithm you have been assigned, and contrast it with the one
used in this lab. Use the calculation of log5 25 to support your answer. 5 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 III 22
Software Installation
Students may wish to install the same software on their own machine. A CD can be obtained from
Mr. Hall, or the software can by downloaded from their respective sites. To avoid problems, please
install software in the speci ed order. Please note that the OS in the lab is Windows 2000 and it is
installed on the D: drive. Administrative privileges are required for a proper installation. We will
not be able to give support for problems experienced under other operating systems.
Microchip MPLAB IDE Depending on the version, the default installation directory is either
n Program Filesn Microchip ORn Program Filesn MPLAB ORn Program Filesn MPLAB
IDE.
Please create a directory called Third Party within the installation directory. All other
software should be installed here.
CCS PCWH Please install in a subdirectory of your MPLAB installation;
for examplen Program Filesn Microchipn Third Partyn PICC. Installation may complain
that it cannot  nd the REG  les; if so please use the  les in the installation directory of the
CD. If the installation either fails to  nd MPLAB.INI for versions of MPLAB greater than
6.0, or fails to set data in registry, then you should install the plug-in.
CCS MPLAB plug-in Only required for MPLAB versions greater than 6.0. Please Install in the
same directory as the CCS PCWH i.e. Third Partyn PICC
GPUTILS Please install in Third PartynGPUtils
GPUTILS MPLAB plug-in Only required for MPLAB versions greater than 6.0. Please Install
in the same directory as the GPUTILS i.e. Third Partyn GPUtils
GPSIM Please install in Third Partyn GPSIM
SDCC Please install in Third Partyn SDCC
MinGW Make Please install in Third Partyn MinGW
PIC16F877 Visual Simulator Please install in Third Partyn Visual Simulator.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 III 23
In the Lab
Before you start, please ensure that the computer is set up to show  le extensions:
 Double click on My Computer then select the following from the menu(s) and pop-up windows:
Tools -- Folder Options -- View.
 Ensure that the checkbox labelled Hide file extensions is clear.
 Click OK.
Next, create a directory on the D: drive . This is the directory you will work with during the lab.
 In the My Computer window, double-click on the icon for the drive.
 Right click and select New Folder.
 Name the directory uPxxxxxxxx where xxxxxxxx is your ID number.
Finally, unzip the archive ecng2005-uP-lab1.zip into the directory you have just created.
Please ENSURE that you delete your directory at the END of the lab (you can copy it to your Z:
drive or a key  rst!)
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 III 24
Command Line Basics
1. Open a command window. Tick each of the following items as you try it:
 The prompt tells you which directory is currently active. e.g. D:\myfolder
 To change directories we type the command cd followed by the name of the directory to
which we wish to move e.g. cd \anotherfolder. The directory name is the command
line parameter that we supply for the command cd.
 To change drives we type the new drive letter followed by a colon. e.g Z:
 If we do not speci y the full path of the directory then we will move to a subdirectory
of the current directory. We also have directory shortcuts . and .. which stand for the
current and parent directories respectively.
Now lets get to work!
(a) Use the cd command to make the folder you created your active directory. Use the space
below to make notes to help you remember how to do this.
Show your lecturer or TA and have them sign/stamp your script here. 1 mark
(b) Within a directory we can list the  les and subdirectories using the dir command. The
dir command also works if we give it the path to a directory e.g. dir C:\otherfolder
Use the dir command to list the  les in another directory . Use the space below to make
notes to help you remember how to do this.
Show your lecturer or TA and have them sign/stamp your script here. 1 mark
(c) At the command line we often cannot remember all the options for a command. Most
commands have help. To display help for a command, try typing the command name
followed by -h , /h, or /?. Not all of these options will work with all programs, but
usually at least one will.
Try dir /?. Which options are available for the dir command? Choose one and use it.
Write a short description of what you did and what happened.
Show your lecturer or TA and have them sign/stamp your script here. 1 mark
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 III 25
(d) We can print the contents of a  le to the screen using the command type. Print the
contents of any text  le (we’ve supplied lab1read.asc) in the command window.
Show your lecturer or TA and have them sign/stamp your script here. 1 mark
(e) We can even start applications. To start Windows Notepad type notepad. You should
close the instance of Windows Notepad which just opened before proceeding.
If you have a sequence of commands that you execute regularly, it can be tedious to type
and retype them. You can place a sequence of commands in an ASCII text  le called a
script or BATCH  le. When you type the name of the batch  le, the commands will be
executed in sequence.
Use the dir command to look in the directory for  les with the .bat extension. Type
the name of the  le (excluding the .bat extension) at the command prompt.
What happened?
Show your lecturer or TA and have them sign/stamp your script here. 1 mark
Edit the batch  le (you can use Windows Notepad) and change it so that it uses the
commands you have learned to change active directory to your folder, and type out the
contents of lab1read.asc, regardless of the directory from which it is called. Save and
test your batch  le. Write down the  nal batch  le commands here.
Show your lecturer or TA and have them sign/stamp your script here. 2 marks
(f) Type set at the command line to see ALL the environment variables which are currently
set. You can also use the set command to create and alter environment variables.
When you set or clear environment variables at the command line, the changes only
a ect that command window. If you open another window, the changes will not be
there. Close your current command window. Open a new window and have a look.
If you ever need/want to make global changes to environment variables, right-click on
"My Computer","Advanced","Environment Variables".
Create an environment variable called ECNG2005 and then change the value from Test
to Ok. What command(s) did you use?
Show your lecturer or TA and have them sign/stamp your script here. 1 mark
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 III 26
(g) Environment variables are useful; they often specify default values to applications. Some
environment variables actually control how the command window/computer behaves.
For example the PROMPT environment variable controls how the command prompt is
displayed. You may use the prompt or set commands to alter it. Change the prompt so
that it displays your name before the drive letter and path. What command did you use?
Show your lecturer or TA and have them sign/stamp your script here. 1 mark
(h) The PATH environment variable lists all the places (search paths) that the computer
will look for a program. If you type the name of a program which is not in the current
directory, and not in one of the search paths, then the computer will complain. Start
Windows notepad by typing notepad (or notepad.exe at the prompt). Try clearing the
path variable (set PATH=). Does notepad still start from the command prompt?
Show your lecturer or TA and have them sign/stamp your script here. 1 mark
Please close all command windows before proceeding.
Build Tools
Compilers take ASCII text  les, that contain source code, as their input. You can edit any
source code  le in any ASCII text editor. This is IMPORTANT! There is no need to cut and
paste code between di erent editor applications - simply close the  le, then open the same
 le in another editor.
Go ahead any create a Visual C/C++ project. You will be able to open the resulting *.c
or *.cpp  les in Windows Notepad, change them, and then use the changed  le in Visual
C/C++.
If you open a  le that is not ASCII text in any text editor, it will look like \junk".
Windows Notepad is an Editor. Visual C/C++ is an Integrated Development Environment
that includes an editor. When you click build, it passes a command (just like the ones we
have just used) to start the compiler.
We are going to investigate compilers for the PIC16F877.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 III 27
2. Open an ASCII text editor (e.g. Windows Notepad) and examine the code in looplogm.c.
This code will calculate the base-n logarithm of the integer x. The initial value of n should
be 3, and the initial value of x should be 81. If not correct and save the  le.
Make two subdirectories called SDCC and PICC in your working directory. Place a COPY
of looplogm.c in each of your directories.
(a) Open a new command window. In this window we will utilise the CCS PIC-C compiler
to compile the code.
 You need to set the PATH environmental variable so that ccsc.exe to run correctly.
The command will depend on the path to the PICC directory on your computer. A
typical command would look like:
set PATH=%PATH%;D:\Program Files\MicroChip\Third Party\PICC
 You need to change the active directory to PICC subdirectory of your personal
folder. A typical command would look like cd D:\uP12345678\PICC Edit the copy
of looplogm.c in this directory. Be sure to include a #DEVICE line in your  le to
specify the target processor (PIC16F877).
 Run the compiler command: ccsc.exe looplogm.c +FM +P. What was displayed? 1 mark
If you have any errors, ask your lecturer/TA for assistance.
 Once your  le has compiled correctly, look at your folder. List the names and ex-
tensions of the  les which have been created. For each  le: Indicate whether it is
an ASCII text  le, and if it is, write out one/two lines from the  le. (Try to choose
interesting ones!)
Filename Filesize ASCII? Extract (if ASCII)
looplogm.c
looplogm.sym
looplogm.hex
Show your lecturer or TA and have them sign/stamp your script here. 2 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 III 28
 We could also create a batch  le to run CCSC compiler. In your batch  le called
myccsc.bat place the commands you have just learned to set the path, change the
directory, and run ccsc on looplogm.c.
If you replace the name of your  le within the batch  le with %1 then you can specify
the  le to be compiled along with the name of the batch  le at the command line.
For example myccsc myfile to compile myfile.c.
Test your  nal batch  le and write it below. Test your batch  le in a new command
window.
Show your lecturer or TA and have them sign/stamp your script here. 1 mark
Close all command windows.
(b) Open a new command window. In this window we will utilise the SDCC compiler to
compile the code.
 You need to set the PATH environmental variable for sdcc.exe to run correctly.
How are you going to do this? 1 mark
 You need to change the active directory to the SDCC subdirectory of your personal
folder. How are you going to do this? 1 mark
 Make sure there is a copy of looplogm.c in the SDCC directory. No preprocessor
directives are needed for this compiler. Unlike the #DEVICE needed for PICC, we
will specify the processor on the command line for SDCC.
 Run the compiler command: sdcc -S -V -mpic14 -p16F877 myfile.c. Please
note that the command line options are case sensitive. What was displayed? 1 mark
If you have any errors, ask your lecturer/TA for assistance.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 III 29
 Once your  le has compiled correctly, look at your folder. List the names and
extensions of the  les which have been created. For each  le: Indicate whether it is
an ASCII text  le, and if it is, write out one/two lines from the  le. (Try to choose
interesting ones!)
Filename Filesize ASCII? Extract (of ASCII  les)
looplogm.c
looplogm.asm
looplogm.d
Show your lecturer or TA and have them sign/stamp your script here. 2 marks
 We could also create a batch  le named mysdcc.bat to run the SDCC compiler.
Try it below!
Show your lecturer or TA and have them sign/stamp your script here. 1 mark
Close all command windows.
The  nal  le which is required for \download" to the PIC16F877 microcontroller is an Intel
HEX format  le *.hex".
You should have noticed that although both compilers generated  les, only CCSC created a
".hex  le. CCSC accomplished the compiling, assembling, and linking in one step.
SDCC generated an assembly language  le *.asm. To complete the job for SDCC we would
need to use a separate assembler and a linker.
Assemblers and linkers can also be used for code which you write directly in assembly.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 III 30
3. Alter your SDCC batch  le so that the search path for gpasm.exe and gplink.exe is added.
They are located in Third Party\GPUtils\.
Add the following lines at the end of your batch  le: ( These commands are available in the
 le batfileadd.txt; you may need to change looplogm to %1 if you have used that option
previously).
 set GPUTILS_HEADER_PATH=D:\Program Files\MicroChip\Third Party\GPUtils\header
 set GPUTILS_LKR_PATH=D:\Program Files\MicroChip\Third Party\GPUtils\lkr
 gpasm -c looplogm.asm
 gplink -m -o looplogm.hex looplogm.o (line wraps)
\D:\Program Files\MicroChip\Third Party\SDCC\lib\pic\libsdcc.lib"
Run the SDCC batch  le in a new command window. If you have any errors, ask your
lecturer/TA for assistance.
(a) Once your  le has compiled, assembled and linked correctly, look at your folder. List
the names and extensions of the ADDITIONAL  les which have been created by gpasm
and gplink. For each  le: Indicate whether it is an ASCII text  le, and if it is, write
out one/two lines from the  le. (Try to choose interesting ones!)
Filename Filesize ASCII? Extract
looplogm.hex
looplogm.o
Show your lecturer or TA and have them sign/stamp your script here. 2 marks
4. Compare the  les generated by the two processes from the same original C  le. Are they
identical? Did you expect them to be? Explain why you did or did not think so.
Show your lecturer or TA and have them sign/stamp your script here. 4 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 III 31
Build Tools Roundup - Make Utility
5. (a) The batch  le works well for a single  le, but where we want to establish rules for
compiling  les it is often more productive to use a make le. Using this make le we can
specify di erent targets, and how they come together to form the  nal executable. We
will only use a simple make le here (an electronic copy of makefile is available).
%.asm: %.c
sdcc -S -V -mpic14 -p16F877 $<
%.o: %.asm
gpasm -c $<
%.hex: %.o
gplink -m -o $@ $< \\
D:\Program Files\MicroChip\Third Party\SDCC\lib\pic\libsdcc.lib
Each rule starts in the  rst column (no space or TAB) and speci es the Target. Our
targets all start with % which is a wildcard i.e. the rule will match any  le which has
the same su x. This is followed by a list of dependencies ( les that must exist in order
to produce the target) . Then the following lines (each starting with a TAB) list the
command(s) used to make the target ($@) from the dependencies($<).
 Check the rules in the  le called "make le" (no extension) in the SDCC directory.
 Open a command window and add the paths for sdcc, gputils, and mingw\bin to
the search path.
 Make the SDCC directory the active directory.
 Test your make le is syntactically correct by using the command:mingw32-make. If
you have any errors, ask your lecturer/TA for assistance.
 Type the following command: mingw32-make myfile.hex
Delete myfile.hex and try it again. 4 marks
Show your lecturer or TA and have them sign/stamp your script here.
You should notice that make automatically builds only the  les that need to be updated.
This is one advantage over using scripts or batch  les.
(b) Challenge(Bonus 10 marks): Place a make le in the CCSC sub-directory that could be
used to build a hex  le using the ccsc compiler. Write your rule(s) down here.
Show your lecturer or TA and have them sign/stamp your script here.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 III 32
Development Tools
We have looked at tools used to build the hex  le from the source code. Now we will look
at tools which assist in the development of correctly functioning programs. The  rst of the
development tools we will look at is the simulator.
Even though we have compiled the code on a HOST PC, the output  le needs to be loaded
onto a TARGET microprocessor in order to run.
The HOST PC cannot normally run this  le by itself. A simulator is a program which will
allow the HOST PC to interpret the machine code intended for the TARGET processor.
This is a convenient alternative for the developer, as software development can start even
though the hardware is not available.
6. GPSIM is a Windows based simulator for the PIC16F877. To start gpsim,
 Open a new command window.
 Change the active directory to SDCC (or CCSC for next question!).
 Add the search paths for gpsim, and gputils.
 Use the following command to start gpsim: gpsim -p pic16f877 looplogm.hex
You will see text printed in the command window and a button bar will pop up.
(a) Open the Program Memory window, using the menu on the button bar. This has two
tabs, one tab shows the assembly language code, with each instruction next to it’s stored
address, and equivalent instruction code, while the other shows the bytes of instruction
code stored in the program memory in a matrix form. What range of instruction memory
locations are occupied by looplogm.hex.
Show your lecturer or TA and have them sign/stamp your script here. 1 mark
(b) Open the RAM window, using the menu on the button bar. This is a matrix view of
the data memory.
You can step through the assembly language code using the step button. The command
window reports the status of the processor with each step. By looking at the RAM
window while you step, you can see how information moves from one register to another
in response to the instructions.
How many data memory locations are being used by looplogm.hex?
Show your lecturer or TA and have them sign/stamp your script here. 1 mark
(c) It is inconvenient to keep stepping through a program. To move more quickly, set a
BreakPoint by double clicking on a program line in the Program Window. Reset the
processor (using the button on the button bar), then run. The processor should stop
before executing the line you have indicated.
Show your lecturer or TA and have them sign/stamp your script here. 1 mark
(d) Open the StopWatch Window. This is used to determine how many cycles the code
takes to run. Play with the Stopwatch and the breakpoint to see how long the code
takes to execute.
How many cycles are being used by looplogm.hex?
Show your lecturer or TA and have them sign/stamp your script here. 1 mark
Close GPSIM when you are done.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 III 33
7. Repeat the process described in 6 to measure the number of instruction memory locations,
data memory locations, and cycles required by the version of looplogm.hex generated using
CCSC (instead of the one generated using SDCC). 3 marks
Show your lecturer or TA and have them sign/stamp your script here.
8. The PIC16F877 Visual Simulation was written to illustrate what goes on inside the micro-
controller on each clock cycle i.e. it is a cycle-level simulator. GPSIM is an instruction-level
simulator. It does not show us what goes on within the instruction-cycle.
When you launch the PIC16F877 Visual Simulation, the main window will look like the layout
diagram shown in your data-sheet book.
Just like the GPSIM simulator, this simulator loads in the Hex  le output by the compiler.
 Launch the PIC16F877 Visual Simulation from the Windows Menu.
 To load in a hex  le, Click on TASKS, DECODE HEX File; a window named
\Decode Hex File" will come up; you should Choose Select File to Decode. Locate
either of your compiled hex  les and press OK. In the \Hex Data" Field, you will see
the actual contents of the HEX  le.
 When parsed (using the Intel Hex format) the byte-codes to be loaded at particular
addresses are displayed in the \Parsed data" Field.
 Click the DECODE button, to interpret the instruction op-codes & operands as mnemon-
ics.
(a) Does this match the GPSIM views of program memory? Is that what you expected?
Explain your answer. 1 mark
(b) Click the Back to the interface button and run the simulator by Clicking the start
processing button. You will see two dots moving around the screen.
 Blue Dot represents information  ow due to the fetch phase of the pipeline.
 Red Dot represents information  ow due to the execute phase of the pipeline.
Each phase is made up of 4 sections, Q1 to Q4, the duration of which is set by the
clock. The currently executing phase is displayed on the left. Pay particular attention
to what happens when a goto instruction is executed. How is pipeline execution a ected? 2 marks
You should close/pause the PIC16F877 Visual Simulation when you are  nished.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 III 34
Build & Development Tools Roundup -
The Integrated Development Environments(IDE)
An IDE allows us to perform all the tasks we have looked at from a single interface. Some
IDE’s are supplied with their own tool-chain, while others contain no tools and must be
con gured to work with your tools. Environmental variables, batch  les, scripts, make les,
and command line parameters can all be used with an IDE to make it operate seamlessly with
a tool-chain. Instead of manually calling separate programs to translate from .c to .asm to
.o to .exe; we can automatically call appropriate programs.
The MPLAB IDE is an example of an IDE that comes with it’s own tools, but allows you
the option to choose other build tools (like the CCS compiler), and customize environmental
variables, and command line parameters.
9. (a) Start the MPLAB IDE from the menu. Like most Windows based IDE’s there is a menu,
toolbar, workspace and a status bar. Create a new project in your CCSC subfolder.
 Click PROJECT on the menu bar and select PROJECT WIZARD. In the
PROJECT WIZARD box click NEXT. From the DEVICE drop down menu
select PIC16F877 and click NEXT. Select CCS C COMPLIER form the AC-
TIVE TOOLSUITE dropdown box and then click NEXT.
 Enter \myproject" in the box named PROJECT NAME. Then browse and  nd
the CCSC folder you created for the PROJECT DIRECTORY box and click
NEXT.
 A window will now come up with the buttons ADD and REMOVE, add your  le
looplogm.c from the CCSC directory and then click FINISH in the new window.
 Then go to WINDOW on the menu bar and select \project1.mcw". You will see
\looplogm.c" . Now double click the \looplogm.c" link and your program will
appear.
Show your lecturer or TA and have them sign/stamp your script here. 1 mark
(b) The MPLAB IDE has a built in editor. It can be used to view and edit any ASCII-type
 le. Try opening di erent  les in the CCSC directory.
Show your lecturer or TA and have them sign/stamp your script here. 1 mark
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 III 35
(c) The MPLAB IDE also has a built in simulator. Go to the menu bar and select DEBUG-
GER > SELECT TOOL > MPLAB SIM. This means that you will be executing
your code on the simulator. You should be able to use the  les which have already
been generated. To display the Program Memory click on the VIEW > PROGRAM
MEMORY menu item. The buttons at the bottom left of the window, allow you to
access three di erent views of the Simulator Program Memory. State the name of each
the view, and identify how the views di er from each other. Are these views similar to
the ones we had in the other two simulators? 2 marks
(d) The PC value displayed in the status bar is set to 0x00. Let us investigate how to
advance the PC to the next instruction location. To advance to the next instruction
location click on the DEBUGGER menu and choose STEP OVER from the menu.
What do we see happening? Is this similar to what happened in the other two simulators?
Show your lecturer or TA and have them sign/stamp your script here. 2 marks
(e) The MPLAB IDE project has been set up to use the same CCSC compiler we used at the
command line. Click on the PROJECT > BUILD ALL menu item (or press F10),
to compile the project. A build window should appear stating the build was successful.
Have a close look. Can you see the command used to launch the compiler? Write it
below.
Show your lecturer or TA and have them sign/stamp your script here. 1 mark
(f) In the GPSIM simulator we used breakpoints, a view of RAM and a stopwatch. Locate
the equivalent functions in MPLAB.
Show your lecturer or TA and have them sign/stamp your script here. 3 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 III 36
Performance Comparison
So far we have compared the sizes of  les, the number of instruction and data memory
locations used, and the number of cycles that the program requires to run.
The performance of the  nal *.hex  le depends on:
 the algorithm implemented
 the compiler/assembler used to generate the *.hex  le
 the language and data types used for implementation
We will investigate each of these in turn, using the skills we have acquired during this lab.
The comparison criteria to be used are:
I the size of the source code  le in bytes
II the number of instruction memory locations used by the  nal program
III the number of data memory locations used by the  nal program
IV the number of instruction cycles required to execute the program on test case A
V the number of instruction cycles required to execute the program on test case B
The test cases to be used are A: log3 81 and B: log5 25.
10. Challenge Question: (Bonus 10 marks) Use the test cases to discuss WHY the algorithm you
researched for the pre-lab is likely to perform better or worse than the algorithm implemented
in looplogm.c. In addition to the speci ed comparison criteria, you could also consider the
accuracy/suitability of the  nal solution.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 III 37
Item I II III IV V
Units bytes instruction-words data-words cycles cycles
looplogm.c with CCSC
looplogm.c with SDCC
liblogm.c with CCSC
liblogm.c with SDCC
asmlogm.asm
Table 1: Performance comparison results
11. Use data from previous pages, and modify the programs and re-run them to obtain data to
update the  rst two rows in Table 3. 2 marks
Demonstrate the changes made to your TA/lecturer, who will sign/stamp your script as
con rmation.
12. The program liblogm.c uses the compiler provided math libraries to calculate the answer.
Compile and/or assemble programs as necessary to  ll the second two rows of the Table 3. 10 marks
Demonstrate this to your TA/lecturer, who will sign/stamp your script as con rmation.
13. Make a new project in MPLAB to assemble asmlogm.asm. You may use either the MPASM
or the GPASM assembler. Make the measurements needed to update Table 3. 5 marks
Demonstrate this to your TA/lecturer, who will sign/stamp your script as con rmation.
14. Use your data to identify which is the most e ective implementation of logarithm. Justify
your choice, and explain whether this is what you had expected. 3 marks
Total marks 100.
This exercise is worth 5% of your ECNG2005 lab mark.
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
 assistance in troubleshooting circuitry and/or using/operating the IDE/debugger
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
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ECNG2006 (EE25M) March 11, 2008 III 38
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1Unit 10 Write the number you have been assigned here .
Answer the following questions for the TARGET platform whose number you were assigned.
1. What is the name of the target platform? .
2. Where did you  nd the information? .
3. Who is the manufacturer of the tool-chain? .
4. What is the price of this tool-chain? .
5. Which HOST platform/operating system(s) can use this toolchain? .
6. Which of the following items does the tool-chain contain?
 IDE
 make utility OR make les
 batch/shell  les OR other scripts
 assembler
 disassembler
 linker
 compiler
 pre-de ned header  les
 pre-de ned library  les OR librarian
 simulator
 downloader
 debug kernel + monitor
 other
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this lab exercise.
{ utilize the MPLAB IDE, CCS compiler, and other software tools, to develop software for
the MicroChip PIC16Cxxx series of microcontroller, in C/C++ and assembly language.
 Which aspect of this lab exercise did you have the most di culty understanding?
 Which aspect of this lab exercise did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this lab exercise.
 Identify one way in which this lab exercise could be improved.
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ECNG2006 (EE25M) March 11, 2008 III 39
PIC16 Introduction
In this section, we have looked at the basic features of the PIC16F877(memory layout, instruction
cycle, instruction set), and the MPLAB/CCS tool-chains (directives and keywords). While these
are speci c to the particular items, they are typical of the features you will  nd available in many
microprocessors/microcontrollers, and their associated tool-chains.
In examining these features we have furthered the  rst learning objective, as you can now: "given
relevant diagrams and speci cations for a particular system, identify, and describe
the role of, the components of a microprocessor, a microprocessor-based system, a
development suite (hardware, software) for a microprocessor-based system."
In the following section, we will look at how data of various types can be represented and manipu-
lated by microprocessors.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) Introduction to microprocessors
c DECE, UWI, St. Augustine, Trinidad
original author: Feisal Mohammed; updated: March 11, 2008{ CLR
Part IV
Numbers & Data
There are several issues connected with the manipulation of real numbers, strings and other forms
of data by computers.
The  rst is how such data should be represented; the second is the means by which the data should
be manipulated; and the third is the bit/byte/word/record-ordering of data within memory to
accommodate the representation.
In this section, we will address these issues for two’s complement, binary-coded decimal, and  oating
point representations of numbers, ASCII representation of strings, and storage of composite records.
11 Real numbers
At the end of this unit the student will be able to:
represent real numbers using  xed and  oating point binary representations.
Two’s Complement The most commonly found representation of integer numbers in computers
is the two’s complement representation. In this representation, the full range of binary numbers (2n
for n bits) is split between positive and negative numbers ( 2n 1 to 2n 1 1). Positive numbers
are the same as their unsigned representations. To  nd the representation of a negative number,
invert the bits of the unsigned binary representation and then add 1. Using this representation, all
subtractive operations become additions with a negative number. Because the uppermost bit of a
two’s complement number is always 0 for positive numbers22, and 1 for negative numbers, this bit
is often referred to as the sign bit.
Changing the bit width of a two’s complement number can be done by sign reduction or extension
i.e. removing locations which are the same as the sign bit, or copying the sign bits to any additional
bit locations. In practice, representation of a number using more bits involves the use of multiple
data words.
Binary Coded Decimal Binary coded decimal representation of numbers, uses a 4 bit binary
number to represent the decimal digits 0-9. It is clearly an ine cient representation, as 6 of the
numbers which could be represented by 4 bits are never used. The advantage of using such a scheme
lies in the fact that most human I/O involves the use of decimal numbers. The use of BCD reduces
the overhead of conversion at the interface-level. The disadvantage is that mathematical operations
become more complex, as the manipulation of individual binary digits may yield an invalid 4 bit
BCD code. Further there is no accommodation for signed quantities. Subsequently BCD arithmetic
22Zero is assumed to be a positive number.
ECNG2006 (EE25M) March 11, 2008 IV 2
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ECNG2006 (EE25M) March 11, 2008 IV 3
operations are generally restricted to addition and subtraction of unsigned quantities. In packed
BCD, two digits are continued within a single byte. Other representations pad the extra bits in
the byte/word (i.e. leave the upper bits of the data word either unused or  lled with a particular
pre x) e.g. the ASCII23 character set de nes a representation of characters as 7 bit numbers. The
digit characters 0 through 9 are represented as 011 00002 thru 011 10012.
Fixed point representations Integer representations are speci c cases of  xed point represen-
tations i.e. numeric representations in which the position of the decimal point is assumed to be
 xed; the digits above and below the decimal point are speci ed.
Floating point representations represent numbers using an assumed numeric base i.e. all
numbers in a particular system, are presumed to have the same base. Numbers are represented by
an exponent of the base, multiplied by a normalised number called the mantissa or signi cand. i.e.
 S B E. Normalised numbers are justi ed so that the  rst signi cant digit appears at a  xed
place e.g. 0.00001 and 0.1 and 100 may be expressed as 1 10 5, 1 10 1 and 1 102.
The presence of sign in the signi cand or exponent may be represented separately, or by use of a
biased numeric representation. A biased numeric representation is one which includes a  xed o set
e.g. for an 8 bit number with a bias of 0x7F, the number -4 would be represented as 0x7B and the
number 4 would be represented by 0x83.
The base used, the use of biased numbers vs. explicit sign representation, the number of bits used
to represent the mantissa and exponent, and the ordering of bits within the representation depends
on the particular system/designer/programmer. In computer systems the conventions are:
 the base is usually 2,
 the exponent is biased, and
 the sign of the signi cand is explicitly represented.
All  xed/ oating point representations, regardless of the details, share the following  aws:
Representation error The precision with which a  oating point number can be represented is
determined by the number of bits used in the exponent and signi cand. Where the number
of bits is insu cient, the number be be approximated by roundo , or truncation. In either
case there is some discrepancy between the number and it’s representation.
Error propagation Arithmetic operations often magnify the e ects of errors in the input quan-
tities. e.g. 2.51 * 2.32 = 5.8232 BUT 2.5 * 2.3 = 5.75.
Non-unique 0 The  oating point representation scheme is such that there is a choice of represen-
tations for zero.
23American Standard Code for Information Interchange
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ECNG2006 (EE25M) March 11, 2008 IV 4
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 IV 5
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 IV 6
Review Questions
1. What range of unsigned integer numbers can be represented in 6 bits?
(a)  25 to 25 1
(b)  25 + 1 to 25 1
(c)  26 to 26 1
(d)  26 + 1 to 26 1
(e) 0 to 26 1
2. If the byte 110101102 represented a packed BCD number, the BCD number would be:
(a)  2A16
(b)  5610
(c) D616
(d) invalid
(e) none of the above
3. If the dataword 11100010 represented a signed integer (two’s complement representation
scheme), the number would be:
(a) +1E16
(b) +E216
(c)  1716
(d)  1E16
(e) none of the above
4. Convert the following numbers (show your working):
(a) 2510 to 2’s complement representation, written as a hexadecimal number.
(b)  7310 to 2’s complement representation, written as a binary number.
(c)  3716 to 2’s complement representation, written as a binary number.
(d)  1216 to 2’s complement representation, written as a hexadecimal number.
(e)  12110 to 2’s complement representation, written as a hexadecimal number.
(f) 9716 read as 2’s complement representation, to a signed hexadecimal number.
(g) 5:610 to a binary  xed point representation with 3 bits (integer), and 5 bits (fraction).
(h) 2:A16 to a binary  xed point representation with 3 bits (integer), and 5 bits (fraction).
(i) 9716 read as signed binary  xed point representation with 2 bits (integer), and 6 bits
(fraction); to a signed decimal number.
(j) 9716 read as unsigned binary  xed point representation with 2 bits (integer), and 6 bits
(fraction); to a signed hexadecimal number.
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ECNG2006 (EE25M) March 11, 2008 IV 7
11010101111100102
5. Show how you would derive the base 10 equivalent of the number shown above, using diagrams,
and/or calculations, if we interpret the value as:
(a) an unsigned 16 bit integer number
(b) a signed 16 bit integer number
(c) a signed 16 bit  xed point number (9,13)
(d) a 16 bit  oating point number (explicit sign, biased exponent(base 2), signi cand nor-
malised just after numeric point)
6. A PIC16F877 program needs to use certain non-integer numbers. For each number, suggest
an appropriate way in which the number can be represented in a single  le register, write down
your representation, and determine what error there is between the actual and represented
values.
(a) 2.5
(b) 0.333
(c) 10000.345
7. You need to sort a list of signed integer numbers on a PIC16F877. Suggest an appropriate
numeric representation for numbers in the list, and justify your answer if:
(a) we wish to sort numbers by magnitude. e.g. (-5,4,3,-2,1,0)
(b) we wish to sort from most negative to most positive. e.g.(-5,-2,0,1,3,4)
8. Any  xed/ oating point representation used in a computer can represent only certain real
numbers exactly; all others must be approximated. If A’ is the stored value approximating
the real value A then the relative error r is expressed as: r = A A0A .(Stallings 2000; 8.21{8.24)
(a) Represent the decimal quantity +0.7 in the following formats, and determine the relative
error for each representation.
i.  oating point format: base = 2; exponent: biased 4 bits; signi cand 7 bits.
ii.  xed point format: 2 bits integer; 6 bits fraction.
(b) If A = 1:427,  nd the relative error if A is truncated to 1.42 and if it is rounded to 1.43.
(c) Numerical values A and B are stored in the computer as approximations A0 and B0.
Neglecting any further truncation or roundo errors, show that the relative error of the
product is approximately the sum of the relative errors in the factors.
(d) One of the most serious errors in computer calculations occurs when two nearly equal
numbers are subtracted. Consider A = 0:22288 and B = 0:2221. The computer trun-
cates all values to four decimal digits. Thus A0 = 0:2228 and B0 = 0:2221.
i. What are the relative errors for A0 and B0?
ii. What is the relative error for C0 = A0 B0?
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ECNG2006 (EE25M) March 11, 2008 IV 8
9. Typically  oating point numbers are represented using a minimum of 32 bits.
The IEEE standard 754 standard de nes a "single" format 32 bit  oating point number as a
sign bit followed by an 8 bit biased exponent, followed by a 23 bit signi cand (representing
the normalised 24 bit binary fraction 0.1xxx xxxx xxxx xxxx xxxx xxxx).
Floating point numbers in the CCS compiler for the PIC16F877, place a biased 8 bit exponent
 rst, followed by the sign bit and then a 23 bit signi cand (representing the normalised 24
bit binary fraction 0.1xxx xxxx xxxx xxxx xxxx xxxx).
The di erence is in the location of the sign bit. Suggest at least TWO possible advan-
tages/disadvantages of placing the sign bit in either position.
10. Role Play/Challenge: Prove that
(a) it is possible to write down the binary/octal equivalent of a hexadecimal number by
individually translating each digit.
(b) the n’s complement can be calculated for any base-n number (Hint: 2’s complement is
for base 2 numbers).
(c) it is possible to write down the 2’s/8’s complement of a hexadecimal number by individ-
ually translating each digit of the 16’s complement.
(d) (Stallings 2000; 8.29) every real number with a terminating binary representation ( -
nite number of digits to the right of the binary point) also has a terminating decimal
representation ( nite number of digits to the right of the decimal point)
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 IV 9
Assignment B 24 ID#
1. Choose two di erent representation schemes for real numbers. Explain how each scheme
operates, using the number  8:4610 as an example. Compare your chosen representation
schemes in terms of the possible representation error and storage space requirements. 12 marks
24Students are advised that there is no online version of this assignment, and that electronic submissions will not
be accepted without the explicit permission of the course lecturer.
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ECNG2006 (EE25M) March 11, 2008 IV 10
2. Draw a diagram to show how to, and then write a routine (snippet) for the PIC16F877 which
will, interpret the upper nibble of the working register as a BCD digit, and return (end) with
the digit value in the lower nibble of the working register. Your code should test for/reject
impossible values, in which case the value in the working register should be set to 0x0F.
Please comment your code appropriately. 12 marks
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3. Draw a diagram to show how to, and then write a routine (snippet) for the PIC16F877 which
will, add two  oating point numbers, where the  oating point numbers are represented using
4 bits (lower nibble) for the unsigned mantissa, and 4 bits (upper nibble) for the signed ex-
ponent. Please explain any choices/assumptions you make about your representation. Please
comment your code appropriately. 16 marks
This assignment is worth 2% of your ECNG2006 mark. It contains a total of 40 marks.
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 IV 12
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BUnit 11 Your lecturer will write an 8-bit binary number, and a decimal number on the board.
Write the letter you have been assigned here . The representation schemes are:
A: signed integer (2’s cmplt)
B: BCD unsigned integer
C:  xed point (4 bits before point; 4 bits after point)
D:  oating point (1 bit sign; 3 bits biased exponent; 4 bits signi cand)
1. Interpret the binary number using your assigned representation scheme.
2. Express the decimal number using your assigned representation scheme.
3. What range of numbers can you represent using this scheme?
4. What is the smallest di erence you can represent using this scheme?
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ represent real numbers using  xed and  oating point binary representations.
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
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ECNG2006 (EE25M) March 11, 2008 IV 13
12 Integer arithmetic
At the end of this unit the student will be able to:
perform integer arithmetic operations using binary representations of the operands.
Addition Two or three bits can be added together to generate a sum and a bit-carry; two unsigned
binary numbers can be added together by considering each pair of bits in turn, and adding them
together with the bit-carry from the previous pair.
Subtraction Two’s complement notation was introduced in the previous unit. Using this no-
tation, both positive and negative numbers can be expressed. The convenience of this notation,
is
 that addition of two’s complement numbers can be accomplished using the same method as
for unsigned binary numbers.
 that subtraction of two’s complement numbers can be accomplished by negating one number,
and then performing an addition.
Over ow is not Carry The bit-carry from the  nal pair of bits is often used to set the carry
 ag. The interpretation of a set carry  ag di ers depending on the arithmetic operation carried
out. If an unsigned addition generates a carry, it implies that the data-word was not big enough
to hold the result (i.e. over ow).
For two’s complement numbers, it is not so simple:
 for valid two’s complement numbers of opposing sign, the carry and sign bits of the addition
result will always be of opposing polarity, and over ow will never occur.
 for valid two’s complement numbers of the same sign, the carry and sign bits of the addition
result will be of opposing polarity i over ow has occurred, and of the same polarity i no
over ow has occured. (as shown below)
Sign Bit No Carry Carry
Reset No Over ow Over ow
Set Over ow No Over ow
As a result, some processors keep separate carry and over ow  ags, others de ne just one which
behaves di erently depending on the instruction carried out. Other variants of the carry  ag,
are the mid-word carry  ags. These are set to re ect particular bit-carry’s in the middle of the
data-word.
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ECNG2006 (EE25M) March 11, 2008 IV 15
Multiplication There are two ways to implement the multiplication of two unsigned numbers,
one is by repeated addition and the other is via partial sums (works like primary-school arithmetic).
Unlike addition and subtraction, the unsigned methods will not work for multiplication involving
negative two’s complement numbers (Stallings 2000; Ch. 8). They will however work for pairs of
positive two’s complement numbers. One procedure is:
 check the sign of the result (same sign: result is positive, di erent sign: result is negative),
 make all numbers positive,
 perform an unsigned multiplication
 negate the result if necessary
Alternatively, Booth’s algorithm, a variant of the partial sums method, may be utilised. This
method exploits the fact that transitions from 0->1 and 1->0 (reading right to left) within the
stream of bits of a two’s complement number re ect a subtraction and addition (respectively) of a
power of 2.
Division is the opposite of multiplication. In it’s simplest form, for unsigned numbers, the divisor
may be repeatedly subtracted from the dividend and the quotient incremented each time. Unsigned
numbers may also be divided using a partial remainder method. Two’s complement numbers may
be converted to unsigned form for division (note that in addition to adjusting the quotient sign,
the reminder sign needs to be adjusted), or the partial remainder method may be modi ed for use
on two’s complement numbers (note that the method involves an element of "back-tracking").
Multi-byte operations Because the ALU is generally structured for single data word operations,
the manipulation of multiple-word integer numbers i.e. 16-bit, 24-bit, etc. on an 8-bit ALU requires
extra work. Addition and subtraction may be accomplished simply by propagating the carry to
the addition of the lowest bits of the next data-word pair. Multi-byte multiplication may be
accomplished using a partial sums method, which involves shifting by words instead of by bits
before adding. Multi-byte division may be accomplished by starting with the highest order word,
shifting the result, and propagating the remainder.
BCD operations Binary coded decimal numbers are speci ed so that individual digits are spaced
just as far apart as their normal representations, in the increasing direction only. It follows that
two digits can be added/subtracted as long as we ensure that we have not generated an invalid
BCD as a result. If an invalid BCD is detected, it should be corrected, and a carry/borrow set for
use by the next pair of digits.
Multiplication/Division of BCD numbers is best done by conversion to binary and then reconversion
to BCD on completion. An alternative is to use lookup tables to generate the desired result.
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ECNG2006 (EE25M) March 11, 2008 IV 17
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ECNG2006 (EE25M) March 11, 2008 IV 18
Review Exercises
1. The working shows an attempt to perform addition on 8 bit signed numbers, using 4 bit
unsigned addition.
 1210 + 2210
 00011002 + 00101102
100011002 + 000101102
100100102
 00100102
 1810
Identify the  aw (if any) in the method shown:
(a) arithmetic error in lower nibble
(b) arithmetic error in upper nibble
(c) carry was not transferred from lower nibbles to upper nibbles
(d) incorrect representation
(e) none of the above
2. The working shows an attempt to perform subtraction using two’s complement representation.
F16 416
11112 01002
11112 + 11002
110112
 001002
 416
Identify the  aw (if any) in the method shown:
(a) arithmetic error
(b) incorrect extension of the representation
(c) incorrect interpretation of result
(d) insu cient bits used to represent the number
(e) none of the above
3. Using two’s complement arithmetic, calculate the result of the following expressions and
determine the values of the carry and over ow  ags (presuming an 8 bit data-word). Show
all working.
(a) 1010 + 1610
(b) 7A16 1016
(c)  8210 16710
(d) 2216 8416
(e) 616 2A16
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ECNG2006 (EE25M) March 11, 2008 IV 19
4. \Many CPU’s provide logic for performing arithmetic on packed decimal numbers. Although
the rules for decimal arithmetic are similar to those for binary operations, the decimal results
may require some corrections to the individual digits if binary logic is used. Consider the
decimal addition of two unsigned numbers. If each number consists of N digits, then there
are 4N bits in each number. The two numbers are to be added using a binary adder.
 Suggest a simple rule for correcting the result.
 Perform addition in this fashion on the numbers 169810 and 178610.
" (Stallings 2000; 9.1)
5. \The tens complement of the decimal number X is de ned to be 10N X, where N is the
number of digits in the number.
 Describe the use of tens complement representation to perform decimal subtraction.
 Illustrate the procedure by subtracting (0326)10 from (0736)10." (Stallings 2000; 9.2)
 What purpose does ten’s complement arithmetic serve in BCD arithmetic?
6. To perform the multiplication of two 8-bit unsigned integers, we can employ:
(a) Booth’s algorithm
(b) partial sums
(c) repeated addition
(d) all of the above
(e) none of the above
7. Booths algorithm is carried out on two 4 bit numbers. This will involve:
(a) 4 arithmetic shifts and 4 or less add/subtract operations
(b) 4 arithmetic shifts and a total of 8 add/subtract operations
(c) 4 logical shifts and 4 or less add/subtract operations
(d) 5 arithmetic shifts and a total of 4 add/subtract operations
(e) 8 logical shifts and less than 4 add/subtract operations
8. (a) Using repeated addition, determine: B16  916. Show all working.
(b) Using partial sums, determine: F16  416. Show all working.
(c) Using Booth’s algorithm, determine: F16  A16. Show all working.
9. Explain why -7/3 and 7/-3 yield the same quotient but di erent remainders.
10. Role Play/Challenge:
(a) Write programs for the PIC16F877 to perform integer multiplication using shifting, and
addition operations only!
(b) Role Play/Challenge: Suggest a method to correctly add and subtract two ASCII digits.
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ECNG2006 (EE25M) March 11, 2008 IV 20
Tutorial Exercise 5A O ine Version25 ID#
1. Explain the di erence between "carry" and "over ow". 2 marks
2. Name and explain one alternative to Booth’s algorithm for multiplication of signed numbers. 2 marks
3. Use Booth’s algorithm to determine 810  210. 6 marks
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
25Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
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ECNG2006 (EE25M) March 11, 2008 IV 21
T
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5
AUnit 12 Write the letter you have been assigned here .
The numbers assigned to each letter are: A: F16; 316 B: 216;F16 C: 216; 316 D: 816;216
Use Booth’s algorithm to multiply the numbers provided.
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ perform integer arithmetic operations using binary representations of the operands.
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
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ECNG2006 (EE25M) March 11, 2008 IV 22
13 Real number arithmetic
At the end of this unit the student will be able to:
perform  xed/ oating point arithmetic operations using binary representations of the
operands.
Any real number X may be re-expressed as
X = Xs BXE
where: Xs is the signi cand or mantissa, B is the base and XE is the exponent.
The relevant arithmetic operations on such numbers may be written as:
X Y =
 
Xs BXE YE  Ys
 
 BYE;8XE  YE
X Y = (Xs Ys) BXE+YE
X Y = (Xs Ys) BXE YE
Fixed point, same format Fixed point numbers can be interpreted as scaled integers (i.e. the
two’s complement integer multiplied by some factor; the base B = 2, the signi cand Xs is the two’s
complement integer and the exponent XE is a negative integer).
If two  xed point numbers are of the same format, it follows that standard integer addition and
subtraction will generate the correct  xed point result. Multiplication and division however will
requires correction in order to increase/reduce the factor. This can be accomplished by the use of
arithmetic shifts26 after performing the standard integer operation.
Fixed point, di erent formats Fixed point numbers of di erent formats will always be of
the same base B = 2, and must be arithmetically shifted to the same format, prior to addi-
tion/subtraction. Multiplication/division however, may be carried out with the unaltered numbers,
and the required shift performed afterwards on the result.
Floating point Arithmetic using  oating point numbers (assuming the same base) involves ma-
nipulating both the exponent and the signi cand as for  xed point notations. However, because
 oating point formats di er not only in the number of bits assigned to each component (sign, signif-
icand, exponent), but also in the order in which they appear in the representation, in the formats
used to represent each component (biased, two’s complement,  xed point), and in the assumed
base, it is not possible to manipulate  oating point numbers using standard integer arithmetic
methods. In order to perform  oating point arithmetic, the number must  rst be decomposed into
it’s components, the components manipulated separately as integer or  xed point, and then the
 oating point number is re-assembled. As a result,  oating point manipulation takes large amounts
of code/time.
26Arithmetic shifts di er from logical shifts in that the sign bit is maintained.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 IV 23
Review Exercises
1. In order to add two real numbers using  oating point representation/arithmetic, we:
(a) add the exponents, and subtract the signi cands.
(b) equate the exponents, and multiply the signi cands.
(c) equate the exponents, and then add the signi cands.
(d) equate the signi cands, and add the exponents.
(e) subtract the exponents, and add the signi cands.
2. In order to multiply two real numbers using  oating point representation/arithmetic, we:
(a) add the exponents, and multiply the signi cands.
(b) equate the exponents, and multiply the signi cands.
(c) equate the signi cands, and add the exponents.
(d) multiply the exponents, and add the signi cands.
(e) multiply the exponents, and then multiply the signi cands.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 IV 24
3. 10:45 1012 3:47 10 7 =
(a) (10:45 3:47) 1012+7
(b) (10:453:47 ) 1012+7
(c) (10:453:47 ) 1012 7
(d) (10:45 3:47) 1012 7
4.
 5:2 82
1:02 814
  1
=
(a) 5:21:02 102 14
(b) 1:025:2  82 14
(c) 5:21:02 814 2
(d) 1:025:2  814 2
5. Show how the following  xed/ oating point calculations are performed (where signi cands
are truncated to 4 decimal digits) (Stallings 2000; 8.25{8.27)
(a) 0:5566 103 + 0:7777 103
(b) 0:3344 102 + 0:8877 10 1
(c) 0:7744 10 2 0:6666 10 2
(d) 0:8844 10 2 0:2233 100
(e) (0:2255 102) (0:1234 101)
(f) (0:8833 103) (0:5555 105)
6. Perform the following  xed/ oating point calculations using addition, increment, shift, and
complement operations(show your working).
(a) 0:112310 1:5610
(b) 0:0012 + 1:012
(c) 0:1012 2112  0:112 2102
(d) 0:1012 2112  0:112 2102
7. For a fractional number F, the relative error of its representation F0 is expressed as F F0F . If
the fractional numbersA = 0:236 andB = 0:135 are represented by the computer as truncated
numbers A0 = 0:23 and B0 = 0:13; What is the relative error between the calculated value
C = A B and it’s represented value C0 ?
8. Role Play/Challenge: With regard for the ease of performing calculations and manipulations
using the representation, which representation of real numbers is to be preferred:  xed or
 oating point?Explain your answer, using sample code for the PIC16F877.
9. Role Play/Challenge: With regard for the accuracy/precision with which a non-integer num-
ber can be represented, which representation of real numbers is to be preferred:  xed or
 oating point?Explain your answer, using sample code for the PIC16F877.
10. Role Play/Challenge: Write an assembly language program for the PIC16F877, to perform
 oating point addition of a number with a sign bit, and 2 bit exponent.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 IV 25
Tutorial Exercise 5B O ine Version27 ID#
Write an assembly-language routine for the PIC16F877, which will multiply two 8-bit  xed point numbers,
where the point is located between bits 6 and 5.
 You may call any of the "standard" code snippets for multiplication of integers { there is no need to
re-write them.
 The routine should return the answer as a  xed point number with the point located between bits 6
and 5.
 On over ow the routine should return 0, with the Carry  ag set.
 Your answer should show how your routine would work with the numbers 01:1010012 and 01:0010002. 10 marks
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
27Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 IV 26
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BUnit 13 Write the letter you have been assigned here .
The values and calculations are: Q: 536.5 Z: 23.875 A:(Q*Z) B:(Q/Z) C:(Q+Z) D:(Q-Z)
What steps would you have to go through in order to perform this calculation:
1. using a  oating point representation of the numbers.
2. using a  xed point binary representation of the numbers.
Challenge: can you write a PIC16F877 routine to do so?
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ perform  xed/ oating point arithmetic operations using binary representations of the
operands.
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 IV 27
14 Data structures
At the end of this unit the student will be able to:
describe how data (and data structures) can be represented and manipulated, with ref-
erence to alignment and byte/bit ordering issues.
Multiple data-words may be used to represent a single number (e.g. 8 bit machine may have 16 bit
integers and 32 bit  oating point) or a single data-word may represent multiple digits (e.g. packed
BCD). It follows that the ordering of bits and bytes may a ect the interpretation of a particular
data structure.
The  rst issue is bit numbering: Is the lowest numbered bit presumed to be bit 0 or bit 1? Are the
bits numbered from left to right or right to left? There are four possible interpretations: 76543210,
01234567, 12345678, 87654321. We have been using the  rst by default (as bit n represents 2n
within the unsigned binary number), however the others do occur.
The second is bit ordering: Is the least signi cant bit assigned to the lowest numbered bit or the
highest numbered bit? For example, do we represent the sequence of bits for the number 0xFFA2
as 1111 1111 1010 0010 or 0100 0101 1111 1111. We have been using the  rst by default, but the
other is also valid.
A related issue is byte ordering. Both of the solutions presented above, presume that the byte and
bit orders are the same i.e. if the least signi cant bit appears  rst, the least signi cant byte appears
 rst. This is also not necessarily the case, especially where structures span multiple data words.
The issues of ordering of bits in the byte, bytes in a data structure, and data structures in arrays
or compound structures are referred to as "endian" issues.
Finally, the issue of alignment concerns cases where the data structure to be stored is smaller than
the word size (e.g. a digit in Packed BCD). In order to facilitate the e cient retrieval of such
data from memory, it is stored so that data "chunks" start on word boundaries. One example is in
storage of video data. Single lines of pixel data are stored, such that each new line starts with a
new data word. Any space at the end of the last data word is left unused.
Why are these issues important?
 Serial transfer data between computers which have di erent endian and alignment conven-
tions, will result in data being read di erently
 Reinterpretation of data (as a di erent type) will result in a di erent values being obtained
if di erent endian and alignment conventions are observed.
 Indexing into a block of memory (or lookup table) requires that the programmer know not
the inherent size of the structure, but the actual size produced after observing alignment.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 IV 28
Review Exercises
1. For the following data structures, draw the big-endian and little-endian layouts, using the format of
Figure 9.18 (see handout) and comment on the results. (Stallings 2000; 9.17)
(a) struct {
double i; // 0x1112131415161718
} s1;
(b) struct {
int i; // 0x11121314
int j; // 0x15161718
} s2;
(c) struct {
short i; // 0x1112
short j; // 0x1314
short k; // 0x1516
short l; // 0x1718
} s3;
2. Based on the above, write a small C program which determines the endianness of a machine and
reports the results. (Stallings 2000; 9.19)
3. It is possible to store a data structure which is either wider or narrower than the data word size, by
simply storing the bits of the data structure consecutively. What are the implications for retrieving
data from memory?
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 IV 29
4. Boundary alignment is the practice of starting new blocks of data in a new data-word block, regardless
of the space remaining in the previous data-word block. What possible advantage does such a scheme
o er?
5. A particular data structure contains an 10 bit unsigned integer, two 7 bit characters, and two status
 ags. Draw a diagram to show how the 26-bit data structure can be stored in consecutive 14-bit
data-words using:
(a) packing with boundary alignment (i.e. 2 data words per structure)
(b) packing with no boundary alignment (i.e. overlapping datawords)
(c) individual program words for each structure element (i.e. 5 data words per structure)
6. A certain C-struct was de ned as containing a character, an integer array of size 3, and a precision
integer number, in that order. The structure is made part of a program, which is compiled to run on
a machine. The machine in question:
 aligns it’s data on 4-byte boundaries,
 stores the most signi cant byte at the least numbered address, and
 stores the most signi cant bit in a byte at bit position 7.
Presuming that:
 a character requires 1 byte of storage space,
 an integer requires 2 bytes of storage space,
 a precision (long) requires 4 bytes of storage space;
Draw a memory layout showing how the record f‘A’(0x41), f10(0x000A),15(0x000F),299(0x012B)g,
1250(0x000004E2)g when stored at location 0x20 will appear.
7. Draw a diagram to illustrate how you would choose to store a set of 3-bit  elds on the PIC16F877.
You should justify your choice by making reference to the amount of  elds stored, and ease of stor-
age/retrieval.
8. (a) Draw a diagram to illustrate how the order of bits in a byte can be reversed in the PIC16F877
using not more than two  le registers, and the rotate operations.
(b) Write a program for the PIC16F877, which performs the algorithm you illustrated in your dia-
gram for reversing the order of bits in a byte.
9. Role Play: Describe the operation of the four di erent methods of implementing lookup tables in the
PIC16F877
(a) PCL manipulation with DT/retlw (no bank crossing)
(b) PCL manipulation with DT/retlw (bank safe)
(c) data EEPROM
(d) program EEPROM
10. Role Play/Challenge: How would you implement record storage and retrieval on the PIC16F877?
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 IV 30
Tutorial Exercise 6A O ine Version28 ID#
One hundred 3-bit  elds need to be stored in Bank 0 of the register  le in the PIC16F877.
1. Which is the most appropriate storage method? 1 mark
(a) each group of 2  elds stored in a  le register
(b) each group of 8  elds stored in 3 consecutive  le registers
(c) each  eld stored in it’s own  le register
2. Justify the choice you made in Question 1 3 marks
3. For your chosen storage method (Question 1), write a routine to retrieve the nth 3-bit  eld
where n is between 0 and 99. n is value in the working register when the routine is called.
The answer should be placed in bits 2-0 of the working register when the routine returns.
Your answer should show how your routine will work for n=57. 6 marks
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
28Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 IV 31
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AUnit 14 Write the letter you have been assigned here .
The concepts are: A: bit numbering B: bit ordering C: byte ordering D: alignment.
1. Write a sentence which explains this concept.
2. A C structure contains a character, and an integer, in that order. The structure is part of a
program. When compiled:
 records are aligned on 2-byte boundaries,
 the most signi cant byte is stored at the least numbered address, and
 the least signi cant bit in a byte is stored at bit position 0
 a character requires 1 byte, and an integer requires 2 bytes of storage space
Show how the record ‘A‘(0x41), 410 is stored starting at address 0x10. Use X to indicate the
unused bits.
bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0
0x10:
0x11:
0x12:
0x13:
0x14:
3. Use another color pen/pencil to amend the table, thus illustrating your concept.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 IV 32
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AUnit 14
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ describe how data (and data structures) can be represented and manipulated, with
reference to alignment and byte/bit ordering issues.
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 IV 33
Numbers & Data
This section has partially addressed the second learning objective for this course i.e. you should
now be able to \ illustrate how data can be represented in (and accessed from) memory".
It should be clear at this stage that there are no "hard-and-fast" rules to representation, however
you need to be aware of what the representation is, in order to make any sense of the data. Further
the rules for manipulation (arithmetic in particular) are dependent upon the chosen representation,
and  nally that the job of manipulating data can be made signi cantly easier or more di cult by
the choice of representation.
In the next section we will look at code for implementing integer arithmetic and common algo-
rithms/ data handling techniques on the PIC16 series of microcontroller.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) Introduction to microprocessors
c DECE, UWI, St. Augustine, Trinidad
original author: Feisal Mohammed; updated: March 11, 2008{ CLR
Part V
Algorithms on PIC16
So far we have been discussing assembly language programming on microprocessors in terms of
generic instructions. This section looks at implementing these manipulations on the PIC16 se-
ries of microcontrollers. It contains sample code for basic operations which have already been
discussed, how pieces of code can be compared, and the limitations of compiler generated code.
15 Basic operations
At the end of this unit the student will be able to:
implement basic programming operations (loops, swaps, lookups), integer arithmetic,
and other computation/numerical methods, in assembly language on the PIC16Cxxx
series of microcontrollers.
The code presented in this section has been gathered from a multitude of sources. It is impor-
tant to recognise that they are "snippets". They will not work by themselves; they are intended
to be used in a program, in which variables have been appropriately declared, and they have
been appropriately placed in the program banks. Thus you should exercise caution in trying
to "cut-and-paste" code. A more appropriate use of these snippets, would be to appreciate the
underlying operations, so that you can reproduce, or improve on, the algorithms in your own
code.
Standard Operations
Indirect addressing The following bit of code29 shows how to clear a section of RAM from 20h
to 2Fh using indirect addressing.
movlw 0x20 ; put the starting address into W
movwf FSR ; ...then into the FSR
Next clrf INDF ; clear the address pointed to by the FSR
incf FSR,F ; point to the next address
btfss FSR,4 ; does the FSR now contain 30h? (is bit 4 set?)
goto Next ; not set, repeat
done ...... ; execution jumps to here when bit 4 is set
29Please note that the code presented in this section uses two features of the PIC assembler which are not actually
translated into opcodes. The  rst is the use of labels to represent the addresses of the following instruction (e.g.
Next and done). The second is the use of labels for the Special Function Registers (e.g. FSR, INDF, F ). These are
pre-de ned in the include  le for the microcontroller using the EQU preprocessor keyword.
ECNG2006 (EE25M) March 11, 2008 V 2
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 3
Swapping data The case where data must be swapped between W and a File register seems
simple enough, just use a few mov instructions and a temporary RAM location. However upon
examining the instructions, it can be seen that both the movwf and the movf instructions would
require the use of the W register. That is there is no instruction that would move data between
two File registers. Therefore, coding the swap routine requires two temporary locations and six
instructions.
movwf tempA
movf data, W
movwf tempB
movf tempA, W
movwf data
movf tempB, W
An alternative routine swaps the working register, W, with a  le register, data, it uses the fact
that two exclusive ORs of the same value cancel out.
xorwf data,W
xorwf data,F
xorwf data,W
Similarly, to swap data between two  le registers where the working register contents are not
needed, the direct method requires a temporary register and 6 instructions.
movf dataX, W
movwf temp
movf dataY, W
movwf dataX
movf temp, W
movwf dataY
This can be reduced to 4 instructions, and no temporary location as follows:
movf dataX, W
subwf dataY, W
addwf dataX, F
subwf dataY, F
Subtraction The subtract instructions (subwf and sublw) work di erently than most people
expect. subwf subtracts W from the contents of the register, and sublw subtracts W from the
literal. Additionally the Carry  ag behaves as Borrow  ag when subtraction occurs i.e. if a borrow
occurs the Carry  ag is not set and vice-versa.
If you want to subtract a literal from W, it is easiest to use the addlw instruction with the two ’s
complement of the literal. For example:
addlw 0feh ; W := W - 2
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ECNG2006 (EE25M) March 11, 2008 V 4
The MicroChip assembler allows this to be written as:
addlw -2
There is no instruction to take the two’s complement of W (like the neg instruction on Motorola
processors), but because of the way the subtract instructions work you can use:
sublw 0
Comparison operations Comparing two numbers need to be treated with care because of the
non-intuitive nature of the Carry  ag.
movlw 2 ; W = 2
subwf Q,W ; W = Q - 2
btfss STATUS,C
goto Less ; Q < 2
goto Gr_eq ; Q >= 2
If Q is less than 2, there will be a borrow therefore the Carry  ag will not be set. The implementation
of the subtraction is done by forming the two’s complement of the subtrahend and then adding. If
Q is 1 then the operation looks like the following
0000 0001 ; Q = 1
1111 1110 ; two’s complement of W (= 2)
---------
0 1111 1111 ^---------------; Carry not set, i.e. borrow
If however Q is 3 then
0000 0011 ; Q = 3
1111 1110 ; two’s complement of W (= 2)
---------
1 0000 0001 ^---------------; Carry set, i.e. no borrow
If now it is required to branch to one spot for Q less than or equal to W, then the subtraction will
have to be turned around, for example
movf Q,W ; W = Q
sublw 2 ; W = 2 - W (Q)
btfss STATUS,C
goto Great ; Q > 2
goto Less_eq ; Q <= 2
When Q is equal to 3, then the operation looks like this
0000 0010 ; 2
1111 1101 ; two’s complement of W (= 3)
---------
0 1111 1111 ^---------------; Carry not set, i.e. borrow
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ECNG2006 (EE25M) March 11, 2008 V 5
and if Q is equal to 1
0000 0010 ;
1111 1111 ; two’s complement of W (= 1)
--------
1 0000 0001 ^---------------; Carry set, i.e. no borrow
The above examples illustrate that care must be taken when using the subtraction routines. It is
better to work out what actually happens than make assumptions based on other CPUs.
Comparison operations form the basis of if-then-else structures and loop termination tests.
Bit masking It is not generally necessary to perform single-bit masking operations on the PIC16
series of microcontrollers, because of the presence of the bit-test-skip instructions. If however you
need to match a multiple-bit pattern, then you will need to make and use a bit mask.
chkallset andwf MASK,W ; to extract just the bits we want
xorwf MASK,W ; to test if all masked bits are set
btfss STATUS,Z; == NOT (any masked bits are clear)
retlw 0 ; the bits were not all set
retlw 1 ; the bits were all set
chkallclr andwf MASK,W ; to test if all masked bits are clear
btfss STATUS,Z; == NOT (any masked bits were set)
retlw 0 ; the bits were not all clear
retlw 1 ; the bits were all clear
setbits iorwf MASK,W ;to set all masked bits in W
return
clearbits andwf NMASK,W ;to clear all masked bits in W
return
togglebits xorwf MASK,W ;to toggle all masked bits in W
return
setupmask movwf MASK ;to change MASK to the contents of W, and work out NMASK
comf MASK,W
movwf NMASK
movf MASK,W
return
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 6
Multi-word operations
Extending bit width 8bit  > 16bit Extension consists of copying the relevant bit into all
the positions of the new upper byte.
E8to16 movlw 255 ; make the accumulator 1111 1111
clrf Out_hi ; clear out the upper result byte
btfsc In, 7 ; if the top bit of the input byte is 1
movwf Out_hi ; put all ones in the upper result byte
movf In,W ; copy the input byte to
movwf Out_lo ; the lower result byte
return
Reducing bit width 16bit  > 8bit Reduction should only occur if the bits of the upper byte
all match the uppermost bit of the lower byte.
R16to8 movlw 255 ; make a byte with the
btfss In_lo,7 ; inverted upper bit of the lower byte
clrw
xorwf In_hi,W ; xor with the upper byte to check if all bits are the same
btfss STATUS,Z ; return an error (non-zero W) if not the same
retlw 255 ;
movf In_lo,W ; otherwise truncate and return
movwf Out
retlw 0
Decrementing a 16-bit variable Given a 16-bit variable with its low byte in countl and high
byte in counth.
Decr movf countl,F ; set the Zero flag if countl is zero
btfsc STATUS,Z ; countl zero? decrement counth (next instr)
decf counth,F
decf countl,F ; always decrement countl
Test a 16-bit variable for zero Using the same 16-bit variable as before, test it for zero.
Iszero movf countl,F ; test for zero in low byte
btfsc STATUS,Z ; if not, don’t test high byte
movf counth,F ; test high byte
btfsc STATUS,Z
goto Bothzero ; Zero? branch to some appropriate place
...
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 7
16-bit Addition A simple work around for 16-bit numbers is to simply increment one of the
numbers if the Carry is set. For example:
; This routine adds the 16-bit number in datalo and datahi, to the
; 16-bit number is resultlo and resulthi, leaving the answer in ;
resultlo and resulthi
Add16 movf datalo,W ; W = datalo
addwf resultlo,F ; resultlo = resultlo + W
movf datahi,W ; W = datahi
btfsc STATUS,C ; If no carry skip
incf resulthi,F ; If carry, add 1 to result
addwf resulthi,F ; resulthi = resulthi + W
There is a problem with this method because the Carry bit does not accurately re ect the results
of the last addition. In particular, if resulthi was 0xFF and the Carry bit was set from the low
bytes then after the last addition the Carry bit would be clear and it should be set. This means
that this method cannot be scaled up to be used with 24-bit and larger numbers.
A slightly longer, but scalable routine that correctly sets the Carry bit on termination is shown
below. You should work out the sequence of operations to ensure that you understand the routine.
Addgood movf datalo,W ; W = datalo
addwf resultlo,F ; resultlo = resultlo + W
movf datahi,W ; W = datahi
btfsc STATUS,C ; If no carry ...
incfsz resulthi,F ; if resulthi is 0 after the increment
; ... don’t do the addition
addwf resulthi,W
movwf resulthi ; resulthi = resulthi + W
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 8
16-bit Subtraction Subtraction is very similar to addition, with the inclusion of the borrow.
The only di erence to look out for is that the PIC16xxx treats the carry bit as a Borrow. Assume
that the operation required is R = Q 1 - Q 2, where Q 1 and Q 2 are two 16-bit numbers. This
can be accomplished by the following code:
Subr movf Q2_lo,W
subwf Q1_lo,W ; W = Q1_lo - Q2_lo
movwf R_lo
btfss STATUS,C ; need to borrow?
decf Q1_hi,F ; get it from hi byte
movf Q2_hi,W
subwf Q1_hi,W ; W = Q1_hi - Q2_hi
movwf R_hi
return
Note that the carry  ag at the end of the routine will not be set correctly if Q1 hi is 0. An improved
routine is as follows
Subrgd movf Q2_lo,W
subwf Q1_lo,W ; W = Q1_lo - Q2_lo
movwf R_lo
movf Q2_hi,W
btfss STATUS,C ; need to borrow?
goto Borrow
subwf Q1_hi,W ; W = Q1_hi - Q2_hi
movwf R_hi
goto Done
Borrow subwf Q1_hi,W ; W = Q1_hi - Q2_hi
movwf R_hi
decf R_hi,F ; do the decrement after
Done return
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 9
Integer Multiplication and Division
8-bit by 8-bit Unsigned Multiplication There are two ways to implement the multiplication
of two 8-bit numbers, one is by repeated addition and the other is via partial sums. The  rst
example multiplies M1 by M2 leaving the result in R hi and R lo. It loops through the addition of
M2 to itself M1 times.
; 8x8 unsigned multiply routine.
; No checks made for M1 or M2 equal to zero
clrf R_hi ; clear result location
clrf R_lo
clrw
M8x8 addwf M2,W ; add M2 to itself
btfsc STATUS,C ; if carry set
incf R_hi,F ; ... increment high byte
decfsz M1,F
goto M8x8
movwf R_lo
...
This second routine, does the same thing but loops only 8 times instead of M1 times as the previous
case. M1 is added to the result which is then shifted. It makes use of the fact that multiplying a
number by one results in the same number so all that is required is either addition if the bit, in
M2, was one or no addition if it was zero.
; 8x8 unsigned multiply routine.
; No checks made for M1 or M2 equal to zero
clrf R_hi ; Clear result location
clrf R_lo
movlw 0x08 ; setup loop count
movwf cntr
movf M1,W ; initialize W with M1
Umul rrf M2,F ; rotate into carry to check bits
btfsc STATUS,C ; if carry set i.e. bit was 1
addwf R_hi,F ; ... we add
rrf R_hi,F ; shift over for the next addition
rrf R_lo,F
decf cntr,F ; check the loop count
btfss STATUS,Z
goto Umul
...
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 10
16-bit by 8-bit Unsigned Division
; 16-bit unsigned division. Quot = (Div_hi, Div_lo) / Divsor
clrf Quot
Divide movf Divsor,W ; setup for subtraction
subwf Div_lo,F ; Div_lo = Div_lo - Divisor
btfss STATUS,C
goto Borrow
goto Div_2
Borrow movlw 0x01
subwf Div_hi,F ; use subtract instead of decf
btfss STATUS,C ; ... because it sets the carry
goto Done ; generated a borrow so finish
Div_2 incf Quot,F ; add one and loop again
goto Divide
Done ....
Booth’s algorithm: 8-bit by 8-bit signed numbers It is presumed that the result will be 16
bits wide, and that the arguments are valid.
; we wish to multiply M*Q. Result will appear in A:Q
bthloop movf Q,W
andlw 0x01
xorwf STATUS,F ; check the pair of bits
btfss STATUS,C
goto arshft
movlw M
btfsc Q,0 ; if the Q_0 bit is 1 then subtract
sub8 subwf A,F
btfss Q,0
add8 addwf A,F
arshft bcf STATUS,C
btfsc A,7
bsf STATUS,C
rrf A,F
rrf Q,F
decfsz count,F ; check if we are done
goto bthloop
....
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 11
BCD operations
Packed BCD Validity checking Adding 6 to a 4-bit nibble produces a carry out of that nibble
if the nibble is an invalid BCD code. The following returns 1 in the working register if either code
in Reg are invalid.
PBCDchk movlw 0x66
addwf Reg,W
btfsc STATUS,C
retlw 1
btfsc STATUS,DC
retlw 1
retlw 0
Packed BCD Single Digit extraction To extract a single digit from a BCD number, a bit
mask may be used:
PBCDext movlw 0xF0
andwf In,W
movwf Nibble_hi
movlw 0x0F
andwf In,W
movwf Nibble_lo
return
Packed BCD Addition The addition of BCD numbers involves consideration of the decimal
carry. The addition plus 6 to each nibble will produce a carry exactly when the decimal addition of
two digits in the operands would produce a carry, and these are the carries we are concerned with.
But the digits in positions that did not produce a carry will contain an excess 6. The problem,
then, is to remove the extra 6 from each digit that did not generate a carry. The following routine
leaves the carry set if the upper digit over ows:
PBCDadd movlw 0x66
addwf RegA,W
addwf RegB,W
rrf Out,F
btfss STATUS,DC
addlw -0x06
btfss Out,7
addlw -0x60
rlf Out,F
movwf Out
return
{ From: Scott Dattalo (http://www.piclist.com/techref/microchip/math/add/bcdp2.htm)
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ECNG2006 (EE25M) March 11, 2008 V 12
Packed BCD Subtraction Similarly to two’s complement arithmetic for binary number, sub-
traction of decimal numbers can be performed by adding the ten’s complement (subtract from all
nines and add 1) e.g.
810 210 810 1610
0810 + 9810 0810 + 8410
1 0610 9210
610  810
Note that 0 to 99 represents the numbers +49 to -50 in ten’s complement notation. The following
code produces the ten’s complement of a packed BCD number:
PBCDCmp movlw 0x99 ;
movwf Out ;
movf In, W ;
subwf Out,F ; Out=0x99-In
incf Out,F
return
Conversion Packed BCD to unsigned binary To convert from BCD to an unsigned binary,
multiply the upper nibble by ten and add the lower nibble. This can be done by shifting the upper
nibble right 3 places (multiply by 2), adding the upper nibble shifted 1 place (multiply by 8) and
adding the lower nibble.
PBCDbin rrf In,W
andlw 0x78 ; 0111 1000
movwf Out
bcf STATUS,C
rrf Out,F
rrf Out,F
addwf Out,F
movlw In
andlw 0x0F
addwf Out,F
return
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ECNG2006 (EE25M) March 11, 2008 V 13
Conversion unsigned binary to Packed BCD To convert binary to BCD, check that the 7th
bit is not used, and set the carry if the number exceeds 99(i.e. between 99 and 128).
BinPBCD btfss In,7 ; check if > 128
retlw 1 ; return an error
clrf Out
movlw 156 ; Check if the number will overflow
addwf In,W ;
rrf Out,F ; Store the Carry in Out
movlw 100 ; subtract 100 if needed
btfss Out,7 ;
subwf In,W ;
iorwf Out,F ; Store the reduced number
swapf Out,W ; sort out the bottom bits
addwf Out,W
andlw 0x0F
btfsc STATUS,DC ; correct for the DC
addlw 0x10
btfsc Out,4 ; correct for bit 4
addlw 0x06
btfsc Out,7 ; correct for the Carry stored in bit 8
sublw 0x08
addlw 6 ; make sure lower nibble is 0-9
btfss STATUS,DC
addlw -6
btfsc Out,6 ; now the upper bits
addlw 0x60 ; If the 64 bit is on store BCD 60
btfsc Out,5
addlw 0x30 ; If the 32 bit is on add BCD 30
btfsc Out,4
addlw 0x10 ; If the 16 bit is on add BCD 10
rlf Out,F
movwf Out ; restore the carry, set Out and return
return
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 14
Data Storage
MPASM recognises the following formats for numbers/ASCII characters:
Type Syntax Example
Decimal D‘<digits>’ D‘100’
Hexadecimal H‘<hex digits>’ , 0x<hex digits> H‘9f’, 0x9f
Octal O‘<octal digits>’ O‘777’
Binary B‘<binary digits>’ B‘00111001’
ASCII ‘<character>’, A‘<character>’ ‘C’, A‘C’
String \<character string>" \Cat"
Any of the above formats will automatically be translated into the appropriate binary code by the
assembler.
Data values may be stored as part of the program i.e. explicitly loaded. Where it is necessary to
assume a sequence of values, a lookup table may be implemented using a sub-routine with multiple
return values. On some occasions, it is necessary to retrieve constant values from either the program
or data ROM’s.
Table Lookup When there is a write to the PCL, for example in a table lookup, the PC is
moved to point to a particular place in memory based on some input parameter. In the following
section of code, the routine returns di erent binary numbers based on the value of the W register.
W can range from 1 to 4. The routine shown will increment the program counter with the value in
W, thus jumping to the relevant number of locations forward. The next instruction will return the
8-bit value (retlw has room for an 8-bit operand) at that position of the table.
table addwf PCL, F ; add W to the PCL and store it in the PCL
retlw B’00000001’ ; return 1 in W if W = 0
retlw B’00000011’ ; return 3 in W if W = 1
retlw B’00000110’ ; return 6 in W if W = 2
retlw B’00000010’ ; return 2 in W if W = 3
end
Please note, that this example presumes that no program bank switching is required. Since the PCL
is only 8-bits wide the table has a restriction in that it cannot cross a 256 word boundary. The
PCL and PCLATH are another pair of Special Function registers that are also assigned multiple
addresses so that access can occur regardless of the value of the RP0, RP1 bits.
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ECNG2006 (EE25M) March 11, 2008 V 15
A similar example follows where the data to be retrieved is an ASCII string.
BatLkup addwf PCL,F
retlw A‘B’
retlw A‘a’
retlw A‘t’
retlw 0x00
retlw 0x04
retlw 0x06
The DT directive may be used as a shorthand. For this directive, each value is pre xed with retlw
and stored in the program memory. Strings are stored as a sequence of ASCII character codes.
The above example could be re-written as:
BatLkup addwf PCL,F
DT "Bat",0x00,0x04,0x06
This method is ine cient because, the upper 6 bits of program memory go unused in each entry,
and an extra location is required for the addition instruction for the table.
Lookup Table data values may be stored into any location of the program memory using the DA
directive. Because the program memory is 14 bits wide, this directive will store ASCII (7 bits per
character) strings with characters packed two-to-a-word. Numeric data will be stored one value to
a word. For example:
ORG 0x20 DA "abcdef",0xFFFF,0x80
will put the following values in the program memory:
Address Value
0x20 30E2
0x21 31E4
0x23 32E6
0x24 3380
0x25 3FFF
0x26 0080
where the ASCII codes for ’a’ thru ’f’ are 0x61 thru 0x66, and the data value 0xFFFF is truncated
to  t 14 bits.
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ECNG2006 (EE25M) March 11, 2008 V 16
To read a single data word from the program memory, an indirect read must be performed (note
that this feature while available on the PIC is not normally available on Harvard architectures).
From the data sheet (PIC 2001)
The steps to reading the FLASH program memory are:
1. Write the address to EEADRH:EEADR. Make sure that the address is not larger
than the memory size of the PIC16F87X device.
2. Set the EEPGD bit to point to FLASH program memory.
3. Set the RD bit to start the read operation.
4. Execute two NOP instructions to allow the microcontroller to read out of program
memory.
5. Read the data from the EEDATH:EEDATA registers.
BSF STATUS, RP1 ;
BCF STATUS, RP0 ;Bank 2
MOVF ADDRL, W ;Write the
MOVWF EEADR ;address bytes
MOVF ADDRH,W ;for the desired
MOVWF EEADRH ;address to read
BSF STATUS, RP0 ;Bank 3
BSF EECON1, EEPGD ;Point to Program memory
BSF EECON1, RD ;Start read operation
NOP ;Required two NOPs
NOP ;
BCF STATUS, RP0 ;Bank 2
MOVF EEDATA, W ;DATAL = EEDATA
MOVWF DATAL ;
MOVF EEDATH,W ;DATAH = EEDATH
MOVWF DATAH ;
The indirect read of the EEPROM method for storing tables, while more e cient on storage space,
takes longer to access and requires more overhead code to read. It is best used for particularly
large tables/data values.
Similar methods may be used to store and read data from the Data EEPROM. The directive in
this case is DE which stores each character or value in a single byte. When using the DE directive
be sure to initialise the address to 0x2100 (programming address of the Data EEPROM).
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ECNG2006 (EE25M) March 11, 2008 V 17
Review Exercises
1. What does the following code-fragment for the PIC 16F877 do:
swapf Reg,W
andlw 0x0F
movwf Reg
(a) swap the higher and lower nibbles of Reg
(b) perform a signed division Reg by 8
(c) logical right shift Reg 4 bit-places
(d) rotate Reg 4 bit-places
2. Label the statements as True or False, in relation to the following code for the PIC16F877:
Lp movlw 255
btfss In_lo,7
clrw
xorwf In_hi,W
btfss STATUS,Z
retlw 255
movf In_lo,W
movwf Out
retlw 0
(a) This code can be initiated by the statement call Lp.
(b) This code only returns the values 0 or 255 in W.
(c) This code produces the 8-bit equivalent of the 16-bit number, only if possible, otherwise
it returns 255.
(d) This code tests the sign of the 16 bit number In, and returns 0 for positive numbers,
and 255 for negative numbers.
(e) This code uses the values In_lo, In_hi and modi es Out.
3. xorwf data,0
xorwf data
xorwf data,0
What is the function of the above code fragment for the PIC16F877?
(a) clear the working register, and the  le register data
(b) indirectly address the location whose address is in  le register data
(c) logical XOR the working register and  le register data and leave the result in data
(d) move the value from  le register data into the working register without a ecting the
 ags
(e) swap the values in the working register and  le register data
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ECNG2006 (EE25M) March 11, 2008 V 18
4. BatLkup addwf PCL,F
retlw A’B’
retlw A’a’
retlw A’t’
retlw 0x00
When the subroutine BatLkup is called with the value 2 in the working register, the value
returned in the working register is:
(a) 0x00
(b) 0x02
(c) A’B’
(d) A’a’
(e) A’t’
5. What does the following code fragment for the PIC16F877 do?
movlw 0x80
movwf FSR
Next movwf INDF
incf FSR,F
btfss FSR,6
goto Next
(a) initialises memory locations 0x20 to 0x2F with the value 0x00
(b) initialises memory locations 0x80 to 0xBF with the value 0x80
(c) initialises memory locations 0x80 to 0xFF with the value 0x80
(d) initialises memory locations 0x80 to 0xBF with the value 0x00
(e) initialises memory locations 0x80 to 0xFF with the value 0x00
6. my_sub movlw 0x66
addwf Reg,W
btfsc STATUS,C
retlw 1
btfsc STATUS,DC
retlw 1
retlw 0
Label the statements as True or false. The above piece of code:
(a) adds 0x66 to the contents of the working register.
(b) returns 1 if there is a carry or digit carry.
(c) returns with 0x66 in the working register.
(d) should be run with the instruction goto my_sub.
(e) will be useful for checking the validity of BCD numbers.
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ECNG2006 (EE25M) March 11, 2008 V 19
7. Basic programming practice:
(a) Write a piece of assembler code to branch to program location 1010h, presuming that
the location is outside of the current program bank. (Note: the relevant bits must be
set in PCLATH before the branch).
(b) The movf instruction copies a byte from a register to the accumulator and a ects the
Zero  ag. Write a piece of assembler code which will copy a byte from a register to
accumulator and does not a ect any of the status bits. (Hint: use swapf).
(c) The notes include a code example which clears all memory locations between 20h and
30h. Write a similar piece of code which will set the values of memory locations 40h to
60h with the value that was in the accumulator at the start of the code.
(d) Role Play/Challenge: The parity of a byte is even ’0’ if there is an even number of bits
set, and odd ’1’ if there is an odd number of bits set. Write a piece of assembler code
that will  nd the parity of a byte in a register, and leave the result in another register.
8. Lookup table/Indirect addressing practice:
(a) Explain the operation of the indirect addressing mechanism on the PIC16F877 using the
code snippet provided in the notes.
(b) The notes contain example code for a lookup table using the DT directive, which pre-
sumes that there was no need to switch banks within the table. Write similar code which
will operate correctly even when the table spans a program bank boundary.
(c) Write a program containing a lookup table for multiplication by 7 in the data EEPROM.
The lookup table should be accessed with a subroutine called mul7 which will take the
value in the working register as it’s argument, and return the answer in the working
register. Assume that mul7 is only called with inputs from 0 to 15.
(d) Role Play/Challenge:
i. One of the drawbacks of the Harvard architecture is that instructions cannot directly
execute reads of program memory. How does this a ect the implementation of
lookup tables, and the performance of lookups?
ii. The PIC16F877 allows an indirect read of the program memory to be performed.
Describe the mechanism used, using an assembler code example.
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ECNG2006 (EE25M) March 11, 2008 V 20
9. Representation & Mathematical practice:
(a) BCD is not the only means of representing decimal numbers. The ASCII30 character set
de nes a representation of characters as 7 bit numbers. The digit characters 0 through
9 are represented as 011 0000 thru 011 1001. Using this information write routines to
correctly add and subtract two ASCII digits. (Note: The carry  ag should be set for
over ow, or no borrow respectively)
(b) Presuming a two’s complement 8-bit integer representation, write an algorithm and
implement it in PIC assembly language to count the number of negative integers in a
set of twenty numbers starting at address 30h. [Past Paper 2001, No. 1(c)]
(c) Write an algorithm and implement it in PIC assembly language to subtract two 24-bit
numbers, Q 1 and Q 2, and store the result in R. Ensure that the carry  ag re ects the
result of the subtraction. [Past Paper 2001, No. 2]
(d) Write code to decrement a 16-bit variable and test the result for zero, branching to
Bothzero if the result is zero. Can this be done using fewer lines of code and/or faster
execution than simply concatenating the two routines shown previously?
(e) Design an algorithm and implement it in PIC assembly language to perform unsigned
division of a 24-bit number by an 8-bit number and produce a 16-bit quotient and 8-bit
remainder.
(f) Design an algorithm that works more e ciently than repeated subtraction for unsigned
8-bit by 8-bit division. Implement it in PIC assembly language.
(g) Role Play/Challenge: Implement Booth’s algorithm for two signed 8 bit numbers in PIC
assembly language. It is presumed that the result will be 16 bits wide.
(h) Role Play/Challenge: Using PIC assembly, implement Booth’s algorithm for the multi-
plication of two 16-bit signed integers.
(i) Role Play/Challenge: Design a Booth’s-like algorithm for the division of a signed 16 bit
number by a signed 8 bit number, and implement it in PIC assembly language.
30American Standard Code for Information Interchange
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ECNG2006 (EE25M) March 11, 2008 V 21
10. Data Structure practice
(a) Role Play: It is necessary to implement a stack in software using indirect addressing.
It must be 16 bytes long and can be located in any part of the GPR space. A variable
stkptr is allocated as a pointer to the stack.
i. When the microcontroller is reset, does stkptr need to be initialized? If so, what
should its initial value be?
ii. Assuming that the stack is never popped when empty, is it necessary that it be
cleared when initialized?
iii. Assuming that more than 16 items are never pushed onto the stack, write an algo-
rithm for the push routine and implement it in PIC assembly language.
iv. Do the same for the above for the pop routine.
(b) Role Play: Another type of data structure is the queue i.e. a  rst-in,  rst-out data
structure. The size of the queue in RAM is to be 16 bytes and an additional three bytes
(variables) are allocated for tracking the data in the queue. The three variables are
inptr Points to the next available input location.
outptr Points to the next available output location.
qcount Number of bytes of data in the queue.
i. When the CPU is reset, how should each of the 19 bytes be initialized, if at all?
ii. Assume , simplistically, that more than 16 bytes will never be stored in the queue.
Write an algorithm for the put routine that puts a byte of data on the queue and
implement it in assembly language.
iii. Write and implement an algorithm for the get routine that retrieves a byte of data
from the queue. It may be assumed that a get will never be executed unless qcount
is non-zero.
iv. In the queue put routine, add additional functionality that checks if the end of the
queue has been reached and \wraps" the input and output pointers back to the
beginning of the queue.
v. In the queue get routine, add a check to prevent retrieval of data if the queue is
empty.
vi. In the queue put routine, add a check to prevent insertion of data if the queue is
full.
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ECNG2006 (EE25M) March 11, 2008 V 22
Lab 2
ID#
You should complete the pre-lab before coming to your lab session. Your Teaching Assistant/Demonstrator
may refuse to allow you into your lab session if your pre-lab is incomplete, or if you are unduly
late.
You will have 3 hours in the lab to complete the exercises. Answers should be written on this lab-
sheet in PEN. Please do not attach any extraneous pieces of paper unless SPECIFICALLY asked
to do so.
Please bring your PIC16F877 datasheet book (provided in ECNG2006) to the lab with
you. You may need to refer to it in order to complete the pre-lab and lab exercises.
Your lab submissions will be checked for unwarranted collusion, and unreferenced use
of Intenet-available/other resources.
At the end of this unit the student will be able to:
implement basic programming operations (loops, swaps, lookups), integer arithmetic,
and other computation/numerical methods, in assembly language on the PIC16Cxxx
series of microcontrollers.
Pre-Lab
1. Please identify
 your lab group letter (E,F,G,H):
 your ECNG2006 uP group designator (e.g. A3):
2. Review basic operations/instructions for the PIC16F877 and answer the following questions
about the PIC16F877:
(a) What is a  ag? 1 mark
(b) Where are the  ags stored? 1 mark
(c) Give ONE example of when the Carry  ag is cleared for an addition instruction. 1 mark
(d) Give ONE example of when the Carry  ag is set for a subtraction instruction. 1 mark
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ECNG2006 (EE25M) March 11, 2008 V 23
(e) Identify another arithmetic  ag (apart from the carry  ag) and give ONE example of
when it is either set(Lab Groups E,F) OR cleared (Lab Groups G,H) by an addition
instruction. 2 marks
(f) Use an example to explain the behaviour of your assigned conditional instruction:
Lab Group E: btfss, Lab Group F: btfsc, Lab Group G: decfsz, Lab Group H: incfsz.
Your answer should state the conditions which exist BEFORE the instruction is executed,
the operands of the instruction, and the conditions which exist AFTER the instruction
is executed. 3 marks
(g) Use the number 2516 to illustrate the di erence between the swapf, rlf, and rrf in-
structions and explain how these instructions can be used to implement multiplication
and division by powers of 2. 4 marks
(h) Use the number 2516 and the bit mask 000000012 to illustrate the di erence between the
andlw, iorlw, and xorlw instructions and explain how these instructions can be used
to extract and/or alter the state of a single bit (without a ecting the state of other bits
in the register). 4 marks
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ECNG2006 (EE25M) March 11, 2008 V 24
3. Review the basic functionality of the MPASM assembler (use the manual/help  les), paying
particular attention to the topics discussed below:
(a) What is a directive? Is it the same as an instruction? Explain your answer in your own
words. 1 mark
(b) Di erentiate between the EQU and ORG directives, in your own words. 1 mark
(c) In your own words, explain why 8-bit registers cannot be used to perform two’s comple-
ment arithmetic with 8-bit numbers. Use an example for clarity. 1 mark
(d) Does MPASM automatically do two’s complement arithmetic? Explain your answer. 1 mark
(e) MPASM is able to recognize numbers written in several di erent bases. Illustrate how
the number equivalent to 279 would be written in an MPASM assembly language pro-
gram using the base you have been assigned:
Lab Grp E: base-10, Lab Grp F: base-16, Lab Grp G: base-2, Lab Grp H : base-8. 1 mark
(f) How does MPASM interpret a number whose base is not speci ed? 1 mark
(g) C compilers allow the programmer to re-use variable-names inside a subroutine, with-
out interfering with data values of the same name held outside the subroutine. Does
MPASM similarly allow us to have two variables of the same name? Explain your answer. 1 mark
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ECNG2006 (EE25M) March 11, 2008 V 25
4. In lab 1, you were provided with C code in looplogm.c and assembly language code asmlogm.asm
which each calculated the base-n logarithm of a number, by repeatedly comparing the number
with an increasing power of n. The code is shown on pages 26 and 25.
(a) The following questions about the C language code should be answered by annotating
the code supplied on page 25.
i. Circle places in the C code where there are loops and function calls. 2 marks
ii. Underline and label, C language instruction(s) used to implement loop initalization,
loop completion testing, value change for loop variable, and loop repetition. 4 marks
iii. Underline and label, C language instruction(s) used to de ne, call, and return from
a function? 3 marks
iv. Draw arrows to indicate the values to be changed if we wanted to change the loga-
rithm being calculated from log3 81 to log5 25. 2 marks
v. How many C-language instructions are in the program? 1 mark
vi. Can the range(s) of data memory locations being used be determined from the source
code? If so, what are they? 1 mark
(b) The following questions about the assembly language code should be answered by anno-
tating the code supplied on page 26.
i. Circle places in the assembly language code where there are loops and function calls. 2 marks
ii. Underline and label, assembly language instruction(s) used to implement loop inital-
ization, loop completion testing, value change for loop variable, and loop repetition? 4 marks
iii. Underline and label, assembly language instruction(s), MPASM directive(s) and
MPASM labels used to de ne, call and return from a function. 3 marks
iv. Draw arrows to indicate the values to be changed if we wanted to change the loga-
rithm being calculated from log3 81 to log5 25. 2 marks
v. How many assembly language instructions are in the program? 1 mark
vi. Can the range(s) of data memory locations being used be determined from the source
code? If so, what are they? 1 mark
unsigned char logn(unsigned char x, unsigned char n)
f
unsigned char i,j;
j = 0;
if(x==0) return 0xFF; // error - 0 has no logarithm
for (i=n; i<=x; i=i*n) j++; // keep comparing with increasing powers
return j;
g
int main()
f
unsigned char a,b, answer;
a = 81;
b = 3;
answer = logn(a,b);
g
Figure 6: looplogm.c
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INCLUDE <P16f877.inc>
LIST p=16f877
ORG 0x00
Num1 EQU 0x51
Num2 EQU 0x03
X EQU 0x20
N EQU 0x21
j EQU 0x22
i EQU 0x23
i2 EQU 0x27
count EQU 0x24
check EQU 0x25
check2 EQU 0x26
Answer EQU 0x28
ORG 0x20
main
movlw Num1
movwf X
movlw Num2
movwf N
call log
movwf Answer
sleep
log movf X,F
btfsc STATUS,Z
retlw 0xFF
clrf j
movf N,W
movwf i
compare subwf X,W
btfss STATUS,C
goto stop
incf j,F
next movlw 0x04
movwf check
clrf i2
movf N,W
movwf check2
movf i,W
movwf count
next2 btfsc check2,0
addwf i2,F
bcf STATUS,C
rlf count, F
movf count, W
bcf STATUS, C
rrf check2, F
decfsz check, F
goto next2
movf i2,W
movwf i
goto compare
stop movf j,W
return
END
Figure 7: asmlogm.asm
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5. (Bonus: 5 marks) Algorithms are descriptions of how a function can be achieved. The
algorithm used in the code provided on pages 26 and 25 is based on the principle of comparing
the number to an incrementally increasing exponent of the base. This is not the only possible
algorithm.
Groups A1,B1,C1,D1,C4 Borcharts algorithm
(e.g. http://www.dattalo.com/technical/theory/logs.html)
Groups A2,B2,C2,D2,B4 Exploits  oating point representation
(e.g http://wehner.org/fpoint/)
Groups A3,B3,C3,D3,A4,D4 Uses a lookup-table
(e.g. http://www.dattalo.com/technical/software/pic/piclog.html)
You were previously asked to read/describe the alternative algorithm you have been assigned
in Lab 1. You should now implement it using the C language and build the code for the
PIC16F877, using the techniques you practiced during Lab 1. Use the calculation of log3 81
log5 25 to test your code.
Write down your initial try at the C language routine in the space below. Please use appro-
priate comments.
Bring an electronic copy of the  le to the lab. Filename is L2Chlnge.c. The  rst line of the
 le should contain a comment with your name and ID #.
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Helpsheet: Writing and Testing Assembly Language code
When writing assembly language code, try to follow the following \Code formatting guidelines"
 labels in  rst column;
 code in second column;
 operands (where appropriate) in 3rd column;
 brief comments (where appropriate) in the 4th column
 directives in capital letters;
 assembly language in common letters;
 appropriate variable names/labels lower case (optional  rst capital letter).
 separate routines with a blank line
What is the purpose of these guidelines? Assembly language is inherently cryptic, and di cult
to read. Following the guidelines \strange" instructions, or directives (e.g lacking an operand or
placed in the wrong order) will be more easily spotted. This makes it easier to trouble shoot and
maintain the code.
In keeping with the need to easily maintain your code, you should choose your variable (register)
labels with care. It is a lot easier to  nd loops whose loop variables are named loopvari than
those named temp, myvar or fred.
You will notice that brief comments should be included where appropriate. If you have chosen
variable (register) labels appropriately, most code lines will not need explanation. You should use
comments to explain the general intent of the program rather than the speci cs of a particular
instruction. Remember that at a later time, you will still be able to  gure out what the instruction
does, but you will have no idea why you placed it there. For example: andlw 0x0F ; bit-wise AND
with 0x0F does not express intent, but andlw 0x0F ; extract lower nibble has clear meaning.
Using comments in this way has the advantage that you can spot errors if the comment does not
match the instruction { for example: andlw 0xF0 ; extract lower nibble clearly needs to be changed
whereas andlw 0xF0 ; bit-wise AND with 0xF0 will not help.
Finally, when you are using test cases for your code, de ne the test values using labels at the top of
the program, then use the label throughout. This way you can change the test cases in one place,
without having to search for all the places that need to be changed.
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In the lab
Before you start, please ensure that the computer is set up to show  le extensions:
 Double click on My Computer then select the following from the menu(s) and pop-up windows:
Tools -- Folder Options -- View.
 Ensure that the checkbox labelled Hide file extensions is clear.
 Click OK.
Next, create a directory on the D: drive . This is the directory you will work with during the lab.
If you create this directory on the D: drive, please delete it at the end of the lab (you can copy it
to your Z: drive or a key  rst!) :
 In the My Computer window, double-click on the icon for the drive.
 Right click and select New Folder.
 Name the directory uPxxxxxxxx where xxxxxxxx is your ID number.
During the lab you should create subdirectories for each question. The subdirectories should be
named according to the lab and the question for example: Lab1Q1
Next, open MPLAB IDE. You will be asked to create new projects throughout this lab.
In Lab 1 you learnt to create a C language project. Refer to those instructions if needed.
To create an assembly language project:
 Click PROJECT on the menu bar and select PROJECT WIZARD. In the PROJECT
WIZARD box click NEXT. From the DEVICE drop down menu select PIC16F877 and
click NEXT. Select MPASM or GPASM from the ACTIVE TOOLSUITE dropdown
box and then click NEXT.
 Enter an appropriate name, for example \Lab1Q1" in the box named PROJECT NAME.
Then for the PROJECT DIRECTORY box, browse and  nd the sub-directory you created
for the question and click NEXT.
 A window will now come up with the buttons ADD and REMOVE. If you have already
typed and/or saved your assembly language  le, you may add your  le. If not then you will
need to crate it and add it to the project later. Click FINISH in the new window.
 Go to the menu bar and select DEBUGGER > SELECT TOOL > MPLAB SIM. This
means that you will be executing your code on the simulator.
 To display the Program Memory click on the VIEW > PROGRAM MEMORY menu
item.
Electronic copies of the code in this script are provided. Please note that there is no guarantee
that the code will operate as required. Troubleshooting code is a skill which you are expected to
use during this lab.
Call your TA or lecturer for assistance if you are stuck at any time.
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PIC Basics
1. This question will investigate how to test if the value in a certain register is zero or not and
how it may be used to our advantage. Create a new assembly language project. Use the
MPSIM simulator windows to examine the data memory and the instruction memory. Build
and step through the code below, using the MPLAB simulator:
LIST p=16f877
INCLUDE <P16F877.inc>
q EQU 0x29
ORG 0x00
goto main
ORG 0x30
main movlw 24
movwf q
loop movf q,F
btfsc STATUS,Z
sleep
goto loop
END
(a) Use the simulator windows to identify ONE di erence between the data and the instruc-
tion memories. 1 mark
(b) What location in the data memory is the variable q stored at? Suggest an alternative
and explain your choice. 2 marks
(c) What location in the instruction memory is the instruction indicated by the label ’main’
stored at? Suggest an alternate value and explain your choice. 2 marks
(d) Did the Zero (Z)  ag change in response to movlw or movwf instructions? Explain why. 2 marks
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(e) Was the Z  ag set(1) or reset(0) by the movf instruction in the program? Explain why. 1 mark
(f) Describe how the registers,  ags and program counter change when you step through
the program. 1 mark
(g) Replace the line movlw 24 with movlw 0. Run the program again. Was the Z  ag set
or reset by the movf instruction? 1 mark
(h) Describe how the registers,  ags and program counter change when you step through
the program. Has anything changed? 1 mark
(i) Change the line btfsc STATUS,Z to btfss STATUS,Z and explain why the program be-
haves the way it does. 1 mark
(j) Change the line btfss STATUS,Z to btfss q,Z and explain why the program behaves
the way it does. 1 mark
(k) Is the value speci ed on the line movlw 24 a decimal value, or a hexadecimal value?
Explain how you checked this. 1 mark
(l) How would you write the instruction to ensure the assembler interprets the value as a
hexadecimal number? 1 mark
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Basic Addition and Subtraction
2. This question will investigate how addition & subtraction instructions are performed. We
will begin by attempting to add to simple unsigned 8 bit numbers. Let us try to add the
numbers 1916 and 1716 and store the result in a register.
Firstly perform the addition manually in the space given. 1 mark
In the code shown below, choose appropriate values for q EQU and ORG . 2 marks
Choose values which are di erent to those given in the previous question.
Create a new assembly language project. Enter the code, and step through it in the MPLAB
simulator. Use watch windows to observe the working register, and the variable register q.
Make sure you understand what the code does.
LIST p=16f877
INCLUDE <P16F877.inc>
q EQU _______
ORG 0x00
goto main
ORG _______
main movlw 17
movwf q
movlw 19
addwf q,0
sleep
END
(a) Explain how you chose an appropriate value for q. 1 mark
(b) Explain how you chose an appropriate value for the ORG statement. 1 mark
(c) In the line addwf q,0, what does the "0" mean? 1 mark
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(d) Run the program and check the result of the addition. Is the value what you expected?
If not why not? How do you  x the program so it behaves as expected?
(e) Change the line addwf q,0 to addwf q,1. How does this a ect the program behaviour? 1 mark
(f) Replace the addwf instruction with subwf, step through the code, and examine what
happens. How does this a ect the program behaviour? 2 marks
(g) Underline the correct de nition of subwf.
((value in w) - (value in f)) ((value in f) - (value in w))
and explain what w and f are.
2 marks
(h) Replace the subwf instruction with sublw, step through the code, and examine what
happens. How does this a ect the program behaviour? 2 marks
(i) Replace the sublw instruction with addlw, step through the code, and examine what
happens. How does this a ect the program behaviour? 2 marks
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Arithmetic routines
3. Two’s complement arithmetic can be used to perform subtraction, by adding the two’s com-
plement representations of
 the minuend and
 the negated subtrahend
(a) The following code will convert the 7 bit number in q to the two’s complement repre-
sentation of q’s negated value:
LIST p=16f877
INCLUDE <P16F877.inc>
q EQU _______
ORG 0x00
goto main
ORG _______
main movlw _______
movwf q
movf q,W
sublw 0xFF
addlw 0x01
movwf q
sleep
END
Fill in the blanks in the code, create a new assembly language project, enter the code,
build the project, and step through the program to see the  nal value of q. Use the
space at the side below to verify that the answer the program delivers is correct.
Demonstrate your program to the lecturer/TA who will sign & mark your e ort. 2 marks
(b) Let us move our code into a short routine called tcmp which will take the 7-bit value
contained in the working register, and return with the 8-bit two’s complement represen-
tation of the negated value. Fill in appropriate comments. 4 marks
tcmp andlw 0x7F ;____________________________________________________
sublw 0xFF ;____________________________________________________
addlw 0x01 ;____________________________________________________
return ;____________________________________________________
(c) Let us write a program which will perform subtraction by calling the subroutine tcmp.
The program must start with:
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LIST ________________
INCLUDE ________________
Minud EQU ________________
Subtd EQU ________________
ORG _______
goto _______
ORG _______
______ movlw Subtd
call tcmp
addlw Minud
sleep
; place tcmp subroutine here!
END
Fill in the blanks in the code. Create a new assembly language project, enter the
code, build and step through the program to see the  nal value in the working register.
Demonstrate your program to the lecturer/TA who will sign & mark your e ort. 2 marks
(d) tcmp is a routine which ends with a return instruction. Try using goto tcmp to call
the routine. Did it cause a problem? Explain your answer. 2 marks
(e) You were told to place the routine AFTER the sleep instruction, but before the END
directive. Play around with other places for the routine. Could placing the routine
incorrectly cause a problem? Explain your answer. 2 marks
(f) In this program, are Subtd and Minud used as literal (values) or register addresses? Can
the EQU directive only be used to \name" registers? Explain your answer. 2 marks
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4. Now we shall investigate unsigned 8-bit by unsigned 8-bit multiplication. When an 8-bit by
8-bit multiplication is performed, the result can potentially  ll 16 bits. Therefore the answer
must be stored in two separate registers. The code below will multiply two numbers M1
and M2. The results will be stored in RESHIGH and RESLOW. This code is based on the
principle of repeated addition, and therefore involves a loop.
LIST p=16f877
INCLUDE <P16F877.inc>
RESHIGH EQU 0x56
RESLOW EQU 0x55
M1 EQU 0x0F
M2 EQU 0xA9
ORG 0x00
goto main
ORG 0x20
main clrf RESHIGH
clrf RESLOW
movlw M1
movwf RESLOW
movlw M2
loop decf RESLOW,F
movf RESLOW,F
btfsc STATUS,Z
goto finish
addlw M2
btfsc STATUS,C
incf RESHIGH,F
goto loop
finish movwf RESLOW
sleep
END
(a) What values are being multiplied? and 1 mark
(b) Circle the loop in the code 1 mark
(c) Write in appropriate comments for the code 2 marks
(d) Create a new assembly language project, enter the code, build and step through the pro-
gram. What values ultimately end up in RESHIGH and RESLOW 1 mark
(e) Does your code work? Yes or No (circle) If No, explain why your code does not work.
(f) Are there any values for which your program will fail? How did you  nd these values(if
any), or ensure that there were none? 1 mark
Demonstrate your  ndings to the lecturer/TA who will sign & mark your e ort.
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Binary Coded Decimal (BCD) Validity Testing. Bonus 10 marks
5. (a) BCD uses a 4 bit number to represent the digits 0 (00002)through 9(10012). Any number
higher than 9 will be considered an invalid BCD digit. Fill out the table below:
Binary Nibble Decimal Equivalent Valid BCD digit?
0011 3 Valid
1101 13 Invalid
0001
1010
(b) We already know that the carry  ag is set when an operation causes the result byte to
over ow. Similarly, the digit carry  ag is set when an operation cause the lower nibble
of the result byte to over ow into the upper nibble. The name, digit carry, stems from
the usefulness of this  ag in the manipulation of BCD numbers.
The highest valid BCD digit is 10012 (9) . What is the smallest number that can be
added to 10102 (10) that will cause the DC  ag to be set?
Express this number in BINARY and DECIMAL .
(c) The return with value retlw instruction places the operand into the working register
BEFORE executing a normal return. Just like the regular return it will be the last
statement executed in a routine, but a single routine can have multiple places at which it
return’s. Which return or retlw is used depends on the order in which the instructions
within the routine are executed, and the values in the registers at the time the routine
is executed.
The computed-goto operates by using an arithmetic operation whose destination is the
program counter e.g. iorwf PCL,F, addwf PCL,F. In this way we change the address
of the next instruction that will be executed. The exact value of the new instruction
address will depend upon the value in the working register at the time of execution.
The retlw and addwf PCL,F instructions are often combined to form a lookup table.
The lookup table will return a value from a list of values, depending on the index passed
in the working register. The  rst item in the list will correspond to index value 0. To
make a lookup table, you must identify an appropriate answer for EVERY possible value
of the index. Masking is often used to ensure that invalid index values are not used.
The 5 times table for a 2-bit index could be implemented as:
times5 andlw _____________
lkup addwf PCL, ________
retlw d’0’
retlw _____________
retlw _____________
retlw d’15’
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(d) The routines BCDChkA and BCDChkB will check if the lower nibble of the register Reg
contains a valid BCD digit. If it does, the working register is set to 0, otherwise the
working register is set to 1. The implementations of the routines however are di erent.
One uses the digit carry-test, and the other uses a lookup table. Create a new assembly
language project, and a main program which can be used to compare the two versions.
i. Did your program compile and work straight away? If it didn’t what kinds of errors
did you have to  x in order to get it to work? How did you track down these errors?
ii. Choose two test cases and run them using each version. Did your program give the
correct answer for each test case? If it didn’t what kinds of errors did you have to
 x in order to get it to work? How did you track down these errors?
iii. Print the  nal version of the main program and attach it to this script. Record your
comparison of the two routines in the space below.
Units instruction-words Case A cycles Case B cycles
BCDChkA
BCDChkB
Table 2: BCDChk Performance comparison results
Demonstrate your program to the lecturer/TA who will sign & note on your printout,
whether the program compiled, whether it worked on each test case, and whether
you can explain how each routine works.
BCDchkA movlw 0x0F
andwf Reg,W
movlw 0x06
addwf Reg,W
btfsc STATUS,DC
retlw 1
retlw 0
BCDChkB movlw 0x0F
andwf Reg,W
lkup addwf PCL,F
retlw 0
retlw 0
retlw 0
retlw 0
retlw 0
retlw 0
retlw 0
retlw 0
retlw 0
retlw 0
retlw 1
retlw 1
retlw 1
retlw 1
retlw 1
retlw 1
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Logarithms again ....Bonus: 10 marks
6. Create a new C language project, enter, build and test the C language code you wrote for
your pre-lab exercise, using the MPSIM simulator.
(a) Did your program compile and work straight away? If it didn’t what kinds of errors did
you have to  x in order to get it to work? How did you track down these errors?
(b) Did your program give the correct answer for each test case? If it didn’t what kinds of
errors did you have to  x in order to get it to work? How did you track down these errors?
(c) Print the  nal version of the code and attach it to this script. Demonstrate your program
to the lecturer/TA who will sign & note on your printout, whether the program com-
piled, whether it worked on each test case, and whether you can explain the underlying
algorithm.
(d) Compare your code to previous programs supplied by taking appropriate measurements
to complete the following table, which is based on the same performance criteria used in
Lab 1. Indicate which Build Tool (GPASM/MPASM/SDCC/CCSC) was used in each
case:
Item Build Tool I II III IV V
Units bytes instruction-words data-words cycles cycles
looplogm.c
asmlogm.asm
L2Chlnge.c
Table 3: Logarithms { Performance comparison results
(e) Use your data to identify the \best" performing implementation. How did the language,
the build tool, and the algorithm implemented, contribute to make this the \best"?
Total marks 100. This exercise is worth 5% of your ECNG2005 lab mark.
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
 assistance in troubleshooting circuitry and/or using/operating the IDE/debugger
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
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L
A
B
2Unit 15 Write the number you have been assigned here .
Answer the following questions for the TARGET platform whose number you were assigned.
1. What is the name of the target platform? .
2. What are the key architectural features of the target platform? .
3. List all available addition and subtraction instructions.
4. List all call/return/branch instructions.
5. Write a routine to  nd the two’s complement of a number that is in a speci ed register when
the routine is called. The answer should be in the same register at the end of the routine.
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this lab exercise.
{ utilize the MPLAB IDE and CCS compiler, to develop software for the MicroChip
PIC16Cxxx series of microcontroller, in C, C++ and assembly language.
{ implement basic programming operations (loops, swaps, lookups), integer arithmetic,
and other computation/numerical methods, in assembly language on the PIC16Cxxx
series of microcontrollers.
 Which aspect of this lab exercise did you have the most di culty understanding?
 Which aspect of this lab exercise did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this lab exercise.
 Identify one way in which this lab exercise could be improved.
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16 Code comparison
At the end of this unit the student will be able to:
contrast alternate programming techniques in terms of size/speed of the code produced
for the PIC16Cxxx series of microcontrollers.
When assessing assembly language code, there are three major considerations: is it syntactically
correct? ;is it logically correct? ;is it optimally sized/fast?
Syntax errors are generally picked up by the assembler and reported with a message indicating what
the assembler "thinks" the problem is. These messages while useful, can sometimes be misleading;
remember that they re ect the most likely cause of the error, but not the actual cause. For
example, placing a label on the same line as the BANKSEL pre-processor directive causes MPASM
to erroneously report that the label is not de ned.
Logical errors occur when the program is written to do something that you didn’t intend to do.
Typical Assembly Language bugs include:
 Improper transfer to subroutines
 Using subroutines that wipe out registers
 Transposed commands
 Not initializing registers [or checking initial conditions]
 Modifying condition code before conditional branch
 Using the wrong conditional instruction
 Not stopping the program properly
 The use/change of "magic" numbers
{ summarized from (Cady 1997)
Two pieces of code for the same processor may be compared in terms of the number of program
memory locations needed to store the code, the number of data memory or register locations
required to store constants/variables, and the time the code takes to execute. For simple processors
with a  xed pipeline recovery policy (like the PIC16F877), we can perform rudimentary performance
prediction by counting instructions (and hence instruction cycles). Very often performance of code
can be improved, simply by making a few simple adjustments. For example, when testing for the
end of a loop, there are many failures of the loop tests, and only one success. Therefore, and
improvement in the code in the failure branch will have a greater impact on program execution
time.
Review Exercises
Questions 1{5 refer to the following code snippets:
Snippet A Snippet B
---------- --------------
movwf tmpA
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movf data,W
xorwf data,W movwf tmpB
xorwf data,F movf tmpA,W
xorwf data,W movwf data
return movf tmpB,W
return
1. What do these snippets do?
(a) swap the contents of data and tmpA
(b) apply the bit mask in W to data
(c) swap the upper and lower nibbles of data
(d) swap the contents of data and W
(e) invert all the bits of data
2. The number of instruction cycles these snippets will take to run (disregarding the incoming
branch/call instruction) are:
(a) A: 4 B: 7
(b) A: 4 B: 8
(c) A: 5 B: 8
(d) A: 5 B: 7
3. The number of  le registers required by each snippet:
(a) A: 2 B: 4
(b) A: 1 B: 3
(c) A: 1 B: 4
4. The instruction storage required by each snippet:
(a) A: 3 B: 6
(b) A: 7 B: 4
(c) A: 4 B: 7
5. The  ags a ected by each snippet are:
(a) A: Zero  ag only B: Zero  ag only
(b) A: none B: Zero  ag only
(c) A: Zero  ag only B: none
Questions 6{8 refer to the following code snippets:
Snippet A Snippet B
--------------- --------------
movf datalo,W movf datalo,W
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addwf resultlo,F addwf resultlo,F
movf datahi,W movf datahi,W
btfsc STATUS,C btfsc STATUS,C
incf resulthi,F incfsz resulthi,F
addwf resulthi,F addwf resulthi,W
return movwf resulthi
return
6. Label the following statements as True or False.
(a) Snippet A will always take 8 instruction cycles to run; Snippet B will always take 9
instruction cycles
(b) These snippets perform 16 bit addition of datahi:datalo to resulthi:resultlo, and
leave the answer in the working register
(c) Snippet A may leave the carry  ag in a state which does not re ect the 16 bit addition.
(d) Both snippets use the same number of data registers.
(e) Both snippets return with the low byte of the result in the working register.
7. The number of program memory locations required by each snippet:
(a) A: 4 B: 4
(b) A: 7 B: 8
(c) A: 8 B: 7
(d) A: 8 B: 9
(e) A: 9 B: 8
8. The  ags a ected by each snippet are:
(a) A: Carry Flag only B: Carry Flag only
(b) A: Carry  ag only B: Zero and Carry  ags only
(c) A: Zero and Carry  ags only B: Zero and Carry Flags only
(d) A: Zero, Carry and Digit Carry Flags B: Zero, Carry and Digit Carry Flags
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 45
9. The following code changes the sign of Reg, so that it is always interpreted as a positive
number in the two’s complement representation scheme. How many cycles does the following
code for the PIC16F877 take for negative and positive numbers respectively?
movf Reg,W
btfss Reg,7
goto Done
sublw 0
Done movwf Reg
(a) 7 cycles; 5 cycles
(b) 7 cycles; 4 cycles
(c) 5 cycles; 5 cycles
(d) 5 cycles; 4 cycles
(e) none of the above
10. You are writing a program which loops continuously. Within the loop, you are required to
alternately set and clear bit 4 in an 8-bit register. The shortest way to successfully do so at
each loop iteration is to:
(a) test the bit; if true bit-wise AND with EF16; if false bit-wise OR with 1016
(b) test the bit; if true bit-wise OR with 1016; if false bit-wise AND with EF16
(c) test the bit; if true bit-wise AND with 1016; if false bit-wise OR with 1016
(d) bitwise XOR with EF16
(e) bitwise XOR with 0816
11. Role Play/Challenge: Programming Scenarios.
TIP: The following exercises are best attempted on paper FIRST, then on the computer. The
key is to think about what is right/wrong rather than make \mad" changes.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 46
(a) As part of a design team, you have been asked to complete a piece of assembly language
code for the PIC16F877 that will count the number of capital letters in a null-terminated
variable length string. You are given an initial piece of code, which may have syntactical
and logical errors in it.
i. Identify all syntactical and logical errors in the code, and re-write the routine if
necessary. Use instruction counting to determine how many cycles your routine will
take.
#include <P16F877.inc>
Cnt EQU H’0B’
ORG G’0000’
goto Start
ORG 0x0020
Start BANKSEL Cnt
clrw
loop goto Table ; get next character from Table
btfsc STATUS,Z ; if the value is 0 we are done
goto Done
sublw ’A’ ; if the ASCII value is less than A
btfsc STATUS,C ; goto loop
goto loop
subwl ’Z’-’A’ ; if the ASCII value is between A and Z
btfsc STATUS,C ; increment Cnt register
incf Cnt,W
goto loop
Table addwf PCL,F ; Lookup table to return the string characters
DT "6 Char", D’0’ ; Null-terminated string.
Done sleep
END
ii. Optimize the code for the string given. Print the code, and illustrate how many
cycles your improved code will take.
iii. Optimize the code presuming that the ratio of capital to common letters in a given
string is 10:20. Print the code, and illustrate how many cycles your improved code
will take.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 47
(b) "A simple digital low pass  lter can be implemented using the algorithm: [An 1]
= Sn4 + Sn 12 + Sn 24 where Sn is the nth sample from an eight bit analog to digital
converter located at Port B." (Katzen 2003; p. 134)
As part of a design team, you have been asked to complete a piece of assembly language
code for a low pass  lter on the PIC16F877. At any time the  ltered value should be
placed on Port C. You are given an initial piece of code.
i. Identify all syntactical and logical errors (if any) in the code.
#include <P16F84.inc>
Sn EQU H’0B’
Snm1 EQU H’0C’
Snm2 EQU H’0D’
ORG G’0000’
goto Start
ORG 0x0020
Start
BANKSEL PORTB
clrw
Loop movf PORTB,W ; get value from PortB
movwf Sn ; store the value
addwf Snm2,W ; add Sn to Snm2
rrf W ; divide by 2
addwf Snm1,W ; add Snm1 to (Snm2+Sn)/2
rrf W ; divide by 2
movwf PORTC ; put out the output
movf Sn ; move Sn to Sn-1
movwf Snm1
movf Snm1,W ; move Sn-1 to Sn-2
movwf Snm2
goto Loop
Done sleep
END
ii. Fix the errors and compile the code. Explain how you tested this code. Print the
code, and illustrate using the instruction counting technique, how many cycles this
code will take to run.
iii. Optimize the code presuming that the A/D converter never generates values greater
than 5 bits wide. Print the code, and illustrate how many cycles your improved
code will take.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 48
(c) As part of a design team you have been asked to complete a piece of assembly language
code which produces a \continuous" histogram of sampled data values. The data is 8
bits wide and is sampled at intervals from PORTB. The histogram has 8 bins. Every
128 samples all bins in the histogram are halved. This ensures that no bin over ows,
and that the most recent updates are re ected most strongly in the histogram. The
histogram bin values are stored in 8 consecutive locations in memory.
You are given an initial piece of code(over) which may have syntactical and logical errors
in it.
i. Identify all syntactical and logical errors you can initially see in the code.
ii. Fix the errors and compile the code. Check that it works using the simulator (you
should explain how you tested this code). Print the code, and illustrate using the
instruction counting technique, how many cycles this code will take to run. Run the
program and use the stopwatch to verify your answer.
iii. Optimize the code presuming that the sampled value is never greater than 7 bits
wide, and must still be divided into 8 histogram bins. Print the code, and illustrate
how many cycles your improved code will take.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 49
LIST p=16f877
INCLUDE <P16F687.inc>
counter EQU 0x10
sample EQU 0x11
array EQU 0x12
ORG 0x00
goto Start
ORG 0x20
Start
clrf counter
LpSmpl movf PORTB,W ; get value from PORTB
movwf sample ; store in sample
swapf sample,W ; derive bin # from sample value
rrf sample,F
movlw B’00000111’
iorwf sample,F
movlw array ; access the relevant bin & increment it
movwf FSR
movf sample,W
addwf FSR,W
incf INDF,F
incf counter,F ; add 2 to counter -- if no overflow repeat the sampling
incfsz counter,F
goto LpSmple
movlw array ; initialise FSR
movf FSR
LpRot rrf INDF,F ; divide each bin by 2
incf FSR,F
movlw array+7 ; is this the last bin?
subwf FSR,W
btfsc STATUS,Z
goto LpRot
goto LpSmpl
sleep
END
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 50
(d) Question is based on (Wilmhurst 2001; Q3.8,Q3.9).
As part of a design team you have been asked to complete a piece of assembly language
code for a PIC16F877 with an external Digital-to-Analogue Converter connected to
PORTC < 5 : 0 >. The PIC16F877 must send values to the DAC such that a ramp
waveform is generated. The frequency of the waveform is determined by two switches
connected to PORTC < 7 : 6 >, as follows:
PORTC < 7 > PORTC < 6 > Frequency
0 0 0 Hz
0 1 0.25 kHz
1 0 1 kHz
1 1 4 kHz
A 4MHz oscillator provides the clock signal to the PIC16F877.
You are given an initial piece of code(over) which may have syntactical and logical errors
in it.
i. Identify (using comments on the code sheet) all the syntactical and logical errors
you can initially see in the code.
ii. Fix the errors and compile the code. Check that it works using the simulator (you
should explain how you tested this code). Print the code, and illustrate using the
instruction counting technique, how many cycles this code will take to run. Run the
program and use the stopwatch to verify your answer.
iii. Improve the code so that the ACTUAL ramp frequency is closer to the SPECIFIED
frequency and/or the code takes less space. Print the code, and illustrate how many
cycles your improved code will take.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 51
LIST p=16f877
INCLUDE <P16F678.inc>
ORG 0x00
goto init
ORG 0x20
int
BANKSEL PORTC ; configure PORTC
movlw 0xC0
movwf TRISC
BANKSEL PORTC
loop decf Cnt ; wait until Cnt is 0 then skip
goto lop
decf Out,F ; ramp up output by 1 notch
movf Out,W
movwf PORTC
chk movf PORTC,F ; check to see what switches are set to
andlw 0xC0
btfsc STATUS,Z
goto no_out
movwf Swt ; get switch bits into lsb’s of W
rrf Swt,W
rlf Swt,F
swapf Swt,W
call tbl ; retrieve the correct count
movwf Cnt
goto loop
no_out clrf PORTC ; zero all output to DAC
goto chk
tbl addwf PCL,W ; lookup table
retlw 0 ; no counting for 0Hz
addlw 16 ; count 16 for 0.25kHz;
return 4 ; count 4 for 1 kHz
retlw 1 ; count 1 for 4 kHz
END
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 52
Assignment C ID#
An engineering design company is developing an embedded system involving a non-linear pressure
sensor and a PIC16F877 with a 4MHz clock signal.
The pressure sensor is activated by a falling edge on PORTC<1>. The output of the pressure
sensor on PORTC<2> is held high for a length of time.
As a member of the design team you have been asked to complete a piece of code for the PIC16F877.
The code should determine the width of the logic-high pulse in microseconds ( s), and then convert
it to a pressure reading in pounds-per-square-inch (psi). The code will be assembled using the
MPASM assembler.
The pressure can be determined from the time using: p = 4 log2t
where t is the pulse width in  s, and p is the pressure in psi.
The design team has made an initial decision to represent the pressure reading using a 4 bit unsigned
integer.
A correctly functioning pressure sensor will output logic-high for between 1 and 15 microseconds
after being activated.
Your initial attempt at the code is shown on page 53.
1. Identify (using comments or a separate text  le) all the syntactical and logical errors you can
initially see in the code. 12 marks
2. Fix the errors and compile the code. Check that it works using the simulator (you should
explain how you tested this code). Illustrate using the instruction counting technique,(with
comments or a in separate text  le) how many cycles this code will take to run. Run the
program and use the stopwatch to verify your answer. 16 marks
3. Improve the code so that the accuracy of the RECORDED pressure is improved and/or
the code takes less space. Illustrate (with comments or a in separate text  le) how many
cycles/storage locations your improved code will take. You should also identify how accurate
the pressure reading is. 12 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 53
LIST p=16f877
INCLUDE <P18F452.inc>
Cntr EQU 0x12
ORG 0x10
goto init ; at the reset vector, branch to start of code
ORG 0x80
init
BANKSEL TRISC ; configure PORTC<2> as an input, and PORTC<1> as an output
bcf TRISC, 2
bcf TRISC,1
BANKSEL PORTC
main call read ; repeat the reading
goto man
sleep
read clrf Cntr ; clear counter
bcf PORTC,1 ; output falling edge on PORTC<1>
bsf PORTC,1
loop incf Cntr,W ; count up until the signal goes low
btfss PORTC,1
goto loop
goto convert ; call routine to convert time to pressure
movwf PrRdg
return
convert iorlw 0x0F ; mask to extract lower bits of byte
addwf PCL,F
retlw 0xFF ; return error value
retlw b’0000’ ; 4*log 2(1 microseconds)= 0 psi
retlw b’0100’ ; 4*log 2(2 microseconds)= 4 psi
retlw b’0110’ ; 4*log 2(3 microseconds)= 6 psi
retlw b’1000’ ; 4*log 2(4 microseconds)= 8 psi
retlw b’1001’ ; 4*log 2(5 microseconds)= 9 psi
retlw b’1010’ ; 4*log 2(6 microseconds)=10psi
retlw b’1011’ ; 4*log 2(7 microseconds)=11psi
retlw b’1100’ ; 4*log 2(8 microseconds)=12psi
retlw b’1100’ ; 4*log 2(8 microseconds)=12psi
retlw b’1101’ ; 4*log 2(8 microseconds)=13psi
retlw b’1101’ ; 4*log 2(8 microseconds)=13psi
retlw b’1110’ ; 4*log 2(8 microseconds)=14psi
retlw b’1110’ ; 4*log 2(8 microseconds)=14psi
retlw b’1111’ ; 4*log 2(8 microseconds)=15psi
retlw b’1111’ ; 4*log 2(8 microseconds)=15psi
END
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 54
Assignment C Submission Guidelines:
You will be awarded (up to a maximum of 4) bonus marks, for the use of exceptionally e cient
methods. An exceptionally e cient method is one which is at least 20% shorter or 5% more accurate
than the median program length/accuracy of ALL submissions. i.e. you will compete with your
peers. You will be penalised (up to a maximum of 2) marks if you violate ANY of the program
formatting guidelines.
All submitted printout/programs MUST follow the speci ed format i.e.
 labels in  rst column;
 code in second column;
 operands (where appropriate) in 3rd column;
 comments (where appropriate) in the 4th column
 directives in capital letters;
 assembly language in common letters;
 variable names/labels lower case (optional  rst capital letter).
Your  les should be named Cqnnnnnnnn.* where nnnnnnnn is your ID Number and q is the question
number.
Electronic Submission:
Please type your answers into plain ASCII text  les (no Word documents please!) with the same
name(s) as your other  les, and the extension .asc . Alternatively you may write your answers in
the *.asm  les as comments.
The *.asc, *.hex, *.asm and *.lst  les for all questions should be zipped into a single archive named
Cnnnnnnnn.zip where nnnnnnnn is your ID Number.
The archive should be uploaded to Moodle using the appropriate link.Please remember to electron-
ically submit AHEAD of the deadline in order to avoid problems with system overload.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 55
A
S
S
T
CUnit 16 Write the letter you have been assigned here .
Letters refer either to the code in the  gure opposite, or which your lecturer will put up.
1. What does your code do?
(a) swap the values in W and a  le register named data
(b) lookup a value at the Wth position in the table
(c) determine if all the masked bits are set
(d) add two 16 bit values
2. Using the initial values provided, write down the line #’s for Q in order of execution.
3. Using the initial values provided, what is the  nal value in the accumulator after Q executes
4. Using the initial values provided, what are the  nal values of the Z, C, and DC  ags (write
unchanged if they have not been altered in the code) after Q executes?
5. How many program memory locations will be occupied by each piece of code R & S.
6. On average, how many instruction cycles will it take to run R & S
7. Which piece of code is better, R or S? Explain your answer.
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ contrast alternate programming techniques in terms of size/speed of the code produced
for the PIC16Cxxx series of microcontrollers.
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 56
Q R S
A
1: BatLkupaddwf PCL,F
2: retlw A‘B’
3: retlw A‘a’
4: retlw A‘t’
5: retlw 0x00
Initial value:
W=0x01
BatLkup addwf PCL,F
retlw A‘B’
retlw A‘a’
retlw A‘t’
retlw 0x00
BatLkup addwf PCL,F
DT "Bat",0x00
B
1: chksetandwf MASK,W
2: xorwf MASK,W
3: btfss STATUS,Z
4: retlw 0
5: retlw 1
Initial values:
W=0xA7;
MASK=0x13;
chkset andwf MASK,W
xorwf MASK,W
btfss STATUS,Z
retlw 0
retlw 1
chkset andwf MASK,W
xorwf MASK,W
btfsc STATUS,Z
retlw 1
retlw 0
C
1: Swap xorwf data,W
2: xorwf data,F
3: xorwf data,W
4: return
Initial values:
W=0xFD;
data=0x00
Swap movwf tempA
movf data, W
movwf tempB
movf tempA, W
movwf data
movf tempB, W
return
Swap xorwf data,W
xorwf data,F
xorwf data,W
return
D
1: Add16 movf datalo,W
2: addwf resultlo,F
3: movf datahi,W
4: btfsc STATUS,C
5: incf resulthi,F
6: addwf resulthi,F
7: return
Initial values:
W=0xF4;
datalo=0x40;
datahi=0xCD;
resulthi=0x00;
resultlo=0XFA
Add16 movf datalo,W
addwf resultlo,F
movf datahi,W
btfsc STATUS,C
incf resulthi,F
addwf resulthi,F
return
Add16 movf datalo,W
addwf resultlo,F
movf datahi,W
btfsc STATUS,C
incfsz resulthi,F
addwf resulthi,W
movwf resulthi
return
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 57
17 Compiler limitations
At the end of this unit the student will be able to:
give examples of the limitations placed on compiler-generated code, for the PIC16Cxxx
series of microcontrollers, by the need for generality.
The purpose of a higher level language, is to o er the programmer an environment which will
behave in a standard way regardless of the platform the program is compiled for. This places
constraints on assumptions the compiler can make when translating your code. If you are aware
of the assumptions which the compiler makes in translating code, you can exploit them to obtain
the best possible performance from your C code. Similarly, if you are aware of the assumptions the
compiler makes, you can avoid the generation of invalid code.
Typically problems occur with:
 boundary checking
 excessive code { type translation
 code spanning program banks
 variables located in di erent data banks
Some tips gathered from http://www.microchipc.com/ for PIC16Cxxx C programming, appear
below. These apply to assembly as well, but tend to be forgotten/ignored when programming in
C.
 Use unsigned integers and/or chars where possible.
 Use byte (not word) arithmetic where possible.
 Some variables will not need initialization; you can save code if you leave it out.
 Order operations to take advantage of values already in the accumulator.
 Perform loop termination tests against zero where possible. When doing a multi-word
count-down loop, you only need to test the uppermost byte.
 Avoid multiplication and especially division { replace with shifts where possible
 Initialise/use variables in the same bank together before doing variables located in
another bank.
 Use of functions can reduce your code size, but, may incur a speed penalty.
 Zero, increment and decrement will always generate more e cient code than value
assignment.
 For large projects, there are three approaches to producing code:
{ compile one C  le;
{ compile one main C  le which includes all the other  les;
{ compile each C  les separately, and then link them together
Debugging support for latter two is not as extensive, but they allow the develop-
ment/use of re-usable code modules. First two generate a smaller  nal binary { less
space wasted in-page. The last one o ers incremental compilation.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 58
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 59
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 60
Review Exercises
1. The code listed below was part of a program entered into the PIC16F877.
loop incfsz Var,F
goto loop
The loop never terminates. Which of the explanations are plausible?
(a) Var is set to a special function register.
(b) The initial value stored at location Var was not set correctly.
(c) The register (Var) cannot hold a number greater than 255.
(d) The register value is being changed by an interrupt.
(e) The status  ags are being changed by an interrupt.
2. Compiler generated assembly code, is sometimes larger (in both program and data space)
than hand-generated assembly language code. Which of the following (if any) contribute to
the in ated code size:
(a) standard sizes of di erent variable types
(b) robust translation of loop-testing conditions
(c) failure to recognize(optimize for) supported assembly instruction-operations
(d) placement of code/data in di erent pages/banks
(e) entry and exit code (subroutine stack treatment)
3. You are writing a C program for the PIC16F877 which initialises locations in memory to
the same default value. You are wondering which program would generate the most e cient
assembly code when compiled. Your alternate choices are:
[A:] for (i=0;i<5;i++) c[i]=-1;
[B:] for (i=4;i!=0;i--) c[i]=0;
[C:] c[0]=0;c[1]=0;c[2]=0;c[3]=0;
Label the following statements TRUE/FALSE if they represent a valid/invalid reason for
choosing the respective C code:
(a) Code [A:] because we have an increment instruction, but no decrement instruction.
(b) Code [C:] only a few locations need to be cleared, so it will take the least space when
compiled.
(c) Code [B:] or Code [C:] clearing a location is easily done with clrf.
(d) Code [B:] testing for the ZERO  ag is easily done with btfsc STATUS,Z.
(e) Code [A:] or Code [B:] indirect addressing can be used in a loop.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 61
4. The loop shown was encoded by the compiler as shown below.
void main()
{
int cbuff[3]={1,2,3};
int i;
for (i=0;i<5;i++) cbuff[i]=0;
}
When the C program is compiled for the PC, it runs 5 times. When the C program is compiled
for the PIC16F877, it runs continuously. The best explanation for this observation is that:
(a) Integers are represented di erently by the two compilers.
(b) The code is being optimized for the PIC, and not for the PC.
(c) The compiled program is too big for the PIC16F877 program memory.
(d) The compiled program accesses too much data memory in the PIC16F877.
(e) The compiled program runs out of time on the PIC16F877.
5. Three sets of C code have been written to perform the multiplication of 255 and 27 using
repeated addition.
5-I 5-II 5-III
void main()
{
int k,M1,M2;
long int R;
R=0;
M2=27;
M1=255;
for (k=0;k<=M1;k++)
{
R=R+M2;
}
}
void main()
{
int k,M1,M2;
long int R;
R=0;
M2=27;
M1=255;
for (k=M1;k>0;k++)
{
R=R+M2;
}
}
void main()
{
int k,M1,M2;
long int R;
R=M2=27;
M1=254;
for (k=0;k<=M1;k++)
{
R=R+M2;
}
}
For the compiler used, integers are represented using 8 bits, and long integers are represented
using 16 bits. When run, Program 5-I never stops. Programs 5-II and 5-III, however, run as
expected. The MOST PLAUSIBLE explanation for the failure of Program 5-I is that:
(a) R is not big enough to hold the result of the operation.
(b) R should also be declared as an integer.
(c) the number 255 cannot be represented using 8 bits.
(d) the termination test requires representing the number 256 in 8 bits.
(e) there is a  aw in the compiler { it generates bad code sometimes.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 62
6. We wish to extract bits 6 and 7 of a byte called Data, and place them in bits 0 and 1
of the working register, respectively. The following code snippets have been suggested.
P-I
swapf Data,F
nop
andlw 0x0C
rrf Data,F
rrf Data,W
P-II
movlw 0xC0
andwf Data,F
swapf Data,F
rrf Data,F
rrf Data,W
P-III
movlw 0xC0
andwf Data,F
bcf STATUS,C
rlf Data,F
rlf Data,F
rlf Data,W
(a) Which of the snippets will perform this task correctly?
i. P-I only
ii. P-I and P-II only
iii. P-I and P-III only
iv. P-II and P-III only
v. P-I, P-II and P-III
(b) Presuming that they are all correct, which snippet will take the shortest time to run?
i. P-I
ii. P-II
iii. P-III
iv. P-I and P-II will take the same (shorter) time
v. P-I, P-II and P-III will all take same time
(c) Presuming that they are all correct, these snippets could have been generated by a
compiler from C program code. Which of the following tasks is MOST likely to be the
task performed by the C program code?
i. clear a  ag contained in a byte (Data)
ii. divide an unsigned number (Data) by 64
iii. extract a packed BCD number from a byte (Data)
iv. multiply a number (Data) by 8
v. retrieve an integer from a packed-array (Data)
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 63
7. All questions refer to the following C code, and generated assembly language for the PIC16F877:
void main()
{
int c;
c=9;
do{}while (c++ <= 255);
}
0000 3000 movlw 0x0
0001 008A movwf 0xA
0002 2804 goto MAIN
0003 0000 nop
0004 0184 MAIN clrf 0x4
0005 301F movlw 0x1F
0006 0583 andwf 0x3
0007 3009 movlw 0x9
0008 00A1 movwf 0x21
0009 2809 goto 0x9
000A 0063 sleep
(a) What is the address of the variable c?
(b) Highlight (or circle) statements which comprise loops in the C and assembly code.
(c) What do you expect the C code loop to do?
(d) What will the generated assembly code loop actually do?
(e) In your own words, explain why this assembly language code has been generated.
(f) Write your own equivalent assembly language for the C code.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 64
8. The C code in Figure 8 on page 65 was compiled; the generated assembly language for the
PIC16F877 is also shown:
(a) What is the address of the variable cbu ?
(b) What is the address of the variable i?
(c) What is the address of the return value?
(d) View/Add comments which explain the generated assembly code.
(e) Highlight (or circle) statements which comprise the loops in the C and assembly code.
(f) What do you expect the C code loop to do?
(g) What will the generated assembly code loop actually do?
(h) In your own words, explain why this particular piece of assembly language code has
been generated from the C code.
(i) Write your own equivalent assembly language code for the C code, in the space next to
the code.
9. The C code shown in Figure 9 on page 66 was compiled; the generated assembly language
for the PIC16F877 is also shown:
(a) What is the address of the variable q?
(b) Highlight (or circle) statements which comprise loops in the C and assembly code.
(c) What do you expect the C code loop to do?
(d) Add comments which explain the generated assembly code. What will the generated
assembly code loop actually do?
(e) In your own words, explain why this assembly language code has been generated.
(f) Would you expect similar assembly language code to be generated if we divided by 3 in
each iteration of the loop? Explain your answer.
10. Role Play/Challenge: Find all possible ways in which a \for" loop test can fail when the loop
variable is a  oating point number.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 65
Figure 8: Code sample and generated assembly for Question 8
#include <16f877.h>
int main()
{
int cbuff[3]={1,2,3};
int i;
for (i=0;i<5;i++) cbuff[i]=0;
return 0;
}
0000 3000 movlw 0x0 ; return to Program Bank 0 (clear PCLATH)
0001 008A movwf 0xA
0002 2804 goto main
0003 0000 nop
0004 0184 main clrf 0x4 ; clear FSR
0005 301F movlw 0x1F
0006 0583 andwf 0x3 ; clear out all the banking bits in STATUS by using a mask
0007 3006 movlw 0x6
0008 1683 bsf 0x3,0x5 ; move to data memory bank 1
0009 009F movwf 0x1F ; configure ADCON1 so that shared pins are digital
000A 3001 movlw 0x1 ; first value for initialising the array
000B 1283 bcf 0x3,0x5 ; back to data memory bank 0
000C 00A1 movwf 0x21 ; start initialisation
000D 3002 movlw 0x2
000E 00A2 movwf 0x22
000F 3003 movlw 0x3
0010 00A3 movwf 0x23 ; all initialised
0011 01A4 clrf 0x24 ; for loop -- initial state i=0
0012 0824 movf 0x24,W ; for loop -- tst <5: subtrt 4, tst Carry (borrow-bar) flag
0013 3C04 sublw 0x4
0014 1C03 btfss 0x3,0x0
0015 281C goto 0x1C ; leave the loop if we fail the test
0016 3021 movlw 0x21 ; put address of cbuff in W
0017 0724 addwf 0x24,W ; add the value of i to get the address of cbuff[i]
0018 0084 movwf 0x4 ; place address in the FSR
0019 0180 clrf 0x0 ; clear the location indicated by FSR (cbuff[i])
001A 0AA4 incf 0x24 ; for loop -- increment
001B 2812 goto 0x12 ; go and re-test condition
001C 3000 movlw 0x0
001D 00F8 movwf 0x78 ; return value stored
001E 0063 sleep
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 66
Figure 9: Code sample and generated assembly for Question 9
#include<16F877.h>
int main()
{
char q;
int i;
q=83;
for (i=0;i<8;i++)
q=q/4;
return 0;
}
0000 3000 movlw 0x0
0001 008A movwf 0xA
0002 2804 goto main
0003 0000 nop
0004 0184 main clrf 0x4
0005 301F movlw 0x1F
0006 0583 andwf 0x3
0007 1683 bsf 0x3,0x5
0008 141F bsf 0x1F,0x0
0009 149F bsf 0x1F,0x1
000A 151F bsf 0x1F,0x2
000B 119F bcf 0x1F,0x3
000C 3053 movlw 0x53
000D 1283 bcf 0x3,0x5
000E 00A1 movwf 0x21
000F 01A2 clrf 0x22
0010 0822 movf 0x22,W
0011 3C07 sublw 0x7
0012 1C03 btfss 0x3,0x0
0013 281A goto 0x1A
0014 0CA1 rrf 0x21
0015 0CA1 rrf 0x21
0016 303F movlw 0x3F
0017 05A1 andwf 0x21
0018 0AA2 incf 0x22
0019 2810 goto 0x10
001A 3000 movlw 0x0
001B 00F8 movwf 0x78
001C 0063 sleep
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 67
Tutorial Exercise 6B O ine Version31 ID#
The C code shown overleaf was compiled and the generated assembly for the PIC16F877 is shown:
1. What addresses are used to store the variables q and i? 2 marks
2. Highlight (or circle) statements which comprise loops in the C and assembly code. 2 marks
3. What do you expect the C code loop to do? 1 mark
4. What will the generated assembly code loop actually do? 1 mark
5. In your own words, explain why this assembly language code has been generated. 1 mark
6. How would the code change if we incremented q in each iteration of the loop? Write changes
next to the code overleaf. Explain your answer below. 3 marks
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
31Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 68
#include <16F877.H>
int main()
{
long q;
int i;
q=26;
for (i=0;i<4;i++)
q=q*2;
return 0;
}
0000 3000 MOVLW 0
0001 008A MOVWF 0xa
0002 2804 GOTO 0x4
0003 0000 NOP
0004 0184 CLRF 0x4
0005 301F MOVLW 0x1f
0006 0583 ANDWF 0x3, 0x1
0007 300F MOVLW 0xf
0008 1683 BSF 0x3, 0x5
0009 009F MOVWF 0x1f
000A 1283 BCF 0x3, 0x5
000B 01A2 CLRF 0x22
000C 301A MOVLW 0x1a
000D 00A1 MOVWF 0x21
000E 01A3 CLRF 0x23
000F 0823 MOVF 0x23, 0
0010 3C03 SUBLW 0x3
0011 1C03 BTFSS 0x3, 0
0012 2818 GOTO 0x18
0013 1003 BCF 0x3, 0
0014 0DA1 RLF 0x21, 0x1
0015 0DA2 RLF 0x22, 0x1
0016 0AA3 INCF 0x23, 0x1
0017 280F GOTO 0xf
0018 3000 MOVLW 0
0019 00F8 MOVWF 0x78
001A 0063 SLEEP
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 69
T
U
T
6
BUnit 17 Write the letter you have been assigned here .
Answer the following questions making reference to the two pieces of C code corresponding to your
letter.
1. Identify one di erence between the two pieces of code.
2. What will this di erence mean when the code is compiled?
3. Choose one piece of code, and explain the circumstances under which this would be the better
choice.
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ give examples of the limitations placed on compiler-generated code, for the PIC16Cxxx
series of microcontrollers, by the need for generality.
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 70
A
void main()
{
int b,c;
b=5;
c=4;
c+=b;
}
void main()
{
long int b,c;
b=5;
c=4;
c+=b;
}
B
void main()
{
int i,c[10];
for (i=0;i<10;i++)
c[i]=i*4;
}
void main()
{
int i,c[10];
for (i=9;i!=0;i--)
c[i]=i*4;
c[0]=0;
}
C
void main()
{
int i,c[10];
for (i=0;i<10;i++)
c[i]=i*4;
}
void main()
{
int i,c[10];
for (i=0;i<10;i++)
c[i]=i<<2;
}
D
void main()
{
int i,c[10];
for (i=0;i<10;i++)
c[i]=i*2;
}
void main()
{
int i,c[10];
for (i=0;i!=10;i++)
c[i]=i*2;
}
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 71
Algorithms on PIC16
In this section we have explored assembly code snippets for the PIC16F877, discussed some of the
ways in which both assembly and C code can be manipulated to improve performance. In doing
so, we have partially satis ed the second course objective; to \implement arithmetic and data
manipulation operations in assembly language, and assess how well code will perform in a given
context, given the architecture and instruction set of a microprocessor".
This represents the mid-way point in the course. You should  ll out the mid-course review at this
stage. After this, we will be paying more attention to the ways in which the software can be made
to control or work with hardware devices. To facilitate this, the group mini-projects will be issued
at this stage. Then the next section will deal with the primary means of external interaction, the
I/O port and other peripherals.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 72
Group Mini-project
Objectives
By the end of this project, your group will have achieved/performed the following objectives.
 made an appropriate sub-division of tasks within the group, and assessment of how well group
members performed their assigned tasks
 consideration of an applied  P problem and production of an appropriate hardware/software
design outline;
 interpretation of information presented on data sheets, and application of that information
in microprocessor interfacing;
 development of assembly language code, which utilises at least 2 of the following:
{ Interrupts,
{ Timing,
{ A/D and/or D/A techniques,
{ Communications,
{ Peripherals: possibly to accomplish any of the above.
 identi cation and elimination of circuit and design problems using a prototype, standard lab
equipment, and troubleshooting procedures.
 documentation of the  nal design/prototype, and identi cation of salient features/ aws.
Protocol/Guidelines
The entire class will be divided into groups, with each group consisting of 4-6 persons. The group
will appoint a group leader. Each group will be required to attempt one of the speci ed projects.
The group leader will "pick from a hat" to determine which project the group will perform. A
preliminary package of parts/datasheets will be supplied to each group leader. Groups should
utilise their group forum to communicate with each other about the project and their group wiki
to update the class on their progress. Each group session, randomly selected group(s) will be called
upon to present their work-to-date.
Please note:
 Group leaders will be responsible for obtaining any additional parts required, and for ensuring
that parts are not damaged.
 The group is collectively responsible for making sure that all members participate in the
exercise.
 Any group with damaged parts will be required to carry on using the damaged parts (therefore
resulting in loss of marks in demonstration and implementation)
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 73
Peer Assessment
The mark which you will receive for this project will be calculated based on the group mark, and
the peer assessment made by the rest of your group. The peer assessment form is on page V-74
r = nPnP
i=1
Pi
final mark =
(
rG ;r< 2
2G ;r 2
where P is the peer assessment mark you received (between 1 and 5), n persons are in the group,
G is the group mark and final mark will be recorded. No-one will be allowed to receive more than
twice the group mark.
Completed peer assessment forms should be returned to the technician, TA, or lecturer, anytime
up to the Tuesday following the submission deadline. If you do not complete the peer assessment
exercise, you will not receive any marks for the group project.
Demonstration
The  nal design prototype should be observed in operation by either the TA or lecturer, prior to
submission (please arrange a mutually convenient time). If you make any changes to the operation
of the system before submission, you will need to repeat this.
Presentation
On a selected day, during the regular lecture slot, a randomly chosen member of your group will
be required to make a 1 minute presentation to the class about your project, and answer 2 minutes
of questions. All group members should be present and prepared, as anyone may be called on the
day.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 74
Peer Assessment Form
Based on: SAMPLE PEER ASSESSMENT FRAMEWORK
http://www.oaa.pdx.edu/CAE/FacultyFocus/spring96/bulman.html
Assessment of others is an important skill. You should take time to complete this assessment form, forcing
yourself to be objective and unbiased. Your responses will be kept con dential.
For each member of the group (except yourself), award a mark from 1 to 5 (or X if not applicable) for
your group-mate’s contribution/performance in each of the tasks identi ed. The marking scheme considers
both the quality of your peers’ work (hardware, software, report) and the cooperative-ness of your peers
(participation).
For Participation, consider things like:
 Did this person attend scheduled meetings / keep appointments?
 Did this person meet all assigned deadlines?
 Did this person accept constructive criticism and act on it?
 Did this person contribute and share ideas?
 Did this person do a fair share of the more tedious work (i.e. photocopying, typing, formatting, testing,
debugging etc.)?
Name & ID# Group #
First Name Surname ID Number Hardware Software Report Participation
Please use the following scale:
1 - did not contribute to this task even though a contribution was expected
2 - willing, but not very successful
3 - average, did the basic work successfully but no special e ort
4 - above average, willing, able, successful, and made a special e ort
5 - outstanding: extra e ort, and critical to success of task
X - was not asked to contribute to that task
Marks reported by your group-mates will be averaged and rounded to the nearest integer to give you an
individual peer assessed mark (from 1 and 5).
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 75
Group Assessment
The group will be assessed on the following basis:
5 marks Presentation/Demonstration
5 marks Report
10 marks Design
10 marks Implementation
Up to four (4) extra bonus points may be awarded, at the discretion of the lecturer or TA. One (1)
in each of the speci ed categories, for exceptional performance in that category.
The group report should be submitted in a manila folder. The group number should be marked on
the tab of the manila folder. A disk or CD containing the *.hex, *.asm and *.lst  les for the software
should also be securely  xed inside the folder. One method: you seal the disk in an envelope, and
staple the envelope to the folder.
Please write the course code, the coursework title, and your Group number on both the folder and
the disk/CD. A list of the group members ID#’s should also appear on the report cover.
The following declaration should appear on the  rst page of the report, and MUST be signed by
all group members.
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall
into one of the following categories:
 oral or written discussion, clari cation and/or modi cation of the project goals
 oral or written discussion of alternate solutions
 oral or written assistance in troubleshooting a particular solution.
 oral or written advice on report presentation/structure, and Technical English grammar/sentence construc-
tion.
Department of Electrical and Computer Engineering, UWI
PLAGIARISM Plagiarism is the presentation by a student-group of an assignment which has in fact been copied in
whole or in part from another student-group’s work, or from any other source (e.g. published books or periodicals),
without due acknowledgement.
COLLUSION Collusion is the presentation by a student-group of an assignment as their own which is in fact the
result in whole or part of unauthorised collaboration with persons who are not members of the student-group.
DECLARATION We declare that this assignment is our own work and does not involve plagiarism or collusion.
We have read and understood University Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
Signed: Date:
Signed: Date:
Signed: Date:
Signed: Date:
(Department of Electrical and Computer Engineering)
Report Format
Details of the report format are shown on the following pages.
c DECE, UWI, St. Augustine, Trinidad
 
 
 
P a r t  o f  t h e  
R e p o r t  
S u b - S e c t i o n   C o n t e n t  F o r m a t  P a g i n a t i o n  
G e n e r a l  I n f o r m a t i o n  Q u o t a t i o n s  o f  m o r e  t h a n  t w o  
l i n e s  a r e  s i n g l e  s p a c e d  a n d  
i n d e n t e d  . 5 ”  f r o m  t h e  l e f t  
M a r g i n .  
 
T a b l e s  o r  F i g u r e s  s h o u l d  
a p p e a r  c l o s e l y  f o l l o w i n g  t h e  
t e x t  w h e r e  t h e y  a r e  d i s c u s s e d .   
N o  f u r t h e r  t h a n  a  p a g e  
f o l l o w i n g .  T a b l e s  a n d  F i g u r e s  
m u s t  h a v e  d e s c r i p t i v e  t i t l e s .  
1 . 5  t o  d o u b l e  l i n e  s p a c i n g  
 
S i n g l e  l i n e  s p a c i n g  u s e d  i n  
Q u o t a t i o n s ,  A p p e n d i c e s ,  R e f e r e n c e s  
 
M a r g i n s -  t o p  a n d  b o t t o m  1 ” ;  L e f t  2 ”  
a n d  R i g h t  1 ”  
 
T y p e f a c e  –  1 0 - 1 2  P i t c h  f o n t s  
P r e f e r a b l y  S e r i f  f o n t  ( T N R )  f o r  b o d y  
t e x t  a n d  S a n s  S e r i f  f o n t  ( A r i a l )  f o r  
H e a d i n g s  
E q u a t i o n s  a n d  F o r m u l a e  m u s t  b e  
t y p e d ,  n o t  h a n d  w r i t t e n  
R o m a n  N u m e r a l s  L o w e r c a s e  
 
P a g e  n u m b e r s  o n  T o p  o r  
B o t t o m  R i g h t  o f  t h e  p a g e  
 
T a b l e s  a n d  F i g u r e s  a r e  
n u m b e r e d  u s i n g  A r a b i c  
n u m b e r i n g  a n d  s e p a r a t e  
s e q u e n c e s  
T i t l e  P a g e  S t u d e n t s ’  N a m e  &  I D  
N u m b e r  
L e c t u r e r ’ s  N a m e  
T i t l e  o f  R e p o r t  
D a t e  o f  R e p o r t  
C e n t e r e d  H o r i z o n t a l l y  a n d  V e r t i c a l l y  
o n  t h e  P a g e  a n d  s p a c e d  e v e n l y  
N o  P a g e  N u m b e r i n g  
A b s t r a c t  A n  I n f o r m a t i v e  A b s t r a c t  i s  
r e q u i r e d ,  n o  l o n g e r  t h a n  2  
p a r a g r a p h s .   T h i s  m u s t  
i n c l u d e :  o b j e c t i v e s ,  s c o p e ,  
m e t h o d ,  f i n d i n g s ,  
r e c o m m e n d a t i o n s  a n d  
c o n c l u s i o n s .  
S i n g l e  L i n e  S p a c e d  L o w e r  c a s e  R o m a n  N u m e r a l s ,   
P a g e  s t a r t i n g  f r o m  2   
T a b l e  o f  C o n t e n t s  I n c l u d e s  a l l  s i g n i f i c a n t  p a r t s  
o f  t h e  r e p o r t .   D o e s  N O T  
i n c l u d e  t h e  T O C  
G e n e r a t e d  f r o m  D o c u m e n t  H e a d i n g s  
G l o s s a r y  ( o p t i o n a l )    
L i s t  o f  A b b r e v i a t i o n s  
( o p t i o n a l )  
  
T h e  F r o n t  
M a t t e r  
L i s t  o f  F i g u r e s / T a b l e s   G e n e r a t e d  f r o m  C a p t i o n s  
A l l  f r o n t  m a t t e r  c o n t i n u e s  
l o w e r  c a s e  R o m a n  N u m e r a l s ,  
n u m b e r e d  c o n s e c u t i v e l y  o f f  
t h e  a b s t r a c t   
 
P a r t  o f  t h e  
R e p o r t  
S u b - S e c t i o n   C o n t e n t  F o r m a t  P a g i n a t i o n  
T h e  B o d y   
 
O v e r v i e w   
 
 
H a r d w a r e  L a y o u t  a n d / o r  
C i r c u i t  d i a g r a m  
 
 
 
 
 
S o f t w a r e  s t r u c t u r e  a n d / o r  
A l g o r i t h m  
 
 
T e s t i n g  
 
 
 
 
D i s c u s s i o n  
S u m m a r i z e  t h e  b e h a v i o r  a n d  
s e t  t h e  g o a l s  o f  t h e  s y s t e m .  
 
B l o c k  a n d  c i r c u i t  d i a g r a m s .  
C a l c u l a t i o n s ,  a n d  c o m p o n e n t  
c h o i c e s  a l o n g  w i t h  
e x p l a n a t i o n s .  P l e a s e  d o  n o t  
i n c l u d e  s p e c i f i c a t i o n  s h e e t s .  
 
 
S t a t e  c h a r t s ,  F l o w  C h a r t s ,  U s e  
C a s e  d i a g r a m s ,  P s e u d o  c o d e  
( a s  a p p r o p r i a t e  )  
 
A  l i s t  o f  t e s t s  d e v e l o p e d  b a s e d  
o n  s y s t e m  r e q u i r e m e n t s ,  a n d  
t h e  r e c o r d e d  r e s u l t  o f  e a c h  t e s t  
o n  t h e  f i n a l  s y s t e m .  
 
A n a l y s i s  o f  h o w  w e l l  t h e  
o b j e c t i v e s  w e r e  m e t ,  d e t a i l s  o f  
d e s i g n  t r a d e o f f s  a n d  
s i g n i f i c a n t  p r o b l e m s  
e n c o u n t e r e d  d u r i n g  
d e v e l o p m e n t ,  s u g g e s t i o n s  f o r  
c o r r e c t i o n / i m p r o v e m e n t .    
E a c h  s e c t i o n  o n  a  n e w  p a g e  N e w  S e q u e n c e ,  A r a b i c  
n u m b e r i n g  
R e f e r e n c e s  C h i c a g o  M a n u a l  o f  S t y l e  1 5
t h
 E d .     
( h t t p : / / w w w . m a i n l i b . u w i . t t / d i v i s i o n s / e p s / g u i d e s / e p s c h i c a g o c i t e . p d f )  
 
T h e  E n d  M a t t e r  
A p p e n d i c e s  -  C O D E  A l l  s u b m i t t e d  p r i n t o u t / p r o g r a m s  M U S T  f o l l o w  t h e  s p e c i f i e d  f o r m a t  
i . e .  
•  l a b e l s  i n  f i r s t  c o l u m n ;  
•  c o d e  i n  s e c o n d  c o l u m n ;  
•  o p e r a n d s  ( w h e r e  a p p r o p r i a t e )  i n  3 r d  c o l u m n ;  
•  c o m m e n t s  ( w h e r e  a p p r o p r i a t e )  i n  t h e  4 t h  c o l u m n  
•  d i r e c t i v e s  i n  c a p i t a l  l e t t e r s ;  
•  a s s e m b l y  l a n g u a g e  i n  c o m m o n  l e t t e r s ;  
•  v a r i a b l e  n a m e s / l a b e l s  l o w e r  c a s e  ( o p t i o n a l  f i r s t  c a p i t a l  l e t t e r ) .  
C o n t i n u e  s e q u e n c e  o f  A r a b i c  
n u m b e r s  
ECNG2006 (EE25M) March 11, 2008 V 78
Description
The mini-project for ECNG2006 in 2007/8 will be related to a system for a remotely operated
vehicle, suitable for exploring hazardous environments. The vehicle will require a number of sub-
systems:
1. GP1: Direction Monitor (4)
2. GP2: Motor Control (3)
3. GP3: Voltage Monitor (5)
4. GP4: Remote Communication (4)
Students are expected to locate datasheets for the parts marked *.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 79
GP1 - Direction Monitor
Initial Requirements
 The compass direction should be obtained using I2C interface (compass address 0xC0)
 The LED’s should be arranged so as to give a visual indication of the direction in which the
compass is oriented: the user should be able to visually distinguish between the 8 directions:
N, NE, E, SE, S, SW, W, NW.
 The user should be able to specify the desired direction using the keypad. The keypad keys
should be appropriately labeled to facilitate this task.
 The system should beep at intervals - the interval between beeps should increase as the
compass direction approaches the desired direction.
Parts
 breadboard, PIC/header, zener diode, crystal oscillator & decoupling capacitor
* Devantech magnetic compass mounted on CD w/cable
 2 Yellow and 2 Red LED’s
 8 Ohm Loudspeaker
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 80
GP2 - Motor Control
Initial Requirements
 The DC Motor speed should be controlled using Pulse Width Modulation (PWM).
 The user should be able to select di erent DC motor speeds using the 4-bit DIP switch; at
least one switch setting will stop the motor.
 A visual indication of the current motor speed (from 0 to 999 revs per minute), should be
displayed on the common anode 7-segment displays.
 The motor speed should be stored in the serial EEPROM (address 0xA4) every 1 second after
startup, until the EEPROM memory is full. After the EEPROM memory is full, stopping
the motor should result in playback of the motor readings to the 7-segment displays.
Parts
 breadboard, PIC/header, zener diode, crystal oscillator & decoupling capacitor
 4-DIP switch
* US Digital Encoder S1-1000-NT mounted on a DC Motor
 3 common-anode 7-segment displays
* I2C Serial EEPROM 24C02
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 81
GP3 - Power monitor
Initial Requirements
 The Regulator should be driven with a variable voltage input between 8 and 12 Volts. It
should independently power the joystick and the PIC16F877, so that the joystick can be
connected/disconnected from the power source, using a signal from the PIC16F877.
 The joystick and a voltage monitoring line for the regulator input voltage, should be appro-
priately connected to di erent channels on the A/D converter. Please ensure that you do
not exceed the allowable input voltages for the PIC16F877. The joystick should be
disconnected from the regulator if the regulator input voltage drops below 9 volts.
 The joystick and 10 segment LED bar should be oriented so that the LED Bar is aligned
with one of the joystick axes. Moving the joystick along the aligned axis should result in the
pattern displayed on the 10-segment LED changing in the direction indicated.
 The battery voltage reading should be stored every 1 second after startup, using the serial
EEPROM (address 0xA2) as a circular bu er. Dropping the regulator input voltage below
9 volts, should result in playback of the preceding 30 seconds of voltage readings to the 10
segment LED bar. Dropping the regulator input voltage below 8 V should result in the
PIC16F877 going to ‘sleep’. The PIC16F877 should be reset using a button press or any
other feasible circuitry.
Parts
 breadboard, PIC/header, zener diode, crystal oscillator & decoupling capacitor
 Joystick
* 5 Volt Regulator
 10 segment LED bar
* I2C Serial EEPROM 24C02
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 V 82
GP4 - Remote Communication
Requirements
 The PIC16F877 should be able to send ASCII strings to/receive ASCII strings from a PC
using the 9-pin serial port and the level-shifting IC. The protocol settings to be used are 2400
baud, 8 data bits, 1 stop bit, no parity.
 The 7-segment displays should be used to display a 24-hour clock which increments at 1
minute intervals - e.g. 23:00 represents 11pm at night.
 The clock value can be reported at any time using a serial port command "GET". The
PIC16F877 should return a string indicating the current time e.g. Current time is nn:nn
where nn:nn is the current clock setting. The clock value can be changed at any time us-
ing a serial port command SET nn:nn where nn:nn is the new clock setting. Invalid com-
mands/input times should be ignored.
 Initial access to the PIC16F877 should be controlled using a password, which the user is
prompted for. The password should be stored in the PIC16F877 Data EEPROM memory in
an encrypted form. The user should be able to change the password from the PC using a
serial command.
Parts
 breadboard, PIC/header, zener diode, crystal oscillator & decoupling capacitor
 4 common anode 7-segment displays
* RS232- voltage level shift IC (IN232) + 9-pin male connector to ribbon
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) Introduction to microprocessors
c DECE, UWI, St. Augustine, Trinidad
original author: Feisal Mohammed; updated: March 11, 2008{ CLR
Part VI
Interfacing Peripherals
There are several features of computers/microprocessors/microcontrollers which have not been
formally explored. In particular the interfacing functions, which let the processor interact with
the external environment, and typical peripherals including I/O Ports, Timers, Interrupts, A/D
conversion, PWM and Serial communications.
The most generic peripheral is the I/O port. All other peripheral functions can be simulated using
an I/O port and dedicated code. The use of dedicated peripherals allows the microprocessor to
perform multiple functions simultaneously. One of the features that di erentiates a microcontroller
from a microprocessor is the presence of built-in I/O ports and peripherals. The following sections
address I/O ports and peripherals in general, and discuss how they operate on the PIC16F877 in
particular.
18 Interfacing Idioms
At the end of this unit the student will be able to:
identify alternate ways in which components may be interfaced with a microprocessor
(i.e. common idioms for interacting with hardware): single bit (with debounce), matrix,
map into memory space, etc.
The registers which appear in the data memory space of the PIC16F877 may be roughly classi ed
into 4 sub-groups(Predko 2001):
 processor registers (STATUS, PCL, PCLATH, FSR, INDF),
 variable memory,
 shared (shadowed) variable memory,
 i/o port, or peripheral hardware registers.
So far we have used the processor registers, and the variable memory (un-shared). Because these
are all in Bank 0 we also did not need to do any bank switching. This will change when dealing with
peripheral registers. Peripheral registers may be a di erent size to the processor data-word size,
and may be of two types: control, and data. Although the registers may have di erent addresses,
they work in tandem to a ect peripheral behaviour.
ECNG2006 (EE25M) March 11, 2008 VI 2
Input/Output Ports
The most common/ exible peripheral is the I/O port, also referred to as the Parallel Interface
Adaptor (PIA) or Parallel Port Interface (PPI). The parallel I/O port acts as a bu er for things
on the bus. It can be con gured for input or output (uses tri-state on external side) using the
tri-state or data direction control register; in many cases bits/nibbles can be separately con gured.
In response to the peripheral data address, data from the bus is latched for conveyance to external
devices, or external input is latched for placement on the bus. I/O ports may be either mapped
into the memory space or they may exist in a separate I/O space. The most commonly used I/O
port peripheral on the PC is the Intel 8255 (and variants), which provides two ports.
The PIC16F877 has  ve I/O ports PORTA thru PORTE. Each port has a direction register (also
referred to as the tri-state register TRISx) associated with it.
Particular port pins may be internally con gured to facilitate interfacing. Three facilities found
in the PIC16F877 are the use of Schmitt trigger inputs (debouncing), internal pull-ups (allow
unpowered switch connections), and open drain outputs (require external pull-up { will not power
the connection). Some of the port pins are multiplexed with other peripheral inputs/outputs.
PORTA is a 6-bit wide, bi-directional port. The corresponding data direction register
is TRISA. Setting a TRISA bit (= 1) will make the corresponding PORTA pin an input
(i.e., put the corresponding output driver in a Hi-Impedance mode). Clearing a TRISA
bit (= 0) will make the corresponding PORTA pin an output (i.e., put the contents of
the output latch on the selected pin). { (PIC 2001)
PORTA is at address 0x05 and its direction register, TRISA, is at address 0x85, which means
that PORTA is in Bank 0 and TRISA is in Bank 1. Therefore either indirect addressing or bank
switching must be performed to access the direction register. The following example shows how
PORTA can be initialized:
; assume that we are in Bank 0
clrf PORTA ; Set the output latches of PORTA to zero
bsf STATUS,RP0 ; Switch to Bank 1
movlw 0x0F ; Value for data direction
movwf TRISA ; set RA<3:0> as inputs and RA4 as output
bcf STATUS,RP0 ; switch back to Bank 0
Alternatively using indirect addressing:
clrf PORTA ; Set the output latches of PORTA to zero
movlw TRISA ; Setup for indirect addressing
movwf FSR ; ...by moving address to FSR
movlw 0x0F ; Value for data direction
movwf INDF ; set RA<3:0> as inputs and RA4 as output
All other ports may be initialised in a similar manner.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 3
The BANKSEL pre-processor directive for the MPASM assembler, may be used as shorthand for bank
switching. The direct addressing example listed above may be re-written as:
; assume that we are in Bank 0
clrf PORTA ; Set the output latches of PORTA to zero
BANKSEL TRISA ; Switch to Bank 1
movlw 0x0F ; Value for data direction
movwf TRISA ; set RA<3:0> as inputs and RA4 as output
BANKSEL PORTA ; switch back to Bank 0
PORTB is very similar to PORTA. It is an 8-bit wide bi-directional port with the pin directions
being controlled by the TRISB register. Each of the PORTB pins can have a weak internal pull-up,
which can all be turned on by a single control bit. This bit being the RBPU (OPTION_REG<7>)
bit. The weak pull-up is automatically disabled when the port pin is con gured as an output. The
pull-ups are also disabled by a power-on reset. (PIC 2001)
PORTC, and PORTD are 8-bit wide, bi-directional ports, while PORTE is a 3 bit wide bi-directional
port.
I/O Programming Considerations
Any instruction which writes, operates internally as a read followed by a write operation. For
example, the bsf and bcf instructions  rst read a register into the CPU, execute the bit clear/set
operation and write the result back to the register. Care must be taken when these instructions
are applied to a port that is con gured for both input and output.
A bsf operation on bit 5 of PORTB will cause all eight bits of PORTB to be read into the CPU.
Then the bit set operation takes place on bit 5 and the result is written to the output latches. If
another bit, say bit 0, of PORTB is de ned as an input at this time, the input signal present on
the pin itself would be read into the CPU and rewritten to the data latch of this particular pin,
overwriting the previous content. As long as the pin stays in the input mode, no problem occurs.
However when bit 0 is switched into output mode later on, the content of the data latch is unknown.
An example is shown in the code below.
; Initial port settings: PORTB<7:4>Input PORTB<3:0> Output
; PORTB<7:6> have external pullups and are not connected
; to any other circuitry
;
; Port Latch Port pins
; ---------- ---------
1 bcf PORTB,7 ; 01pp pppp 11pp pppp
2 bcf PORTB,6 ; 10pp pppp 11pp pppp
3 bsf STATUS,RP0 ;
4 bcf TRISB,7 ; 10pp pppp 11pp pppp
5 bcf TRISB,6 ; 10pp pppp 10pp pppp
At line 2, it would seem that the port latch’s high byte should be 00pp but when the instruction is
executed, the port pins are read again, thus the bit clear acts on 11pp pppp.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 4
The actual write to an I/O port happens at the end of an instruction cycle, whereas for reading,
the data must be valid at the beginning of the instruction cycle. Therefore, care must be exercised
if a write followed by a read operation is carried out on the same I/O port-pin. The sequence of
instructions should be such to allow the pin voltage to stabilize (load dependent) before the next
instruction which causes that  le to be read into the CPU is executed. Otherwise, the previous
state of that pin may be read into the CPU rather than the new state. When in doubt, it is better
to separate these instructions with a NOP or another instruction which does not access this I/O
port.
Simple I/O Interfacing
A single bit of an I/O port may be used to interface with a number of devices. Each bit of an
I/O port typically corresponds to a single IC pin. Circuits connected to I/O port pins should be
con gured in accordance with the current sink/source limits of the IC. There are typically three
limits: maximum current sunk/sourced by a) the pin b) all pins in the port c) all ports on the IC.
As IC’s can generally sink more current than they can source, \sink" con gurations are preferable.
The simplest output we can visualise is an LED as an indicator. This will pass current in one
direction only (diode), once the potential di erence exceeds a known value. As a result of the
passing current, the LED will light. An appropriately oriented LED in series with a resistor, can
be tied between either 5V or ground, and an I/O port pin. The resistor value will depend on the
current requirements of the LED and current limits of the I/O pin/port/IC. Depending on the
orientation, the LED will light when the port bit is con gured as an output, and either a 1 or
0(depending on the LED orientation, and tie voltage) is placed on the port bit.
Similarly, the simplest input we can visualise is a single switch. Again it can be con gured so
that it either pulls the output high, or grounds the input. The input port will read ’1’ when the
voltage exceeds the low-high voltage threshold, and ’0’ when the voltage is below the high-low
voltage threshold. When con gured as an input, the IC will draw a minimal amount of current
(high impedance), however a protective resistance/capacitance may be used to \debounce" inputs.
It follows that multiple inputs/outputs can be simultaneously controlled from a port: e.g. a 7
segment display can use pins of a 8 pin port, all con gured as outputs. Where the number of pins
available on the port is limited, inputs/outputs may be combined, or certain pins may be used to
select the active device e.g. matrix keypads/displays.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 5
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 6
Review Exercises
1. PIC16F877 I/O ports have two associated registers TRISx and PORTx. Pins 5 and 6 of a
particular port are connected to LED’s as shown in the diagram.
If the value in the TRISx register is 0x3F and the value in the PORTx register is 0x4F then:
(a) the LED connected to port pin 5 is ON and the LED connected to port pin 6 is OFF.
(b) the LED connected to port pin 5 is ON and the LED connected to port pin 6 is ON.
(c) the LED connected to port pin 5 is OFF and the LED connected to port pin 6 is ON.
(d) the LED connected to port pin 5 is OFF and the LED connected to port pin 6 is OFF.
2. A student interfaces an LED (Voltage drop 1.8V, Max. Current 5mA)to the PIC16F877.
Which of the following is NOT an appropriate method of doing so:
(a) tied to 5V through a 100 Ohm resistor
(b) tied to ground through a 1 kilo-Ohm resistor.
(c) tied to 5V through a 1 kilo-Ohm resistor.
(d) tied to 3V through a 220 Ohm resistor.
(e) tied to ground through a 2 kilo-Ohm resistor.
3. A normally open push button switch needs to be interfaced to the PIC16F877. Which one of
the following methods (if any) is NOT a viable alternative:
(a) tie the switch between PORTB pin and ground with internal pull-ups enabled;
switch-o == pin-high
(b) tie the switch between PORTB pin and ground with an external pull-up resistor;
switch-o == pin-high
(c) tie the switch between PORTB pin and VDD with a pull-down resistor to ground;
switch-o == pin-low
(d) tie the switch between PORTB pin and VDD with a pull-down resistor to ground;
switch-o == pin-high
(e) none of the above
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 7
4. We have a circuit in which a switch is connected to PORTB<6> and to ground. If PORTB pull-ups
are enabled, all PORTB pins are con gured as inputs, and no other PORTB pins are connected
to circuitry, what value is placed in the working register by the instruction movf PORTB,W
when the switch is ON?
(a) 0xFF
(b) 0xFE
(c) 0x7F
(d) 0xBF
(e) 0xDF
5. The MPASM assembler has the BANKSEL preprocessor directive, which switches banks to
the bank of the speci ed register.
(a) Write code to con gure PortB for all inputs using the BANKSEL directive.
(b) Using the BANKSEL directive, write an assembly language routine which will
 initialize PORTB<3> as an input, and PORTB<6> as an output.
 within a loop: read PORTB<3> and set PORTB<6> to the value read.
(c) Identify the pins associated with PORTB<3> and PORTB<6> on the PIC16F877 in 40 pin
DIP package.
(d) Draw circuitry to connect a simple switch to PORTB<3> and an LED to PORTB<6>, so
that when your program from 5b is run, the LED will light when the switch is o , and
vice versa.
6. (a) "A certain [PIC16F877] system needs to be able to both activate eight LEDs and to
read the state of eight normally-open (N.O.) push switches. It has been proposed that
... Port B might be able to combine these functions { the former when set to output
and the latter when set to input. Can you devise a suitable circuit?" (Katzen 2003; Q.
11.4, p. 299)
(b) For the circuit you devised, write a program which will repeatedly read the switches,
and display the two’s complement value of the equivalent byte on the LED’s.
7. A dairy farmer has asked you to develop an "intelligent" milk vat which can report the
level, pH and viscosity of the liquid it contains, when it is remotely polled from a "master"
computer. Each reading should also be displayed as a histogram on a 10 bit LED bar display.
You are given linear sensors which each generate a voltage between 0-5V for ranges of (0 -
100%full, pH 10 - pH 4, 0 - 50 mPas), and a PIC16F877 with a 4MHz oscillator.
(a) Draw appropriate diagrams (and specify components and pin numbers where necessary)
for circuitry such that:
 The three sensors are connected to analog channels AN0, AN1, AN2.
 The bar displays are individually turned on by transistors connected to PORTB<3:1>.
 Bar display LED’s are switched on when the relevant output is high.
 The upper two bits of the bar display are connected to PORTB<5:4> and the lower
eight bits of the bar display are connected to PORTC<7:0>.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 8
(b) Use diagrams to describe, and then write a subroutine called send_port which will place
the upper 6 bits of the 10 bit value contained in registers Val_hi<1:0>, Val_lo<7:0>
onto the Parallel Slave Port <5:0>, without altering bits <7:6> of the Parallel Slave
Port.
(c) Use diagrams to describe, and then write a subroutine called display which will display
the 10 bit value contained in registers Val_hi<1:0>, Val_lo<7:0> as a histogram or
"moving bar" on the active LED bar (your subroutine does not need to activate the
bar).
8. You are a member of a development team which is working on an automated parking system
to help people locate their car in the supermarket parking lot.
Persons entering the car park will be given two battery powered wireless parking "cards":
one to be left in the car, and one which they carry while they shop. The pair of cards must
be returned on exiting the car park.
The cards each contain an array of DIP switches which are set to the same ID#.
The carry-card has a button, a circle of 8 LED’s, a wireless transmitter, and an array of 4
directional wireless receivers. When the button is pressed, a query-message containing the
ID# is transmitted.
The car-card has a single wireless transmitter/reciever. When the car-card receives the query
message, it transmits a reply-message.
The carry-card receives reply-message(s) with varying strength on each of the directional
receivers; if a reply-message with the correct ID# is received, the signal strength at the
receiver is recorded. After waiting an appropriate time, the carry-card lights the LED closest
to the receiver(s) which received the reply (i.e. in the direction of the car).
(a) Draw circuit(s) to show:
 how a normally-closed push-button can be interfaced to the PIC16F877 pin PORTB<
0 > so that when the button is pressed, the PIC16F877 detects a high signal.
 how an LED can be interfaced to the PIC16F877 pin PORTD< 0 >, so that it lights
when the PIC16F877 sends a low signal.
Your answers should include details of how you would con gure the PIC16F877 port
pins, and choose appropriate component values.
(b) The 8 LED’s are connected to the pins of PORTD. In order to light a single LED, you
need to write a byte with one bit set to PORTD.
Your team is discussing whether to generate this byte from the bit # by using a lookup
table, or by rotating the number 0x01 the appropriate number of bits.
Write subroutines which will take the bit# in the working register, and return the
appropriate byte in the working register, using each of these methods. Your code should
return 0x00 if the number passed in the working register is not between 0x00 and 0x07.
(c) You are using a module independent of the PIC16F877 to detect the RF ID# for each
receiver. Each of these modules has an output that will stay high for a time proportional
to the signal strength received. The 4 module outputs are tied to PORTB< 7 : 4 >.
i. Describe using a diagram how you will determine which LED needs to be lit based
on the outputs of the 4 modules.
ii. Implement this algorithm using polling on the PIC16F877.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 9
9. You would like to interface a keypad to the PIC16F877 as shown in the  gure below.
(a) Write a routine called KeyInit which will con gure PORTB with pull-ups enabled,
RB0:RB3 as inputs, RB4:RB7 as outputs, and clear PORTB. You should return at the
end of your routine.
(b) After this initialisation routine has been executed:
i. if no keys are pressed, what value will be transferred to the working register by the
instruction movf PORTB,W.
ii. if the ’5’ key is pressed, what value will be transferred to the working register by
the instruction movf PORTB,W.
iii. identify the three other keys that could be pressed to give the same answer as in
9(b)ii
(c) Presuming that internal pull-ups were not available, how could we interface a keypad to
the I/O port, so that the keypad would function in the same way? Design an appropriate
circuit showing one of the keypad buttons. Give adequate consideration to the voltage
and current requirements/loads of the PIC16F877.
(d) The keypad columns must be individually activated in order to uniquely determine which
key is pressed. Write a routine called KeyScan which loops through all four columns until
a keypress is detected, and then returns with the value from PORTB in the working
register.
(e) Draw up a table with the following headings in your exam booklet:
 Key Pressed
 ASCII value
 KeyScan return value
and  ll it in for all 16 keys.
ASCII values: ’*’ 0x2A; ’+’ 0x2B; ’-’ 0x2D; ’/’ 0x2F; ’.’ 0x2E; ’=’ 0x3D; ’0’ to ’9’ 0x30
to 0x39
(f) Comment on how your routine KeyScan will work if two keys are pressed simultaneously.
(g) Suggest a suitable strategy for converting the KeyScan values to ASCII codes, and write
a new routine KeyConvert which will convert the value in the working register, and
return the converted value in the working register.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 10
10. You have been asked to design a PIC16F877 based tra c light controller for an intersection.
Requirements
 There are 2 sets of tra c lights. One facing North/South and the other facing East/West.
Each set consists of a pair of units. Each unit has a red, yellow, and green light. Each
light requires 0.5A, and runs at 12V. Lights of the same color in a set are wired in parallel
with each other.
 Six output lines (North/South PORTD<7:5>; East/West PORTD<2:0>) from the PIC16F877
are used to switch the lights on/o .
 The tra c lights are controlled by four re ective infrared sensors which detect waiting
cars. The sensor deliver a 5V signal when a car is detected, and 0V when no car is
detected. The sensors are connected to PORTB<7:4>.
 An analogue light sensor is connected to the A/D converter channel 0. which is con g-
ured for a left justi ed result. During the night, ADRESH falls below 0x80.
 During the night, the tra c lights switch (in either direction) if a car has been waiting
for more than one (1) minute, otherwise they remain green in the same direction.
 During the day, the lights are green for 5 minutes in one direction before switching.
 The sequence of lights is red, yellow (10 seconds), green, yellow (10 seconds), red.
 The tra c light controller also acts as a survey system counting the number of cars that
pass each sensor. The four 16-bit counts are written to the data EEPROM, and then
cleared, every half-hour.
Conceptual Design
main: configure PORTD pins for outs (lights)
configure A/D channel 0
configure PORTB pins for ins (sensors)
loop: if daytime
for 5 minutes
for each sensor
if change from 0 to 1,
increment car count
switch lights
else
for each sensor
if change from 0 to 1,
increment car count
if waiting more than 1 minute
switch lights
if 30 minutes have passed
write counts to EEPROM
goto loop
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 11
(a) Design a circuit to interface a PIC16F877 to a pair of tra c lights (e.g. Red on
North/South to PORTD<0>). You should explain/show calculations for component val-
ues.
(b) Write a routine named LgtSwitch that will switch the lights. You should return at the
end of your routine.
(c) i. Draw a diagram to explain how to increment a 16 bit integer number, stored in
successive bytes, using indirect addressing.
ii. Suggest an alternate method of storing counts that would require less than 16 bits.
What are the disadvantages of your method?
(d) Write a routine named CntStore which stores the four 16-bit counts in the data EEP-
ROM. You should return at the end of your routine.
(e) Write an instruction-based delay routine named dlym10 which will supply a delay, in
multiples of 10 seconds, based on the value in the Working register (e.g. calling this
routine with a value of 2 in the working register will supply a 20 second delay).You
should return at the end of your routine.
Your answer should explain how you determined/veri ed all values, presuming that the
PIC16F877 is being driven by a 4MHz clock signal.
(f) Write a routine named RdAD which will start, wait for, then read the A/D converter
Channel 0, and return the value of ADRESH in the working register.You should return
at the end of your routine.
(g) Identify and explain one way in which the counter/timer peripheral could be used in this
application.
(h) Criticise the system you have just produced, and subsequently suggest at least two
improvements.
(i) Write an assembly language program for the PIC16F877 which performs the algorithm
provided. Your program should indicate where the preceding routines RdAD, CntStore,
dlym10, LgtSwitch, are to be placed, but you do NOT need to rewrite them.
You should also indicate where the con guration routine for the ports and A/D converter
Cfg should be placed, but you do NOT need to write this routine.
You should also indicate where the ISR, and handler for PORTB should be placed, but
you do NOT need to write them.
Your main program should use appropriate preprocessor directives.
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Tutorial Exercise 7AO ine Version32 ID#
The Bank of Nowhere has recently installed a Passive Infra-Red sensor at the doorway to their
bank vault. The sensor generates a 5V signal when a person is standing in front of the doorway,
and 0V when no-one is at the doorway. The bank wishes to use a PIC16F877 to ring a bell for
as long as someone is standing in the doorway. The sensor is connected to PORTB<0>. The bell is
connected to PORTC<2> via some interface circuitry. The bell rings when it receives a signal greater
than 3V, and draws a peak current of 100mA.
1. Draw an appropriate circuit to interface the bell to the PIC16F877 pin PORTC<2>. Explain
why you chose/designed this circuit. 4 marks
2. For your circuit, write a routine called RingChg which will toggle PORTC<2>. Your code should
be appropriately commented. 3 marks
3. Write a code snippet which will repeatedly reads the pin PORTB<0> and call RingChg ONLY
when a person enters/leaves the doorway. Your code should be appropriately commented. 3 marks
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
32Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
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ECNG2006 (EE25M) March 11, 2008 VI 13
T
U
T
7
AUnit 18 Write the letter you have been assigned here .
A: Count the number of times a single pole double throw switch is closed.
B: Turn an output pin on/o when a non-latching push-button is pressed.
C: LED  ashes at a constant frequency
D: Speaker beeps at a constant frequency
For the task which you have been assigned:
1. Draw the circuitry you would use
2. Describe how the software would work ( ow charts or code snippets may be used)
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ identify alternate ways in which components may be interfaced with a microprocessor
(i.e. common idioms for interacting with hardware): single bit (with debounce), matrix,
map into memory space, etc.
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
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19 Interrupt Basics
At the end of this unit the student will be able to:
explain the operation of interrupts, the di erent types of interrupts (h/w, s/w), how
interrupts may be prioritized (peripheral interrupt controller), and identify common
applications of interrupts.
Computer systems do not exist in isolation but are usually interfaced to the external world via I/O
devices. The problem with I/O devices is that they are usually much slower than the computer
system, therefore it is necessary to synchronize the interactions between the I/O devices and the
computer system. With an input device, such as a keyboard, there will be times when the CPU
has to wait for new data to be ready. Only when the data is ready, can it be read by the CPU
and processed. If the time to create new data is longer than the processing time then the system is
said to be I/O bound. If the data is ready before the CPU is ready, then the system is CPU bound.
Systems can be unbu ered where the I/O writes/reads directly to the CPU or bu ered where a
bu er or memory storage exists between the I/O device and the CPU. There are several ways
through which synchronization can be accomplished.
 Blind cycle
 Busy polling
 Interrupts
 Direct Memory Access (DMA)
Blind Cycle
This type of synchronization is appropriate when the delay due to I/O is  xed and known. It
is called blind because it provides no feedback from the I/O device back to the computer. For
example, if a printer can print at 40 characters/sec but cannot inform the computer when the last
character has been printed. Figure 10 shows a diagram of how this synchronization is achieved.
Blind cycle synchronization is simple and predictable, however if the output/input rate is variable
Send character to printer
Start
Wait 25 ms
Figure 10: Blind cycle  owchart for a printer
or unknown then this method cannot be used. Computing time is wasted while the CPU is waiting
and, because no feedback is available, error checking cannot be performed nor can special conditions
be handled.
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Busy polling
With this method of synchronization, the I/O device has a status bit that can be checked by the
CPU to determine if data transfer can occur. Figure 11 shows how this type of synchronization
is achieved. Busy polling can accommodate variable or unknown output/input rates, however the
Send character to printer
Start
printerready? No
Yes
Figure 11: Busy wait  owchart for a printer
CPU still wastes time waiting for the I/O device to become ready. Therefore, the CPU is unable
to do any other task while polling is taking place. A better method is to have the external device
signal the CPU when servicing is required i.e. interrupts.
Direct Memory Access (DMA)
All the interfaces, seen so far, between the I/O device and the CPU have been controlled by software.
In this type of control, if it is required to transfer data from an input device to RAM, the data
must  rst be transferred to the CPU registers, then from the registers to RAM. Performance can
be greatly improved if the data can be transferred directly between I/O devices and RAM. Direct
Memory Access (DMA)allows this type of direct transfer without CPU intervention. DMA can be
read or write and in both cases, the CPU is halted and the data transferred between memory and
I/O device. The PIC16F877 does not support DMA.
Interrupts
Interrupts are a way of signaling the CPU in an asynchronous fashion that servicing is required.
When an interrupt occurs the CPU  nishes execution of its current instruction, saves any needed
registers and the program counter (PC) on the stack and transfers execution to the Interrupt Service
Routine (ISR). The CPU then executes the ISR and when  nished, restores the PC and continues
execution of the main program. This sequence is shown in Figure 12. The CPU is only interrupted
when a service request is received, so no waiting is takes place and useful work can be done between
interrupts. Interrupts can be vectored which means that each interrupt has a unique interrupt vec-
tor, and therefore a separate ISR. With vectored interrupts, the CPU will automatically execute
the appropriate ISR when an interrupt occurs and there is no need to determine which device gener-
ated the service request. PC’s have a standard interrupt vector structure. An interrupt subroutine
may be "installed" by changing the instruction at the appropriate vector. The PIC16F877 does not
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ECNG2006 (EE25M) March 11, 2008 VI 16
main ISR
PC
Stackpointer
(a) Before interrupt
main ISR
PC
Stackpointer
savedregistersold PC
(b) During interrupt handling
main ISR
PC
Stackpointer
(c) After interrupt handling
Figure 12: Main memory and stack when an interrupt occurs
support multiple vectored interrupts, but the same e ect can be achieved by polling the interrupt
sources to determine who raised the interrupt and then executing an appropriate sub-routine.
When multiple interrupts occur simultaneously, the CPU must have some way of determining
which interrupt to service  rst, i.e. which interrupt has priority. Further most CPU’s have a
single interrupt line, which is shared by all the interrupt sources. To implement hardware priority
ordering, a peripheral interrupt controller (PIC) may be used. This is in turn a peripheral, which
accepts all the hardware interrupt signals, and asserts the CPU interrupt line i an enabled signal
is received. The PIC control registers are used enable signals, and set their relative priorities. The
PIC data registers report which of the many signals have been exerted, and the location of the
ISR vector. If a PIC is not used (or does not support priority ordering) then the priority of the
interrupt is determined by the order in which the interrupt sources are polled in the ISR, and the
sequence in which the interrupts are enabled/disabled.
The interrupts which we have discussed, are all triggered by a hardware signal. It is also possible
to trigger an interrupt handler from software (typically using a special interrupt instruction). In
essence, this is the same as a subroutine call. Typically hardware interrupts are generated by
peripheral reporting task completion or requesting CPU service. Software interrupts may be used
to access BIOS (and other low level) functions, as well as to allow programs to access shared
memory and peripherals in a structured or \known" fashion.
Software interrupts are not supported by the PIC16F877.
The PIC16F877 family has 14 sources of interrupts, including Timer Over ow, and
signals/changes on Input Ports. The interrupt control register (INTCON) records in-
dividual interrupt requests in  ag bits (Note that the  ag bits will be set whether or
not the interrupt is enabled). It also has individual and global interrupt enable bits. A
global interrupt enable bit, GIE (INTCON<7>) enables (if set) all unmasked interrupts,
or disables (if cleared) all interrupts. When bit GIE is enabled, and an interrupts  ag
bit and mask bit are set, the interrupt will vector immediately.
Individual interrupts can be disabled through their corresponding enable bits in various
registers. Individual interrupt bits are set, regardless of the status of the GIE bit. The
GIE bit is cleared on RESET. The \return from interrupt" instruction, RETFIE, exits
the interrupt routine, as well as sets the GIE bit, which re-enables interrupts.
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ECNG2006 (EE25M) March 11, 2008 VI 17
The RB0/INT pin interrupt, the RB port change interrupt, and the TMR0 over ow
interrupt  ags are contained in the INTCON register. The peripheral interrupt  ags
are contained in the special function registers, PIR1 and PIR2. The corresponding
interrupt enable bits are contained in special function registers, PIE1 and PIE2, and
the peripheral interrupt enable bit is contained in special function register INTCON.
When an interrupt is responded to, the GIE bit is cleared to disable any further in-
terrupt, the return address is pushed onto the stack and the PC is loaded with 0004h.
Once in the Interrupt Service Routine, the source(s) of the interrupt can be determined
by polling the interrupt  ag bits.
It is the responsibility of the ISR to clear the interrupt  ag bit corresponding to the
interrupt that caused the ISR to be executed, otherwise the CPU would assume that
another interrupt of the same type had occurred during execution of the ISR. The
interrupt  ag bit(s) must be cleared in software before re-enabling interrupts to avoid
recursive interrupts.
For external interrupt events, such as the INT pin or PORTB change interrupt, the
interrupt latency will be three or four instruction cycles. The exact latency depends
on when the interrupt event occurs. The latency is the same for one or two-cycle
instructions. Individual interrupt  ag bits are set, regardless of the status of their
corresponding mask bit, PEIE bit, or GIE bit.
If interrupt occurs during the execution of the ISR it is not serviced until after the ISR
completes and the GIE bit re-enabled. At this point, if the postponed interrupt’s enable
bit is set, it is serviced. { (PIC 2001)
Timing & Timers
One use to which the PIC can be put to is to control the times when events happen. A typical
example is reading a port or A/D at speci ed intervals. If polling or blind cycle synchronization is
used then what is required is some means of generating a delay to control the loop that reads the
port or A/D.
There are several ways in which  xed delay times can be obtained. One of the most straightforward
is by using the times taken by instructions to create the delay. Another is to use the built-in timer
facility available.
It can be assumed for the rest of this discussion that a 10 ms delay time is required. If the clock
frequency is 4 Mhz, then the number of cycles required is 10,00033. Let us see how to generate
smaller delays, for example 1 ms. This can be done by counting down a value in the W register
until it becomes zero. A  rst pass at the delay routine can be:
delay_1ms movlw <unknown>
:Loop4 addlw 0xFF ; add -1, cannot use decrement
btfss STATUS,Z
goto :Loop4
return
33The cycle frequency is the 1/4 the clock frequency
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Now to count how long it takes to do the :Loop4 loop. The addlw instruction takes 1 cycle, the
btfss takes 1 and the goto takes 2. So a single traversal of the loop takes 4 cycles. The last
traversal of the loop will take one more cycle when the bit test jumps to the return. Since it is
only one cycle in a millisecond, the error is small. The  nal part is the setting of the <unknown>
value.
The call to the delay routine will take 2 cycles and the loading of W will take another cycle, and
since the loop takes 4 cycles, we need to round out the overhead to 4 cycles and compensate for it
in the initial value of W.
The  nal routine that generates 1 ms delay is show below
delay_1ms movlw 0xF9 ; 0xF9 = 249
nop
usec4 addlw 0xFF ; add -1, cannot use decrement, why?
btfss STATUS,Z
goto usec4
return
This routine can be expanded through the use of a variable to allow times in multiples of 1 ms.
For example
; routine is passes the amount of ms to delay in W ; it uses an
additional register variable msec_count
delay_ms movwf msec_count
msecloop movlw 0xF8 ; 0xF8 = 248
call usec4 ; 248 * 4 + 2 (call) = 994
nop
nop
decfsz msec_count,F
goto msecloop
return
The overhead for the call to delay ms is neglected as well saving W in msec count.
Go through the routine carefully making sure that you understand its operation.
Using built-in timers
The Timer 0 of the PIC will generate an interrupt, if enabled, when an over ow occurs. This can
be an advantage over the previous type of delay because the CPU can be doing other useful work
during the waiting. In non-time critical applications, the use of ISR’s are not necessary and simple
polling will su ce. It is fairly straightforward to generate small delays by initializing Timer 0 to
speci c values and letting it timeout while continuously checking for the timeout. Normally Timer 0
runs continuously whenever the PIC is turned on, but there may be times when the user may want
to set the time when it starts. This can be done by ensuring that the RA4/T0CK1 pin is grounded
and setting Timer 0 in counter mode. Since the input to the counter is zero, no counting will take
place and initialization of Timer 0 can take place without timing being started. When timing is
required Timer 0 can then be switched to timer mode.
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Simple timing (no interrupts)
The following section of code illustrates how simple timing can be done.
Init bsf STATUS,RP0 ; bank 1
movlw B’00100110’ ; set in counter mode, prescaler = 128
movwf OPTION_REG
bcf STATUS,RP0 ; back to bank 0
bcf INTCON,T0IF ; clear to be sure
.
call Delay
.
Delay movlw 106 ; time for 256 - 106 = 250 cycles
movwf TMR0
bsf STATUS,RP0 ; switch to bank 1 to access OPTION_REG
bcf OPTION_REG,T0CS ; start timing
bcf STATUS,RP0
Loop btfss INTCON,T0IF ; keep checking for overflow
goto Loop
bcf INTCON,T0IF ; clear flag
return
Writing an Interrupt Service Routine (ISR)
The PIC16F877 has only one interrupt vector, located at 0x004, and as a result, polling must be
done at the start of the ISR to determine which interrupt source generated the interrupt. Once the
source is determined the appropriate handler can be executed. Within the handler, the interrupt
 ag bit must be cleared before re-enabling interrupts. Using the template given previously, an ISR
designed to handle Timer 0, external and Port B change interrupts looks like the following.
ISR movwf w_temp ; save W and STATUS
movf STATUS,W ;
movwf status_temp ;
Poll btfsc INTCON,TOIF ; test if TMR0 overflow occurred
call Timer_hndlr ; call handler for TMR0
btfsc INTCON,INTF ; test if external interrupt occurred
call Extern_hndlr ; call handler for external interrupt
btfsc INTCON,RBIF ; test if PORTB change interrupt occurred
call Change_hndlr ; ...etc
ISRDne movf status_temp,W ; restore pre-isr STATUS register
movwf STATUS ;
swapf w_temp,F ; restore pre-isr W register contents
swapf w_temp,W ; ... without affecting the zero flag
retfie ; return from interrupt
Timer_hndler
bcf INTCON,TOIF ; clear the overflow flag
; do processing
return
Extern_hndlr
; clear appropriate flag etc
return
Change_hndlr
; clear appropriate flag etc
return
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Simple timing (with interrupts)
If a 10 ms delay is required then there is no combination of TMR0 and prescaler that will give
that time repeatedly so other methods must be devised. It can be seen that using a prescale value
of 8, Timer 0 will over ow every 2.048 ms, and counting this  ve times will give 10.24 ms or
approximately 10 ms. The following routine illustrates how this can be done.
COUNT EQU 0x05
clrf cntr
.
Delay btfss INTCON,T0IF
goto Delay
bcf INTCON,T0IF
incf cntr,F
movlw COUNT
xorwf cntr,W
btfss STATUS,Z
goto Delay
clrf cntr
return
Instead of polling the T0IF  ag, an ISR can be used that is triggered by the over ow interrupt,
in addition the counter, cntr is changed by the ISR. So the routine can now be broken into two
parts and also can be made simpler by doing the check for the  ve times over ow of TMR0 by
decrementing instead of incrementing cntr.
COUNT equ 0x05 ; ISR
; initialization of ISR etc
Poll btfsc INTCON,T0IF
goto Timer
btfsc INTCON,.
.
Timer decf cntr
bcf INTCON,T0IF
goto Poll
; end of ISR
; Main routine
Init movlw COUNT
movwf cntr
.
Delay btfss cntr,7
goto Delay
movlw COUNT
movwf cntr
return
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More precise timing
The time delay achieved in the preceding section is only an approximation to the desired value of
10 ms which means that TMR0 must over ow every 2 ms. Now the total count in TMR0 with a
prescaler of 8 is 256 8 = 2048 and 2000 = 256 8 6 8. Therefore if TMR0 is incremented by
6 every time it over ows then the cycle time will be 2 ms. This can be done in the Timer handler
of the ISR, for example,
Offset equ 6
Timer movlw Offset
addwf TMR0,F
decf cntr
bcf INTCON,T0IF
goto Poll
Note that whenever a write is done to TMR0, it waits for two cycles to re-synchronize before
counting starts again. In this case the wait is not signi cant and the error in 10 ms is only 0.1%.
Review Exercises
1. Label the following statements about interrupts True/False:
(a) Interrupts may be triggered either by an electrical signal, or a special software instruc-
tion.
(b) Before handling an interrupt, all processor registers which may be a ected by the handler
must be saved, so they can be restored later.
(c) When an interrupt occurs the instruction which is being processed is stopped, and re-
executed after the interrupt has been handled.
(d) It is possible for an interrupt handler to be interrupted by another interrupt.
(e) Interrupt prioritisation may be achieved either by using a dedicated peripheral, or by
checking the interrupt sources in a pre-determined order.
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2. All PIC16F877 ISR routines must save the working register before anything else is done. This
is because:
(a) using the retfie instruction will corrupt the working register.
(b) after 10 instruction cycles without use, the Working register will lose it’s value.
(c) the ISR routine is likely to change the Working register, and the main program needs
the original value.
(d) we can’t poll the interrupt sources unless the Working register is empty.
(e) none of the above
3. Choose the word combination which best completes the following sentence(s):
The interrupt 3I is the address of the instruction executed when the interrupt
occurs. Interrupts are typically used when the 3II needs to communicate information
to the 3III .
(a) 3I: service routine; 3II:CPU ; 3III:peripheral
(b) 3I: vector; 3II:CPU ; 3III:peripheral
(c) 3I: service routine; 3II:peripheral; 3III:CPU
(d) 3I: vector; 3II:peripheral; 3III:CPU
(e) 3I: handler; 3II:peripheral; 3III:CPU
4. (a) In your own words, explain what happens when an interrupt occurs, and when the main
program resumes.
(b) In your own words, explain what is meant by saving and restoring the "context", and
why these processes are needed in interrupt procedures.
5. (a) Di erentiate between hardware and software interrupts.
(b) Which is preferable: software or hardware prioritisation of interrupts? Justify your
answer.
(c) Student A tells Student B that hardware interrupts are interrupts that are prioritised
using hardware. Is student A correct? Explain your answer.
6. (a) What is the reason for having the interrupt sources in a particular order?
(b) When the ISR is entered in response to one speci c interrupt, is it possible that a
di erent source might be serviced  rst? Discuss your answer.
(c) If while one interrupt source is being serviced, a second interrupt source requests service,
will the second source be serviced before the retfie instruction at the end of the ISR?
Discuss your answer.
7. Identify one source of interrupts on the PIC 16F877 (apart from the timers), and locate the
 ag and enable bits for your chosen interrupt source.
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8. \The speed of a rotating shaft can be measured by using a coded disk to generate a pulse on
each angular advance of 10 , which can be used to interrupt a PIC. If the top speed is 20,000
revolutions per minute, what is the absolute maximum duration of the ISR in this worst case
situation to avoid missing pulses? You may assume a crystal frequency of 4MHz. "(Katzen
2003; Q7.4 p.193)
Where code is requested, it is su cient to simply include the relevant snippets/fragments. Use
comments to indicate any assumptions you make about the rest of the code.
9. In a typical ISR, part of the initialization saves the W and STATUS registers, however this
assumes that the main code that will be interrupted will never be in Bank 1 at the time of
the interrupt. If the ISR ever switches banks then the values of W and STATUS that are
restored would be incorrect34. In processors where the Bank 0 addresses are di erent from
the Bank 1 addresses several things can be done to relax this assumption.
 Interrupts can be disabled before switching to Bank 1 and re-enabled after switching
back to Bank 0.
 Use two addresses, one in each bank, to hold the temporary value of W, i.e. W_TEMP. For
example,
W_TEMP equ 0x20
W_TEMP_ equ 0xA0
Now when W is stored in W_TEMP using direct addressing at the beginning of the ISR it
will go in either of two locations: 0x20 or 0xA0, depending on which bank is active at
the moment the interrupt occurs.
(a) Rewrite the initialization of the ISR that saves W and STATUS in this case, switching
to Bank 0 for the duration of the ISR. There would be need only for one location for
STATUS_TEMP de ned in Bank 0.
(b) Rewrite the ending of the ISR to restore the STATUS and W.
10. You are responsible for writing an application for a PIC16F877 with an 8MHz external clock
signal, which reads 8-bit information from a device. The device is con gured so that the most
recent byte appears on PortB every 40ms, and stays there until the next byte is available.
When the byte changes, the device raises the READY signal line for 5ms and then drops it.
(a) In your own words, explain the di erence(s) between using blind cycle polling, busy-
wait polling, and interrupts to accomplish your task.
(b) In your own words, explain how indirect addressing may be used to implement a
circular bu er.
(c) For each of the three approaches(blind cycle polling, busy-wait polling, and interrupts)
 explain where the READY signal line needs to be connected,
 write a program for the PIC16F877 which will store the 8 most recently read bytes
in a circular bu er,
 determine the number of program memory locations needed to store the code and the
number of data memory or register locations required to store constants/variables.
34In the PIC16F8x, Bank 1’s General purpose registers are mapped to Bank 0, so that this argument does not hold
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Tutorial Exercise 7BO ine Version35 ID#
The Bank of Nowhere has recently installed a Passive Infra-Red sensor at the doorway to their
bank vault. The sensor generates a 5V signal when a person is standing in front of the doorway,
and 0V when no-one is at the doorway. The sensor is connected to PORTB<0>. The bank wishes to
maintain a count of the number of people who ARRIVE at the vault doorway.
1. Write a routine called Init which will enable the appropriate interrupts. Your code should
be appropriately commented. 4 marks
2. Write an interrupt handler which will increment a register named Cnt when a person arrives. 6 marks
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
35Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
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ECNG2006 (EE25M) March 11, 2008 VI 27
T
U
T
7
BUnit 19 Write the letter you have been assigned here .
A: Count the number of times a single pole double throw switch is closed.
B: Turn an output pin on/o when a non-latching push-button is pressed.
C: LED  ashes at a constant frequency
D: Speaker beeps at a constant frequency
For the task which you have been assigned:
1. Describe one way in which interrupts could be used to accomplish your task.
2. Identify 2 issues you need to consider in order to decide if using interrupts is appropriate for
your task.
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ explain the operation of interrupts, the di erent types of interrupts (h/w, s/w), how
interrupts may be prioritized (peripheral interrupt controller), and identify common
applications of interrupts.
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
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20 Typical peripherals
At the end of this unit the student will be able to:
explain the operation of commonly used peripherals: PPI/PIA, A/D, Timers/Counters,
PWM, D/A.
In the previous units we looked at the operation of two peripherals on the PIC16F877, the port
and the timer. Peripherals share the following characteristics:
 they have internal latches/registers which can be accessed by the microprocessor. These reg-
isters and latches are of various types: control (telling the peripheral what to do), con gu-
ration (telling the peripheral how to do it), state (reporting what the peripheral is presently
doing) and data (containing the data the peripheral must send/recieve to the microprocessor)
 they are able to indicate their state to the microprocessor through the use of  ags (internal
or maintained by a peripheral controller), or output signals.
 (in conjunction with a PIC) they are able to trigger maskable interrupts i.e. the interrupt
signal may be individually enabled/disabled.
There are a variety of peripherals which are typically used in conjunction with microproces-
sors(Horowitz and Hill 1989; 11.11 Peripheral LSI chips). In this unit we will look at the methods
of implementation, and typical operation of these peripherals.
Please note that all peripherals operate asynchronously w.r.t. the microprocessor i.e. they are not
guaranteed to respond/perform the operation within an instruction cycle, and timing should be
carefully considered in your application. In particular, sampling/generation of rapidly changing
signals will be limited.
Parallel I/O Ports
There is a data register which may either be read (to re ect input values) or written to (to set
output values). If the port can be direction con gured:
 there will be some method of specifying which bits are input and which are output. This may
vary from a data direction register, where each bit re ects the direction of the corresponding
port bit, to bits within a register which indicate a pre-de ned pattern of inputs/outputs.
 there will be two separate data registers, one for input, and one for output. These registers
are frequently mapped to the same internal address for external access.
Ports may also contain con guration registers for interfacing circuitry (e.g. internal pull-ups/ground).
Interrupts generated by ports are generally used to indicate a change of state on the port (i.e.
inputs have changed)
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Timer/Counter
This peripheral has an internal register to hold the present value, and two inputs: one which
increments the register value, and the other which resets the register (note that the register reset
value may not always be 0). Generally, over ow of the internal register also triggers a reset.
Optionally, the value of the register prior to reset is latched into another internal register. Both,
or just one of the above registers may be externally accessible.
The peripheral as described is dual-purpose: When the increment is connected to a regular oscil-
lating signal, and the reset connected to an irregular signal, the register will increment at regular
intervals; indicating the amount of time passed between resets. When the signals are reversed, the
register will count the number of cycles occurring in a particular interval. (Horowitz and Hill 1989;
15.10 Frequency, period and time-interval measurements)
The current state of the counter/timer (running, stopped, over ow occurred) is generally speci ed
by  ags in an accessible internal register.
Control registers associated with the timer/counter peripheral, start/stop the counter, and may
determine the reset value. Con guration registers optionally choose scalers (these are divide-by
counters which reduce the counted/reported values), enable or disable default signals, choose to
count in one-shot or continuous modes.
Watchdog timers
A watchdog timer is a special timer which has pre-determined increment and reset inputs, no
accessible internal registers, and outputs a signal on over ow (rather then  ag and reset). The
increment input is generally the microprocessor clock signal (possibly scaled), and the reset input
must be tied to an instruction generated signal.
Control & Con guration registers enable/disable the timer, and choose the increment source and
the scale values (where appropriate).
A/D and D/A conversion
The purpose of A/D conversion is to generate a digital value, which is proportional to, the ana-
log o set from a known reference. The analog value which needs to be converted is generally
a DC voltage; however DC current, and AC current, voltage, frequency levels may also be con-
verted. The purpose of D/A conversion is to generate an analog value (DC current/voltage; AC
current/voltage/frequency), whose o set from a known reference, is proportional to the original
digital value.
A/D and D/A conversion involve some element of time over which the analog signal must be kept
constant, therefore the control registers must make provision for starting/stopping the sampling
and/or holding of the analog signal. A/D and D/A conversion peripherals have con guration
registers which may be used to specify the reference input(s) to be used and the time associated
with signal acquisition. They will also have data registers which contain the digital value, and the
current peripheral state: i.e. (acquiring), converting, conversion done, (holding). The resolution of
the A/D or D/A process will determine the e ective bit-width of the data, and hence the number
of data registers required to hold the result.
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Please note that resolution and accuracy are not the same thing.
"Accuracy is often confused with resolution. Resolution relates the smallest signal(or
noise) to the full-scale value. Accuracy relates the smallest signal to the measured
signal. Accuracy is given as a percent and describes how close the measurement is to
the actual value. The signal is accurate to within VRESOLUTIONVSIGNAL  100%" { (Cady 1997;
p 207)
Another characteristic property of A/D and D/A converters, is their linearity, i.e. how well they
approximate a straight line relation between analog and digital values.
A/D and D/A conversion circuits may be based on several principles. The most commonly used
with microprocessors are those that deal with DC voltage as the analogue signal. Popular types
include: successive approximation A/D, dual-slope (charge{discharge) A/D, parallel ( ash) A/D
converters (Horowitz and Hill 1989; 9.20 Analog-to-digital converters pp. 622-624), scaled (binary
weighted) resistors D/A, and R-2R ladder D/A (Horowitz and Hill 1989; 9.16 Digital-to-analog
converters). One  nal form of D/A which we will consider separately is Pulse Width Modulation
(PWM).
PWM
This frequency based system is used for data transmission as well as D/A conversion.
"In this technique the digital input code is used to generate a train of pulses of  xed
frequency, with width proportional to the input count ... a simple low pass  lter can be
used to generate an output voltage proportional to the average time spent in the HIGH
state i.e. proportional to the input code .... this sort of D/A conversion is used when
the load is itself a slowly responding system; the pulse-width modulator then generates
precise parcels of energy, averaged by the system connected .." (Horowitz and Hill 1989;
p.618)
Examples of usage are to control motor speeds, and display brightness.
Given a particular PWM frequency, PWM has resolution (like any other D/A or A/D technique)
i.e. the smallest possible change in pulse width. Where the digital value has n possible values, the
best use of PWM will map a change of 1 in the digital value, to the smallest possible pulse width
change (PWM periodp; smallest possible change pn).
PWM peripherals typically o set the digital range, to make use of the fact that a digital value of
0, is equivalent to turning o PWM. This means that the maximum expressible digital value, can
be used to represent a constant "on" (pulse width=PWM period).
Frequency sampling and re-construction
The only signal a port pin is capable of directly outputting is a square wave. The maximum
frequency at which a signal which can be output from a port or other peripheral depends not only
on the processing/response time of the peripheral, but also on the speed with which the voltage
rises/falls on the pin. While this is primarily determined by the pin circuitry, it is also in uenced by
the load which the output pin is driving. The signal which is output will have frequency components
determined by the rise/fall times and the nature of the load (i.e. not a perfect square wave).
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If a continuous signal is to be generated using a D/A converter, please note that the hold time
will also limit the maximum frequency of the constructed signal (period >= 2*hold time), and
introduce high frequency components which can be removed using a low-pass  lter.
The maximum frequency at which change can be detected on an input pin will depend on how
quickly the pin circuitry can respond to the signal. Realistically, however this signal is not recognised
by the microprocessor until the peripheral has communicated with the microprocessor, and this may
further reduce the detection frequency.
If a continuous signal is to be sampled using an A/D converter, the aperture (how long the signal
is sampled over) and conversion (how long the converter takes to operate) times will also a ect the
maximum sampling frequency. The sampling frequency (Nyquist Frequency; Shannon Sampling
Theorem) must be at least twice that of the input signal. A low pass anti-aliasing  lter may be
used on the input to guarantee this.
Programming peripherals
The programs presented so far, have addressed the programming needs of ports, timers and ISR’s
in an ad-hoc fashion. In programs which utilise more than one peripheral, a more rigorous approach
makes it easier to ensure that the peripherals operate correctly.(** optional)
Blind Polling:
Initialise -- configuration
Repeat
**Command -- control
Wait for time
**Read -- data
Busy-wait Polling:
Initialise -- configuration
Repeat
**Command -- control
do
Job
Until -- state
**Read -- data
Interrupts:
Initialise -- configuration
UnMask -- interrupts
Command -- control
Repeat
Job
ISR:
save context
**Command -- control
**Read -- data
**Clear -- Flags
restore context
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Review Exercises
1. (a) Identify the types of registers associated with peripherals, and explain the circumstances
under which each type of register needs to be accessed.
(b) Peripherals may be built into a microcontroller, or external peripherals may be interfaced
via a generic parallel I/O port. Which is better? Why?
2. Choose the word combination which best completes the following sentence(s):
2I Modulation may be considered as a means of 2II .
The frequency/period of the signal is independent of the 2I .
The ratio of 2I to period is referred to as the 2III .
(a) 2I:pulse width; 2II:digital-to-analog conversion; 2III:duty cycle
(b) 2I:pulse width; 2II:analog-to-digital conversion; 2III:duty cycle
(c) 2I:pulse wave; 2II:digital-to-analog conversion; 2III:duty cycle
(d) 2I:pulse width; 2II:digital-to-analog conversion; 2III:cycle time
(e) 2I:pulse width; 2II:analog-to-digital conversion; 2III:cycle time
3. The timer/counter peripheral can be used for all of the following functions except:
(a) counting transitions (lo->hi or hi->lo) on a signal line
(b) estimating frequency of a signal
(c) generating a PWM signal
(d) (integer) frequency division of a signal
(e) determining the time between successive transitions(lo->hi or hi->lo) of a signal
4. The A/D converter sampling time is the sum of the acquisition and conversion time where:
(a) the acquisition time is determined by the circuit properties, and the conversion time is
constant.
(b) the conversion time is determined by the A/D clock, and the acquisition time is deter-
mined by the circuit properties
(c) the acquisition time is determined by the A/D clock, and the conversion time is deter-
mined by the circuit properties
(d) the conversion time is constant, and the acquisition time is determined by the A/D clock.
(e) the acquisition time is constant, and the conversion time is determined by the circuit
properties.
5. (a) In your own words, di erentiate between the accuracy and resolution of an A-D/D-A
converter.
(b) In your own words, explain how PWM may be used as a form of D-A conversion.
(c) "An A/D converter is required to digitize a 1kHz sinusoidal waveform. What is the
maximum allowable conversion time for the A/D? Assume a sample and hold circuit is
being used to give the correct aperture time." {(Cady 1997; 11.8). Explain your answer.
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6. An 8 bit timer/counter is being used with an oscillating signal of 4MHz, and a second source
of indeterminate frequency. The frequency of the second source can be estimated by using
the counter/timer in either the counter or timer con gurations. What are the minimum and
maximum frequencies that can be detected in (a) counter mode, (b) timer mode. Show all
working.
7. Based on (Horowitz and Hill 1989; Exercise 9.4) A 10kHz PWM signal is being output based
on an 8 bit binary value.
(a) What is the length of the PWM period? Explain your answer.
(b) What will the pulse width be if the binary value is 011101102? Explain your answer.
8. For the PIC16F877:
(a) Write an initialization routine so that when a falling edge occurs on the RB0/INT pin,
an interrupt will occur.
(b) Write an RB0 INT interrupt handler that increments a 1-byte variable, CountINT. If
CountINT is incremented to 100, set bit 5 of a variable Flags36, then clear CountINT
and  nish by branching back to the polling routine.
9. On the PIC16F877 we can use Timer0 as a counter to count input pulses on the RA4/T0CKI
pin. Write a program such that Timer0 generates an interrupt when 100 pulses have occurred.
Within the interrupt handler set bit 5 of a variable, Flags and then reinitialize Timer0 so
that another interrupt will occur when 100 more pulses have occurred.
Compare the pros and cons of this approach for counting external events to the approach of
Problem 8.
10. Challenge: For the PIC16F877:
(a) If the PIC is clocked at 10 MHz, TMR0 can be used to obtain a 10 ms loop-time by
counting the number of times it over ows. Write an interrupt handler that uses TMR0
and makes it over ow when 250 cycles have elapsed. When 10,000 interrupts have
occurred then 10 ms would have elapsed. Code a main routine that waits until 10,000
interrupts have occurred.
(b) Why is adding to TMR0 better than loading a value into it?
(c) Note that in Problem (10a), it would seem that adding six to TMR0 after each over ow
changes it into 250 cycle timer. However the normal increment of TMR0 is bypassed by
the addition, suggesting that seven is more appropriate. In addition to which whenever
TMR0 is written to, incrementing is inhibited for the following two instruction cycles
because of the need to re-synchronize the timer with the clock. What would be an
appropriate value to add to TMR0?
(d) The approach used in Problem (10a) makes no use of the prescaler. How can the prescaler
be used to cut down on the number of interrupts per 10-ms interval? What is the e ect
of each write to TMR0 clearing the prescaler on the long-term accuracy of the 10-ms
delay time?
36This is to indicate that 100 input edges have been seen.
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Tutorial Exercise 8AO ine Version37 ID#
The Bank of Nowhere has recently installed a security system at the doorway to their bank vault.
The bank wishes to use a PIC16F877 to ring a bell for as long as someone is standing in the
doorway. The volume of the bell should increase from minimum (after 10 seconds) to maximum
(after 3 minutes). The bell is connected to PORTC<2> via some interface circuitry, such that it is
ON when the pin is at a voltage corresponding to LOGIC-HIGH.
1. Describe how Timer1 could be used to determine how long the person has been standing in
front of the doorway. You may use diagrams if needed. 3 marks
2. Explain how PWM could be used to change the volume of the bell. You may use diagrams if
needed. 3 marks
3. Write a snippet to indicate how the Timer1 value would be used to a ect the volume. You
may assume that all peripherals have been appropriately con gured. Bonus 10 marks for an 4 marks
interrupt-based snippet.
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
37Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
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T
U
T
8
AUnit 20 Write the letter you have been assigned here .
A: Analog to Digital Converter.
B: PWM Peripheral.
C: Digital to Analog Converter.
D: Counter/Timer.
1. Write a sentence to explain the function/operation of the peripheral in a  P based system.
2. Identify 1 timing constraint/speci cation associated with the peripheral.
3. Explain how the peripheral could be replaced by an I/O port and a small program
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ explain the operation of commonly used peripherals: PPI/PIA, A/D, Timers/Counters,
PWM, D/A.
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
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21 Digital troubleshooting
At the end of this unit the student will be able to:
select and apply techniques for troubleshooting digital circuitry using standard laboratory
equipment
When you run into problems with your circuit: {
summarised from http://learnat.sait.ab.ca/ict/digi290_godin/ppt/lab.ppt
1. \Check the obvious" { see common errors
2. \Visually inspect the components and the wiring. Look for clues. Start with points that
contain multiple connections. Check to ensure there are no  oating inputs." {  oating inputs
tend to show up as transient faults. Just because your circuit was working  ne before doesn’t
mean that a  oating input was not the cause. Floating grounds are a particular problem!
3. \Check those things that are the quickest" { you can reduce your troubleshooting time by
doing the quicker tests  rst.
4. \Analyze the inputs and outputs: Using the logic probe, look at the inputs and outputs
starting with those that have the greatest e ect on the circuit. Probe the contacts on the
ICs, not the breadboard." { you can also do this with a scope/meter, and a shielded lead (or
if you have to a bit of wire). Be careful not to short anything!!
5. \Rule of Halves: Where possible, split the circuit in half. ... Apply external signals where
needed."
Common circuit errors {
summarised from http://learnat.sait.ab.ca/ict/digi290_godin/ppt/lab.ppt
See also (Predko 2001; Ch. 14 { PIC Micro MCU Application Debugging Checklist).
Multiple Outputs \Do not wire 2 or more outputs together. This will damage your components
and is not proper practice." { this is a useful rule. On occasion you may wish to drive outputs
simultaneously in order to increase the current drive capability. It is not worth the trouble
... switch using a transistor if necessary.
Driving LED’s \You must use resistors in series with your LEDs. [Minimum] Recommended is
330 ohms. 7-segment displays are especially vulnerable to blow when no resistor is used to
limit current. You must limit current for each segment."
Mis-wired Switches Unless the I/O port is equipped with an internal pull-up to Vdd or pull-down
to Vss, then you should have a resistor in series with the switch and tap o the resistor at the
port.
DIP packaged switches/resistors SIL switches are wired as for single switches. Two way
switches in DIP packages may look the same. Be careful!
Inadvertent component damage \CMOS components are susceptible to damage by static elec-
tricity. Use grounding precautions such as: Static Bags, static foam, etc Grounding straps,
grounded work area, etc". Also when troubleshooting a live circuit with a meter, scope or
probe, be careful not to short circuit any pins with the leads.
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Incorrect power/signal connections Make sure that power and ground are correctly connected
(i.e. no accidental reversal; polarity is correct). Try to use a convention for which rails are
power and ground, and a particular color of wire for external power/signal connections. Make
sure all signals have the correct peak-peak and DC o set voltages, at source and at connection.
Wiring When wiring component packages we often see: o by one; mirrored wiring; chip pin-out
or pin assignment read incorrectly (look for the dot/notch (DIP) or the  at side (T092)).
Input/Output timing The input/output may be set to the right value, but at the wrong time.
Use the scope to make sure that the timing on your signals occur in the correct sequence and
is in-line with the speci cations.
Meeting speci cations \Using the wrong speci cation sheet or device (ICs, capacitors, resis-
tors, etc)", \Applying the wrong input frequency or waveform", "Defective component, bent
contact, or device operating outside of speci cation"
Troubleshooting induced faults You may be testing at the wrong point! Try to test as close
to the signal source as possible, and then work your way out to check the integrity of sig-
nal connections Also note that test equipment can load circuits, and may discharge/charge
capacitors. Choose your test points carefully.
Noise Noise sources can be external, or internal. Common internal sources of noise can be elim-
inated by decoupling lines, keeping wiring runs short, running wiring in parallel (or using
twisted/shielded cables), and isolating high voltage or rapidly switching components.
Inter-Connections For breadboards: \Wires stripped improperly (too short)." { this may leave
the input  oating, or give a partial connection. You should also look out for shorts between
neighbouring breadboard tracks, breaks along a continuous track, and jumpers between rails
that you want tied together. For soldered boards: bad solder joints are often di cult to spot.
Make sure you have continuity across the joint.
Fan-out \Exceeding fan-out, the number of gate inputs an output can handle (use a 1kohm pull-up
or pull-down resistor as a temporary  x)."
In the event that your circuit is functioning as it should, there may be something wrong with
your hardware/software design, or your understanding of how the hardware/software design should
work.
\ Six-step troubleshooting procedure:
 Identifying the problem
 Symptom elaboration
 Identifying the probable faulty functions
 Isolating the faulty function
 Isolating the faulty circuit
 Failure analysis
"{ http://www.but.be/WLN/Industrial%20Computer/DETDE.htm
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In order to follow this troubleshooting procedure, you will need more than the meter and oscilloscope
you used to detect circuit errors. You will need to use your simulator, ICD, and logic analyser as
well. You should also consider using simple indicators/triggers like LED’s and switches to indicate
what is going on when your program is running, and allow you to simulate inputs.
Identifying the problem You are observing some anomalous behaviour ... try to express the
observed behaviour as clearly and unambiguously as possible. For example: \the circuit
doesn’t work" .... is not helpful. \there is a voltage of 1.6V on a pin which is outputting a
low" ... clari es the problem.
Symptom elaboration Does the problem only occur in particular circumstances or is it all the
time? Does the problem appear when you step through the code, and disappear when you
run the code (or vice versa)? The problem appears in which combination of the following: the
simulator, the debug system, the live system? Can you do anything which changes/a ects the
problem temporarily? For example: \It only occurs on the debug system, not the simulation.
The voltage changes when the pot is turned, even though the pot is on another circuit. A
high is transmitted on the pin just  ne."
Identifying the probable faulty functions List the possible causes. For example: \The pin
circuity could be inappropriate, or badly con gured. The system ground could be  oating. I
could be feeding a voltage back to the pin from my circuit. My instruction to put a 0 on the
pin might not be working as I intended it to. The pin might be con gured as an input. "
Isolating the faulty function Eliminate from your list, one at a time, by investigating using all
the tools at your disposal. If you don’t  nd the fault, try the next step or go back to the
previous step! For example: \I haven’t got anything intentionally connected to that output
pin which should feedback. I haven’t got any accidental cross-connections or breadboard
shorts. Even when I disconnect the pin from the circuit, it is still happening. The debug
screen shows a 0 on the pin, and that the pin is con gured as an output. The pin circuitry
looks ok. The system ground tests at 0 Volts on the chip. "
Isolating the faulty circuit (if appropriate) Disconnect the suspect circuitry, and see if the
problem disappears. If it doesn’t go back to the previous steps! For example: \ I disconnected
the circuit and the output went to 0V. Something must be loading/feeding back onto the pin"
Failure analysis Think about what caused this circuit/function to behave improperly. Re-think
your hardware/software design in light of your observations. For example: \ Where is the
signal coming from? Can I move a connection or component, so it doesn’t feedback? Does
my circuit design allow a signal to feedback in a way I hadn’t though about? "
See also (Predko 2000a; Ch. 12 { Debugging your Applications). Finally: Remember that trou-
bleshooting is a very time-consuming process (especially if a re-design is needed), and plan your
time accordingly.
Review Exercises
1. You have built a circuit to connect an LED between a PIC16F877 port pin and ground (via a
resistor). You have written a program which toggles the port pin which the LED is connected
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ECNG2006 (EE25M) March 11, 2008 VI 40
to. When you run the program on the PIC16F877 the LED does not appear to  ash. Which
ONE of the following statements is NOT a plausible explanation.
(a) The LED is faulty or is connected backwards.
(b) The port pin was con gured as an input.
(c) The port pin cannot sink su cient current to light the LED.
(d) The port pin has open-drain output circuitry, and cannot exert a \true" high value.
(e) The port pin is faulty.
2. You have a circuit in which a counter is connected to the output pin of the PIC16F877. The
counter should count on each rising edge; however you notice that it counts twice on each
rising edge. In order to discover the cause of the problem, the most appropriate action is
to:
(a) use a pull-up resistor on the output pin signal
(b) connect an oscilloscope to the output pin and observe the waveform
(c) replace the PIC16F877
(d) replace the counter
(e) use a meter to check the supply voltage
3. You have built the circuit described in question 4 on page 7: \ a circuit in which a switch
is connected to PORTB<6> and to ground ... PORTB pull-ups are enabled, all PORTB pins are
con gured as inputs, and no other PORTB pins are connected to circuitry."
However, when you press the switch, and attempt to read PORTB, the value returned is 0x00.
Which of the following statements is NOT a plausible explanation for this observation?
(a) PORTB was not con gured correctly as inputs.
(b) The switch is faulty, and never makes contact.
(c) The pull-ups were not enabled.
(d) The program instruction is being executed from the wrong bank.
(e) All the pins of PORTB have inadvertently been shorted (or the port input circuitry is
faulty).
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4. You have written a program for the PIC16F877 which uses interrupts from Timer0 to toggle
the state of a port pin connected to an LED. When you start the program, the LED goes
from o to on and then stays on. The port-pin is the only one which is con gured as an
output. Which of the following is NOT a plausible explanation:
(a) The pin is toggling so fast that you can’t see the LED  ashing.
(b) The ISR routine used the return instruction instead of the retfie instruction and only
ran once.
(c) The wrong bit-logical instruction was used to toggle the port-pin.
(d) The LED is connected to the wrong port-pin.
(e) The ISR did not clear the Timer0  ag.
5. You have built a circuit to drive a DC motor using PWM. The DC motor is connected to the
power supply, and is switched on and o using a transistor which grounds the other side of
the motor. The base of the transistor is connected to the PIC via a resistor. There are no
other components. Your circuit works, but the transistor and/or the power supply fuse blows
every now and again. Explain what the problem might be, and suggest a  x.
6. You have built a PIC16F877 based remote-controlled car which is battery powered. There is
an intermittent fault, where where the car stops working. Switching the car o and on again
corrects the problem. You observe that the problem happens when the car is driving into
an obstacle, and when you attempt to accelerate strongly. Suggest a possible cause of the
problem, and an appropriate solution.
7. A programmer writing an ISR for the PIC16F877 accidentally uses the return instruction
instead of the retfie instruction. Presuming that interrupts are initially enabled, what
would you expect to happen when the program runs and interrupt triggers occur? { Based
on (Katzen 2003; Q. 14.2 p.422)
8. \Microchip recommend that where possible channel 0 RA0/AN0 should not be used in the
PIC16C71 due to possible noise problems. Can you see why this is so and can you think of
any way around this problem?"(Katzen 2003; Q. 14.9 p. 423)
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ECNG2006 (EE25M) March 11, 2008 VI 42
9. \A certain PIC running at 20MHz has its Port C connected to LEDs tied high through a 1k 
resistor with a 300pF capacitance to ground. All LEDs are o and the programmer attempts
to turn on LED 7 and LED0 as follows:
bcf PORTC,7 ; Turn on LED7
bcf PORTC,0 ; Turn on LED0
However only LED 0 actually turns on. What is happening? "(Katzen 2003; Q. 11.2 p. 299)
Please draw the circuit, and identify at least two possible sources of the problem.
10. You have designed some hardware/software to interface the PIC16F877 with a device which
will pull a signal line low for a given time (short 10 microseconds or long 20 microseconds),
once it sees a falling edge (within 0.2 microseconds) on it’s input.
LED’s are connected between PORTC<1:0> and the 5V rail. The device input is connected to
PORTC <2>. The device output is connected to PORTC <3>. The PIC is wired to +5V supply
and Ground, and is clocked using a 1MHz clock signal.
The program should check the device, and then light one light-emitting diode for short, and
two light-emitting diodes for long.
....
start BANKSEL TRISC ; configure TRISC
movlw 0xF4
movwf TRISC
BANKSEL PORTC
clrf Cnt
mowlw 0x08
movwf PORTC
clrw PORTC ; falling edge
loop incf Cnt,F
btfsc PORTC,3
goto loop
light movf Cnt,W ;light diodes
sublw 4
btfss STATUS,Z
bsf PORTC,1
bsf PORTC,0
goto done
...
When you run the program, neither diode lights.
(a) Identify the pins used on the PIC16F877, and draw the circuits described.
(b) Identify 3 possible causes of this problem, and outline how you would attempt to elimi-
nate each possible cause.
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Tutorial Exercise 8BO ine Version38 ID#
The Bank of Nowhere has recently installed a security system at the doorway to their bank vault.
The bank wishes to use a PIC16F877 to ring a bell for as long as someone is standing in the
doorway. The volume of the bell should increase from minimum (after 10 seconds) to maximum
(after 3 minutes). Timer 0 is used to determine how long the person has been present. The PWM
peripheral is used to vary the volume. The sensor is connected to PORTB<0>, and triggers an
interrupt only when the person ARRIVES. Departure is detected by polling in the main loop.
Indicate ONE possible cause of EACH of the following symptoms, and indicate how you would test,
and/or eliminate it.
1. The bell never rings. 3 marks
2. When a person is in the doorway, the bell rings intermittently at 10 second intervals. 3 marks
3. When a person leaves the doorway, after standing there for more than 3 minutes, the bell
does not stop ringing. 3 marks
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
38Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
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T
U
T
8
BUnit 21 Write the letter you have been assigned here .
Symptoms:
A LED is o ; nothing happens when button is pressed
B LED is o ; when button is pressed LED comes on and remains on despite further button
pressing
C LED is on; nothing happens when button is pressed
D LED is on; when button is pressed LED goes o and remains o despite further button
pressing
A PIC16F877-based system contains a switch-button that is connected (with appropriate circuitry)
to PORTA<4>. The button state is polled from the main loop, and is used to decide if to light/out
the LED connected (with appropriate circuitry) to PORTC<0>. You can assume there is nothing
wrong with the code. Draw up a troubleshooting plan/ owchart for your assigned symptom.
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ select and apply techniques for troubleshooting digital circuitry using standard labora-
tory equipment
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
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22 PIC16 peripherals
At the end of this unit the student will be able to:
utilize built-in peripherals of the PIC16F877 microcontroller in simple applications.
The PIC16F877 has several built-in peripheral interface features which are multiplexed with the
parallel i/o port pins. Please note: Each of these features could be implemented using a PIC
without peripheral interfaces, ( possibly augmented with custom hardware ). For the sake of
e ciency however dedicated hardware has been provided. Each will be reviewed in turn (NB.
UART and PSP later in course) All information is from (PIC 2001) and (PIC 1997).
Port overrides
PORTC is an 8-bit wide, bi-directional port. The corresponding data direction register is TRISC.
PORTC is multiplexed with several peripheral functions namely: Serial Communications, Pulse
Width Modulation(PWM), Capture and ComPare (CCP), and oscillator input/ouput for Timer1.
When enabling peripheral functions, care should be taken in de ning TRIS bits for each PORTC
pin. Some peripherals override the TRIS bit to make a pin an output, while other peripherals
override the TRIS bit to make a pin an input.
PORTD is an 8-bit port; each pin is individually con gurable as an input or output. PORTD
can be con gured as an 8-bit wide microprocessor port (parallel slave port) by setting control bit
PSPMODE (TRISE<4>).
PORTE has three pins (RE0/RD/AN5, RE1/WR/AN6, and RE2/CS/AN7) which are individually
con gurable as inputs or outputs. The PORTE pins become the I/O control inputs for the micro-
processor port when bit PSPMODE (TRISE<4>) is set. In this mode, the user must make certain
that the TRISE<2:0> bits are set, and that the pins are con gured as digital inputs(as PORTE
pins are multiplexed with analog inputs which are activated by ADCON1). TRISE controls the
direction of the RE pins, even when they are being used as analog inputs.
Watchdog timer
The WatchDog timer generates a device reset whenever it over ows and is normally used to re-
cover from any malfunction that corrupts the contents of the program counter. For example, in
unattended operations if a failure occurs in the program and the device gets into a perpetual loop,
the watchdog timer can be used to reset the microcontroller and thus exit the loop. It can also be
used in low-power battery applications to \awaken" the CPU periodically.
The Watchdog Timer (WDT) is a free running on-chip RC oscillator which does not require any
external components. This oscillator is separate from any other oscillator on the microcontroller,
which means that it will continue running even if the other oscillators have been stopped, for
example during a sleep instruction. During normal operation a WDT time-out generates a device
RESET. If the device is in SLEEP mode, a WDT Wake-up causes the device to wake-up and
continue with normal operation.
The same prescaler that is used with Timer 0 can also be assigned to the WDT but it is used as
a postscaler instead. The clrwdt and sleep instructions clear the WDT and the postscaler (if
assigned to the WDT) and prevent it from timing out and generating a device RESET condition.
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Timer 0
Timer 0 is an 8 bit timer with the following features:
 Internal or external clock select
 Edge select for external clock
 8-bit software programmable prescalar
 Interrupt on over ow from 0xFF to 0x00
When TMR0 uses the internal clock it is said to operate in timer mode and it uses an external
clock it is said to operate in counter mode. Timer mode is selected by clearing the T0CS bit
(OPTION_REG<5>). In timer mode, the TMR0 will increment on every instruction cycle. Counter
mode is selected by setting the T0CS bit. In this mode, TMR0 will increment either on every rising
or falling edge of pin RA4/T0CK1. The incrementing edge is determined by the T0 Source Edge
select bit, T0SE (OPTION_REG<4>). Clearing bit T0SE selects the rising edge.
Timer 0 and the Watchdog timer both share a single programmable prescaler which can be used to
extend their ranges. The prescaler assignment is controlled by bit PSA (OPTION_REG<3>). Clearing
bit PSA will assign the prescaler to Timer 0. When this is done the prescale value is software
selectable using the bits PS2:PS0 (OPTION_REG<2:0>). The prescaler is neither readable or write-
able. When the prescaler is assigned to Timer 0, all instructions that write to the TMR0 register
will clear the contents of the prescaler. When it is assigned to the WDT, a clrwdt instruction will
clear the contents of the prescaler along with the Watchdog Timer.
The prescaler is like an additional counter that is placed in series with Timer 0. Assume that the
prescaler is set to 1:4 and both it and the Timer 0 are initialized to zero. The  rst clock pulse
will increment the prescaler to 1, while the timer will remain on zero. Subsequent clock pulses will
continue incrementing the prescaler only, until it contains 3. The next clock pulse will cause the
prescaler to rollover to 0 and the timer will increment to 1. Since the prescaler cannot be read,
a timer reading of say 10 with a prescale value of 1:4 can correspond to number of clock ticks
from anywhere between 40 and 43. In order to avoid an unintended device reset the following two
routines must be used when switching the assignment from Timer 0 to the WDT and vice versa.
These are only needed if the WDT is used.
; Changing from Timer 0 to WDT (assumes Bank 0 i.e. RP0 cleared)
clrf TMR0 ; clear TMR0 and prescaler
bsf STATUS,RP0 ; switch to Bank 1
clrwdt ; clear Watchdog timer
movlw b’xxxxx1xxx’ ; select new assignment and prescale value
movwf OPTION_REG
bcf STATUS,RP0 ; back to Bank 0
; Changing from WDT to Timer 0 (assumes in Bank 0)
clrwdt ; clear WDT and prescaler
bsf STATUS,RP0 ; switch to Bank 1
movlw b’xxxx0xxx’ ; select Timer 0
movwf OPTION_REG
bcf STATUS, RP0 ; back to Bank 0
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The TMR0 interrupt is generated when the TMR0 register over ows from 0xFF to 0x00. This
over ow sets the TOIF bit (INTCON<2>). The interrupt can be masked by clearing the TOIE bit
(INTCON<5>). The TOIF bit must be cleared in software, by the Timer 0 interrupt service routine,
before re-enabling this interrupt.
Timer 1
Timer 1 is a 16-bit timer whose values are kept in two readable and writeable 8-bit registers, TMR1H
and TMR1L. Timer 1 is controlled by the T1CON (Timer1 Control) register1 and can operate in
one of two modes, timer or counter mode. The operating mode is set by bit TMR1CS (T1CON<1>).
Similar to Timer 0, Timer 1 increments on every instruction cycle in timer mode and on every
rising edge of the external clock in counter mode. Timer mode is selected by clearing TMR1CS
(T1CON<1>). Counter mode There are two sub-modes available, synchronized and asynchronous
mode. In synchronous mode, the external clock input is synchronized with the internal phase
clock. In this con guration, Timer 1 will not increment during SLEEP mode, even if the external
clock is present, because synchronization circuit is shut o . When the bit T1SYNC (T1CON<2>) is
set, synchronization is not done and Timer 1 will continue to operate during SLEEP mode. There
is a special consideration that must be taken into account when reading Timer 1 because of its
16-bit value. Since it takes two reads to get its value, the timer may have incremented between the
reads, thus, the value read may not re ect the actual value in the timer.
Timer 2
Timer 2 is an 8-bit timer with both a prescaler and a postscaler. It can be used as the PWM
time-base in addition to normal timing activities. There is no external clock input so it is driven
solely by the internal processor clock. The prescaler functions just like the prescaler of the other
two timers, Timer 0 and Timer 1, and can have prescale values of 1:1, 1:4, and 1:16. The postscaler
is used in a di erent fashion and gives the timer added  exibility. The other two timers over ow
when they reach 0xFF (Timer 0) and 0xFFFF (Timer 1). Timer 2, on the other hand, over ows
when its value matches the value in the PR2 register. The PR2 register, which defaults to 0xFF on
reset, is readable and writeable. The output of the comparison between Timer 2 and PR2 is then
sent to the postscaler which can have postscale values from 1:1 to 1:16. This gives the timer much
more  exibility to time events, unlike Timer 0 where it was necessary to o set it on each over ow
when timing events could not be mapped to a multiple of 0xFF timer ticks.
A/D Converter
The built-in analog-to-digital(A/D) converter of the PIC16F877 has 8 channels and these are mul-
tiplexed across ports A and E. The  rst  ve channels being on Port A<5,3:0> and the rest on
Port E<2:0>. The ADCON0 register, controls the operation of the A/D module. The ADCON1
register, con gures the functions of the port pins. The port pins can be con gured as analog inputs,
voltage reference, or as digital I/O(whose behaviour is controlled by TRISx and PORTx registers).
The analog input channels must have their corresponding TRIS bits selected as an input.
The A/D module has high and low voltage reference inputs that are software selectable to some
combination of VDD(device voltage), VSS(ground reference), RA2, or RA3. Ideally, the digital
output value is the fraction of the di erence between the reference voltages in which the analog
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voltage falls. The A/D converter produces a 10-bit result.
VA V REF
V+REF  V REF  2
10
After the A/D module has been con gured as desired, the selected channel must be acquired before
the conversion is started. The analog input charges a sample and hold capacitor. There is a
minimum acquisition time which must be observed, to ensure that the capacitor charges to within
0.5 LSB of the actual input voltage. This is estimated in the data sheet as a worst case value of
20 s, however this value is dependent on the temperature, the source impedance, and the supply
voltage related switching resistance. For alternate conditions this acquisition time may be reduced.
During conversion, the sample and hold capacitor is disconnected from the input, and the converter
then generates a digital result of the analog level on the capacitor via successive approximation.
The A/D conversion of the analog input signal results in a corresponding 10-bit digital number.
Some additional wait time is needed between conversions. The minimum wait time is 2TAD where
TAD is the conversion time per bit.
The minimum conversion time per bit is dictated by the internal circuitry of the successive approx-
imation A/D converter. For the PIC16F877 microcontroller this time must be at least 1:6 s. The
signal used to clock the A/D converter is software selectable as either an internal RC oscillator
with a period of between 2 and 6 s, or can be derived from the chip clock frequency (1/2, 1/8,
1/32). For example, a 20 MHz chip clock signal, has a period of 50ns. It follows that the A/D
conversion clock can only be set to 1/32 of the clock frequency, otherwise the 1:6 s limit will be
violated. Total conversion time is 12TAD for 10 bits, plus overhead.
Since the A/D is a 10-bit device, two bytes are needed to store the converted result. These are
ADRESH (A/D RESult High register) and ADRESL (A/D RESult Low register). The extra 6
bits are not used. The A/D can be instructed to have the result left-justi ed. where the six
least-signi cant bits of ADRESL are read as 0 or right-justi ed, where the six most-signi cant bits
of ADRESH are read as 0. When the A/D conversion is complete, the result is loaded into the
ADRES register, the GO/DONE bit (ADCON0<2>) is cleared, and A/D interrupt  ag bit, ADIF, is
set.
To recap, acquisition time is the time that the A/D modules holding capacitor is connected to the
external voltage level. Then there is the conversion time of 12TAD, which is started when the GO
bit is set. The sum of these two times is the e ective sampling time. Please note that relative to the
instruction cycle, this sampling time is slow, and should not be used to measure rapidly changing
signals.
Capture/Compare/PWM
One function which is frequently performed by microcontrollers is the timing of external input
signals, or the regular switching of external output signals. Both can be implemented using interrupt
routines: either record the time when the event occurs or when the timer expires switch the external
signal. The PIC16F877 has dedicated hardware which will perform such switching without the need
for software. This peripheral has 2 channels on each of which it can perform one of three functions:
 automatic capture of the timer value when an external input occurs (CAPTURE)
In Capture mode, CCPR1H:CCPR1L captures the 16-bit value of the TMR1 reg-
ister when an event occurs on pin RC2/CCP1. An event is de ned as one of the
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following:
{ Every falling edge
{ Every rising edge
{ Every 4th rising edge
{ Every 16th rising edge
The type of event is con gured by control bits CCP1M3:CCP1M0 (CCPxCON< 3 :
0 >). When a capture is made, the interrupt request  ag bit CCP1IF (PIR1< 2 >)
is set. The interrupt  ag must be cleared in software. If another capture occurs
before the value in register CCPR1 is read, the old captured value is overwritten
Timer1 must be running in Timer mode, or Synchronized Counter mode, for the
CCP module to use the capture feature. In Asynchronous Counter mode, the
capture operation may not work. (PIC 2001)
 automatic switching of an external signal when the timer value matches a pre-set value (COM-
PARE)
In Compare mode, the 16-bit CCPR1 register value is constantly compared against
the TMR1 register pair value. When a match occurs, the RC2/CCP1 pin is:
{ Driven high
{ Driven low
{ Remains unchanged
The action on the pin is based on the value of control bits CCP1M3:CCP1M0
(CCP1CON< 3 : 0 >). At the same time, interrupt  ag bit CCP1IF is set. Timer1
must be running in Timer mode, or Synchronized Counter mode, if the CCP mod-
ule is using the compare feature. In Asynchronous Counter mode, the compare
operation may not work. (PIC 2001)
 automatic toggling at a pre-de ned signal frequency (PWM) Pulse-Width-Modulation is one
means by which a digital value can be converted into an analog output.
A PWM signal is a repeating signal that is on for a set ... time, [which] is propor-
tional to the [desired output voltage] (Predko 2000c).
i.e. the average voltage/power delivered by such a signal will be proportional to the "on"
time.
The "on" time of the PWM signal is referred to as the pulse width or duty cycle, which may
be expressed either in time, or as a percentage of the PWM period. The latter notation is more
convenient, as the PWM frequency can be independently changed without re-specifying the duty
cycle.
The choice of PWM frequency depends on two factors. Firstly, the frequency must be high enough
that the device does not visibly/audibly/mechanically go on/o . Secondly, the frequency must be
low enough that within the PWM period, there are su cient "steps" available to express all the
digital values which will be converted. This "step" size is referred to as the granularity of the PWM
signal. The PWM frequency and granularity together limit the resolution of the PWM signal i.e.
the maximum number of digital values which may be converted.
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In the PIC16F877, although the Capture and Compare functions use Timer1 to perform their
functions, the PWM function uses Timer2. The period is determined by Timer2 over ow value
held in PR2. The pulse width is determined by the value held in CCPR1L:CCPR1CON<5:4>. The
built-in PWM function granularity is determined by the chip oscillation period. (PIC 2001)
Simulator Limitations
The MPLAB simulator is an instruction-level simulator, i.e. it does not concern itself with events
occurring between instructions. The following list itemizes the functions and peripherals among
the entire PICmicro MCU family of microcontrollers that are a ected by simulation on instruction
cycle boundaries(MPL 2000):
 Clock pulse inputs smaller than one cycle can not be simulated even though timer pre-scalers
are capable of accepting clock pulse inputs smaller than one cycle.
 PWM output pulse resolution less than one [instruction] cycle is not supported.
 Compares greater than 8-bits are not supported.
 In unsynchronised counter mode, clock inputs smaller than one cycle can not be used.
 The oscillator waveform on RC0/RC1 pins can not be shown.
 MPLAB-SIM does not simulate serial I/O.
 Timer0 (and the interrupt it can generate on over ow) is fully supported by the MPLAB-SIM
simulator, and will increment by the internal or external clock. Clock input must have a min-
imum high time of 1 TCY and a minimum low time of 1 TCY due to stimulus requirements.
 Timer1 in its various modes is supported by the MPLAB-SIM simulator, except when running
in counter mode by an external crystal.
 The external oscillator on RC0/RC1 is not simulated, but a clock stimulus can be assigned
to those pins.
 PWM output (resolution greater than 1 TCY only) are not supported in this version of the
MPLAB-SIM simulator.
 The Synchronous Serial Port is supported in SPI mode only. The MPLAB-SIM simulator
currently does not support the I2C mode.
 The simulator does not load any meaningful value into A/D result register (ADRES) at the
end of a conversion.
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In Circuit Debugger (ICD) Limitations
The MicroChip ICD module is supported by the IDE, and provides programming and debugging
facilities. Programming uses the In Circuit Serial Programming (ICSP) facilities of the Microchip
family; debugging operates by adding a debug executive (kernel) to the code programmed on target.
This means that some resources are not available to the system designer/programmer when using
the ICD (ICD 2000):
 MCLR/VPP shared for programming
 RB3 reserved for programming
 RB6 and RB7 reserved for programming and in-circuit debugging
 Six (6) general purpose  le registers reserved for debug monitor: 0x70, 0x1EB to 0x1EF
 First program memory location (address 0x000) must be a NOP instruction
 Program memory 0x1F00-0x1FFF reserved for Debug Code
 One(two) stack levels not available
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Review Exercises
1. In order to use the Parallel Slave Port on the PIC16F877:
(a) the PSPMODE bit in register TRISD must be cleared
(b) the PSPMODE bit in register TRISE must be cleared
(c) the PSPMODE bit in register TRISE must be set
(d) the PSPMODE bit in register PORTE must be cleared
(e) the PSPMODE bit in register TRISD must be set
2. The A/D module may be used to read from one of eight channels. Which of the following
values, will allow the program to sample from analog channel 3, and deliver a right justi ed
result if it is written to ADCON1 using
movlw value
movwf ADCON1
(a) value EQU B’1011 0011’
(b) value EQU B’1000 1110’
(c) value EQU B’1000 0110’
(d) value EQU B’1010 0001’
(e) value EQU B’1111 0000’
3. In order to con gure PORTB so that pins <4:3> are outputs and pins <2:0> are inputs, we can:
(a) move the literal B’00011000’ into TRISB
(b) move the literal B’00011000’ into PORTB
(c) move the literal B’00000111’ into PORTB
(d) bit-wise OR TRISB with B’00000111’ then bit-wise AND TRISB with B’11100111’
(e) bit-wise OR PORTB with B’00000111’ then bit-wise AND PORTB with B’11100111’
4. (a) Di erentiate between the host and target machines with reference to the IDE, the com-
piler, and the generated machine code.
(b) List features of simulators, and debuggers using MPLAB as an example. What are some
of the disadvantages of using simulators to develop application code?
(c) Why do we need a simulator, when we can debug directly on the system?
(d) Why do we need debug facilities when we can just use the simulator?
(e) Suggest some possible advantages of programming in assembly language rather than in
high level languages (e.g. C).
5. One useful feature of the PIC16F877 microcontroller is the fact that it can report what
condition caused it to re-start: Power-on, Brown-out, Watchdog timer reset or Physical reset
(MCLR pin). Are these features supported in the simulator? Can the simulator be used to
generate each of these conditions, in order to test any associated code?
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6. The A/D converter on the PIC16F877 can accept reference voltages.
(a) What range of voltages are appropriate for use as reference voltages?
(b) What advantage could using di erent reference voltages o er?
7. Write a routine called PWMSetup for the PIC16F877 with a 1MHz clock, which will con g-
ure/initialise the PWM peripheral on CCP1, and set the initial PWM duty cycle to 1520  s.
What is the minimum pulse width change that your routine can make in  s?
8. What value must be loaded into T2CON in order to con gure the PIC16F877 as follows: Timer
2 on, prescaler 1:16, postscaler 1:5.
9. (a) For the example code for time delays on page VI 19, calculate the percentage error caused
by the neglected overhead, and the extra cycle on return for delays of 1 ms, 100 ms and
255 ms.
(b) For the example on VI 20, the code timed 250 cycles, with the prescaler set to 128. For
a 4MHz clock, what is the delay in ms?
(c) Another method of obtaining a delay of 10 ms is to count for exactly 10,000 cycles. Now
if the prescaler with TMR0 is set to 1:64 then it will over ow every 16384 cycles. Now
10;000 = 16384 (100 64 16). Design a main delay routine that tests for the over ow
 ag, clears it and bypasses exactly 6384 cycles, so that when it returns, at total of 10,000
cycles have been executed. An outline is shown below. As long as the main routine takes
less than 10 ms to execute then the loop will be done in exactly 10,000 cycles.
ORG 0x000
Init ; initialize Timer0 etc
main ; main routine
call Delay
goto main
Delay ; test for overflow flag
; clear overflow flag
; bypass 6384 cycles
return
(d) Challenge: Devise a routine using TMR0 to get time delays of 15 ms.
10. Challenge: Identify a pair of con icting peripherals on the PIC16F877 (i.e. the peripherals
utilise the same pins). For your con icting peripherals, suggest a work-around appropriate
for an application which requires functions provided by both peripherals.
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Lab 3 ID#
You should complete the pre-lab before coming to your lab session. Your Teaching Assistant/Demonstrator
may refuse to allow you into your lab session if your pre-lab is incomplete, or if you are unduly
late.
You will have 6 hours in the lab to complete the exercises. All answers should be written on this
lab-sheet in PEN. Please do not attach any extraneous pieces of paper unless SPECIFICALLY asked
to do so.
Please bring your PIC16F877 datasheet book (provided in ECNG2006) to the lab with
you. You may need to refer to it in order to complete the pre-lab and lab exercises.
Your lab submissions will be checked for unwarranted collusion, and unreferenced use
of Intenet-available/other resources.
At the end of this unit the student will be able to:
utilize built-in peripherals of the PIC16F877 microcontroller in simple applications.
Pre-Lab
1. In general, a peripheral will have 4 types of information associated with it: con guration,
control, state, and data. The information may be a whole register (as in the case of the
parallel I/O port); it may also be a single bit, or a multi-bit pattern. For each of the following
peripherals:
 PortB
 A/D
 PWM
locate the description of the peripheral in your datasheet, and identify the associated regis-
ters/ ags. Classify each register,  ag, or group of bits (as appropriate) as:
 con guration { tells peripheral how to do task it will do later
 state { peripheral uses to report what doing/did
 control { tells peripheral what to do now
 data {information the peripheral either provides or will work with
Use the table below to summarize your  ndings. 3 marks
Peripheral Page #’s Con guration Control State Data
PortD TRISD, TRISE
(PSPMODE)
TRISE(IBF,
OBF, IBOV)
PORTD
PortB
Timer0 p. 47-9 PSA, PS2:0 T0IE, T0CS T0IF TMR0
A/D
PWM
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 57
Ports
2. All (parallel input-output) ports have at least two registers associated with them: a data
register PORTx and a con guration (data-direction) register TRISx. PORTB is a register
and is inside the PIC16F877. RB0 to RB7 are labels for pins external to the PIC16F877 IC.
Pins RB0 to RB7 are connected to the register PORTB inside the PIC16F877. Since PORTB
holds a byte it is logical to assume that each pin (RB0 to RB7) reads/writes one bit from/to
PORTB. By default, PORTB pins are con gured as inputs
(a) What are the pin #’s on the 40 pin DIP package which correspond to RB0 to RB7? 1 mark
(b) If ideal high/low voltages are applied to pins RB0:RB7 as shown in the table, when
PORTB is con gured as all inputs, what bit pattern will appear in PORTB? 1 mark
Pin Label RB7 RB6 RB5 RB4 RB3 RB2 RB1 RB0
Pin #
Pin Voltage/V 0 5 5 0 0 5 0 0
PortB Bit # <7> <6> <5> <4> <3> <2> <1> <0>
Bit Value
3. If we want to get pins RB0 to RB7 to re ect the values which we write to PORTB,  rst we
must con gure the pins of PORTB as outputs.
(a) The registers that con gure the PORT pins as INPUTS or OUTPUTS are the TRIS reg-
isters. What bank(s) are the TRIS registers located in? 1 mark
(b) How does one select a di erent bank? (Note: BANK0 is the default bank.)
i. use the preprocessor directive BANKSEL.
To change to the bank that contains TRISB what line of code would you use?
1 mark
ii. explicitly set or clear the banking bits in the STATUS register.
To change to the bank that contains TRISB what line of code would you use?
1 mark
(c) If the TRIS bit is set ’0’ then the corresponding Port Pin acts as an output. If the
TRIS bit is clear ’1’ then the corresponding Port Pin acts as an input. Complete the
code below so that it changes all the PORTB Pins to outputs. Try to do it in as few
instructions as possible. 3 marks
BANKSEL _____ ; switch banks
_____________ ; put your code here
_____________ ;
_____________ ;
BANKSEL PORTB
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 58
4. In order to connect a simple switch as an input on register-bit PORTB< 1 > (pin RB1).
Determine the following:
 the pin # on the PIC16F877 40 pin package that we need to connect to is . 1 mark
 the bit-mask and bit-wise-logical operation to change the relevant bit in TRISB to an
input without a ecting the other pins 1 mark
 the input voltage (logic-high threshold) & current requirement(s) of this pin are . 1 mark
 If the maximum current the switch can carry is 100 mA and the resistance of the switch
is 0.2 Ohms design an appropriate circuit so that the pin is logic-high when the switch
is ON, and logic-low when the switch is OFF 2 marks
 For the same conditions, design an appropriate circuit so that the pin is logic-high when
the switch is OFF, and logic-low when the switch is ON 2 marks
5. PORTB supports internal pull-ups. What are these and what are they used for? 2 marks
6. PortB pins RB3, RB6 and RB7 are multiplexed with functions used by the ICD2 to com-
municate with the IDE i.e. for programming, receiving instructions (run, stop, step) and for
returning register values. What do you think will happen if we connect circuitry to these pins
while we are using the ICD2 to \debug" the PIC16F877? 2 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 59
Instruction Timing
The clock cycle time for the PIC16F877 is determined by the oscillator frequency;
Q = TOSC = 1fOSC The instruction cycle time for the PIC16F877 is 8 clock cycles; however
due to pipelining it appears as 4 clock cycles. Tinstruction = 4 TOSC
7. (a) For a 4MHz oscillator, what will be actual PIC16F877 instruction cycle time? . 1 mark
(b) For a 4MHz oscillator, what wil PIC16F877 instruction cycle time appear to be? . 1 mark
Timers(Counters)
The timer (also known as the counter) is a type of peripheral. A basic timer has two
inputs increment, and reset, and two outputs current count and over ow signal.
Timers(Counters) over ow and generates a signal when the value held matches the over-
 ow value. Where no over ow value is speci ed, this is the maximum value that the timer
can hold (i.e. 8 bits { 0xFF). Counter/Timers take signals either from an external source, or
from the instruction cycle time Tinstruction.
A timer whose increment input is connected to a  xed frequency, will over ow at regular
intervals, determined by the input frequency and the over ow value. Sometimes the frequency
of the increment input signal is reduced by a preliminary stage (pre-scaler) which over ows
at a small scale factor. The value held by the timer is then incremented on the rising/falling
edge of the scaled signal. The value of the scale factor is determined by setting bits in the
timer con guration register. Available scale factors are always powers of 2(i.e. 1:2; 1:4; 1:8).
8. (a) What value will Timer0 over ow at? . 1 mark
(b) What pre-scale factors are available for Timer0? . 1 mark
A/D and PWM
The PIC16F877 has several additional peripherals. We will look at the built in Analog to
Digital converter, and the built in Pulse-Width-Modulation generator. These peripherals
automatically use timing, to sample/generate signals on one or more channels.
9. How many channels does the A/D converter support? Can signals on these channels be sam-
pled simultaneously? If so, under what conditions. 2 marks
10. How many channels does the PWM generator support? Can signals on these channels be
generated simultaneously? If so, under what conditions. 2 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 60
Helpsheet { How to use the ICD2
Code should be compiled and tested using the simulator BEFORE you attempt to program/debug the
PIC16F877-based application. When you are ready to change from Simulator mode to ICD mode, you
should do the following:
1. If you built the circuit yourself, or made any changes, then get your circuit checked by the TA,
technician, or lecturer.
2. After your circuit has been checked!! After your circuit has been checked!! Connect one end of
the USB cable to the computer, and the other to the ICD2 module. Then plug the header into the
ICD2 module using the RJ12 cable, and power up your circuit.
3. To use the ICD2 instead of the simulator, choose Debugger > Select Tool > ICD2. In some cases, a
wizard will appear to help you set up the ICD2. If no wizard appears, goto the next step. To use the
wizard, Click Next.
The next form allows you to select a communications method. Under the Com Port select "USB".
Click NEXT.
The next form allows you to select the ICD power source. Select "Target has own power supply".
Click NEXT.
The next form allows MPLAB to autoconnect to the ICD2 at startup. Do not check the box
provided. Click NEXT.
This form allows MPLAB to automatically download the ICD2 operating systems when needed. Check
the box provided and click NEXT. On the next screen click FINISH.
4. To connect to the ICD2 choose Debugger > Connect. A window should now show some messages; the
 nal message should read
MPLAB ICD 2 Ready
Please ask your lecturer/lab assistant for help if any problems are encountered.
5. The ICD2 must now be enabled as the programmer. To do this choose Programmer>Select Programmer>MPLAB
ICD2.
6. The con guration bits must now be set before programming the device. Select Con gure. Now ensure
that the following selections are made:
Oscillator XT
Watchdog Timer O 
Power Up Timer O 
Brown Out Detect O 
Low Voltage Program Disabled
Flash Program Write Enabled
Background Debug Enabled
Data EE Read Protect O 
Code Protect O 
7. The device can now be programmed. Click Debugger>Program, to download the hex- le and debug
code into the device. Programming may take a couple of minutes. During the process, the status bar
should indicate that the device is being programmed then veri ed. When veri cation is complete the
MPLAB ICD2 windows should indicate that the programming was successful and that the device is
ready. be o /disabled.
8. The program can now be executed using Debugger>Run. The Status Bar should now show that the
program is being run. The run process is halted by selecting Debugger>Halt. The circuit is now
running your program.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 61
Helpsheet { A/D and PWM { calculations & programming
A/D Timing
A/D sampling time = min’m acquisition time ( 20 s) + # bits (10) * conversion time/bit (TAD > 1:6 s)
+ min’m wait time (2TAD) where:
 Acquisition time is the time that the A/D module’s holding capacitor is connected to the external
voltage level.
 Conversion time is started when the GO bit is set, and includes conversion time for each bit, and a
minimum wait time.
The signal used to determine TAD is software selectable as either an internal RC oscillator with
a period of between 2 and 6 s, or can be derived from the chip clock frequency TOSC with scale
factor (1/2, 1/8, 1/32). Relative to the instruction cycle, this sampling time is slow, and the A/D
should not be used to measure rapidly changing signals.
How to calculate the voltage represented by ADRESH (w/left-justi ed result):
The A/D answer is 10 bits long. If the user reads the 8 most signi cant bits (i.e. the 8 most left
most bits in ADRESH): 255 will represent VREF+ volts and 0 will represent VREF volts.
In general, ADRESH will represent VREF+ VREF 256  ADRESH volts.
How to calculate the PWM frequency and duty cycle (pulse width)
\The PWM frequency is de ned as 1[PWMperiod]."PIC (2001) To set the PWM period, we write
values to PR2, and set the TMR2 prescaler (bit pattern set in the two LSB of T2CON).
PWM period = [(PR2)+1] 4 TOSC (prescale value)
To set the duty cycle we write values to CCP1RL and two lower bits. If we ignore the lower 2 bits
of the duty cycle then: PWM duty cycle = CCPR1L 4 TOSC (prescale value)
Writing a program to interface with a peripheral
When writing a program which uses a peripheral you will probably want to employ a program
which has the following structure:
configure
wait for and/or reset state
write data
assert control
wait for and/or reset state
read data
to repeat,loop to appropriate point
Sometimes these steps are not possible or not appropriate for particular peripherals, but the general
structure holds in all cases. Where a wait is required we have several options:
busy wait loop until condition detected (nothing else happens)
instruction-based delay loop for a pre-set # of cycles (a specially timed thing happens (if any))
timer-based delay occasionally test if timer has expired (anything else may take place)
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 62
Helpsheet - 7 segment display patterns
A seven segment display consists of seven/eight elements arranged as shown:
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|_|.
Banks of seven segment display modules are typically used to display numeric information e.g. on
calculator displays. A seven segment LED display module uses an individual LED to illuminate
each of the segments. It typically has 10 connection lines: 8 which individually connect to the
LED’s, and 2 which connect to the LED common anode OR cathode.
Multiple seven segment LED display modules can be connected in parallel, with each module
individually activated by a transistor which lets current  ow from power (common anode) OR to
ground (common cathode). If more than one module is activated simultaneously, they will both
receive and display the same patterns. To make it look as though we are simultaneously displaying a
di erent pattern on each display, we must switch rapidly between the displays. Read the Chapter
from Predko (2000d) in your databook to clarify.
In this lab, the following patterns are used to display numbers, characters, and frequently used
patterns on a 7 segment display.
0 1 2 3 4 5 6 7 8 9
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c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 63
In the Lab
In this lab exercise, we will learn about bank switching, changing ports from inputs to outputs,
reading switches and lighting simple LED displays. Then we will practice using three built in
peripherals: Timer, A/D converter, and PWM module. Finally, we will create a scrolling alphanu-
meric display with user adjustable scroll rate and brightness level.
You will be given a circuit that looks like the circuit shown in Figure 13.
Figure 13: Lab 3 Circuit Diagram
Before starting each numbered question, you should read through the entire question, and make
note of any queries that you may have.
Electronic copies of the code in this script are provided. Please note that there is no guarantee
that the code will operate as required. Troubleshooting code is a skill which you are expected
to use during this lab. Use the MPLAB ICD and MPLAB IDE debugging functions, to examine
registers,  nd bugs and  x application software. As you work, try to make use of the IDE features,
i.e. breakpoints, watch windows etc.
Call your TA or lecturer for assistance if you are stuck at any time.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 64
ID#
Please identify
 your lab group letter (E,F,G,H):
 your ECNG2006 uP group designator (e.g. A3):
Micro-controller Circuit
1. The  rst step in using a microcontroller is to double check the circuit connections. The circuit
you will be using in the lab is shown on page 63.
(a) Are the pin numbers for all devices correct? Make adjustments to the diagram if needed. 1 mark
(b) Does the circuit diagram match the circuit? Make adjustments to the diagram if needed. 1 mark
(c) You will notice that there is a switch bank with 4 switches. One side of the switch is
connected to GROUND and the other is connected to the PIC16F877 on PORTB.
i. When a switch is OFF the input pin is: high low  oating? (Circle correct answer) 1 mark
ii. When a switch is ON the input pin is: high low  oating? (Circle correct answer) 1 mark
(d) The LED’s have been connected so they light when the associated pin is at 5 Volts.
Assuming that a typical LED has forward voltage drop of 2V and an 8mA min’m current
requirement. Determine the following for the LED connected to PORTD<7>:
i. the pin # on the PIC16F877 40 pin package is . 1 mark
ii. the maximum current that the pin can source/sink is . 1 mark
iii. the resistance connected in series with the LED is: . 1 mark
iv. Verify that the current requirements of the pin and LED are both met. 1 mark
(e) Each LED modules devices is grounded through a transistor (common cathode). The
bases of the transistors are connected to pins RC1 and RC2 respectively. Explain how
i. pins RC1 and RC2 can be used to control whether the LED devices are ON/OFF. 1 mark
ii. the circuit should be modi ed for common-anode LED devices. 1 mark
Hook up your board and ICD2 now.
N.B Please let the lab technician/demonstrator check your circuit before you
proceed.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 65
Accessing ports in C and Assembly Language
2. To input/output signals from a port, the relevant registers/bits must be changed. We can do
so using either an assembly language program or a C language program
(a) Complete the following code, so that the LED Bar is switched ON.
LIST p=16f877
INCLUDE <p16F877.inc>
test EQU 0xFF
ORG 0x00
nop
goto main
ORG 0x20
main
BANKSEL TRISD ; Select the Bank that contains TRISD and TRISC
movlw 0x00 ; Make PORTD pins all
movwf TRISD ; outputs
clrf TRISC ; Make PORTC pins all outputs
BANKSEL ______ ; Select the bank that contains PORTC and PORTD
movlw ______ ; Literal specifies pin connected to base of LED Bar transistor
MOVWF PORTC ; Move value to PORTC
loop movlw test ; Move this value to PORTD
_______________ ; to prove that the LED bars were chosen
goto loop ; Keep repeating
done sleep
END
Create a new assembly language project for the PIC16F877 in the MPLAB IDE. Compile
the code and ensure that it is built successfully. Simulate it and see if it works.
Next PROGRAM the PIC16F877 and ensure that the code works with the circuit.
Did the LED’s on the LED Bar light up with a bit pattern corresponding to test?
Yes or No (Circle your answer)
If it didn’t what was the problem? How did you  x it?
Use the MPLAB IDE and/or command line tools to determine:
i. The size (in bytes) of the source code  le: . 1 mark
ii. The number of instruction memory locations used by the code: . 1 mark
iii. The number of instruction cycles required to perform the loop: . 1 mark
Demonstrate to the lecturer/demonstrator/TA who will sign your script.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 66
(b) We can achieve the same e ect using a PICC compiler C language program. Complete
the following code, so that the LED Bar is switched ON.
#include <16F877.h>
#define test 0xFF
int main()
{
SET_TRIS_D(0x00); // Make Port D pins all outputs
SET_TRIS_C(0x00); // Make Port C pins all outputs
OUTPUT_C(_______); // Literal specifies pin connected to base of LED Bar transistor
for (;;) // keep repeating
{
OUTPUT_D(test); // move this value to PORTD to prove that LED Bars were chosen
}
}
Create a new PICC project for the PIC16F877 in the MPLAB IDE Compile the code
and ensure that it is built successfully. Simulate it and see if it works.
Next PROGRAM the PIC16F877 and ensure that the code works with the circuit.
Did the LED’s on the LED Bar light up with a bit pattern corresponding to test?
Yes or No (Circle your answer)
If it didn’t what was the problem? How did you  x it?
Use the MPLAB IDE and/or command line tools to determine:
i. The size (in bytes) of the source code  le: . 1 mark
ii. The number of instruction memory locations used by the code: . 1 mark
iii. The number of instruction cycles required to perform the loop: . 1 mark
Demonstrate to the lecturer/demonstrator/TA who will sign your script.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 67
(c) We can combine C and assembly language using multiple source  les which are separately
compiled/assembled and then linked. We can also use the #asm keyword to include
assembly language in a C program.
Complete the following code, so that the LED Bar is switched ON.
#include <16F877.h>
#define test 0xFF
int main()
{
SET_TRIS_D(0x00); // Make Port D pins all outputs
SET_TRIS_C(0x00); // Make Port C pins all outputs
OUTPUT_C(_______); // Literal specifies pin connected to base of LED Bar transistor
for (;;) // keep repeating
{
// move this value to PORTD to prove that LED Bars were chosen
#define PORTD_Address 0x08
#asm
movlw test
movwf PORTD_Address
#endasm
}
}
Create a new PICC project for the PIC16F877 in the MPLAB IDE Compile the code
and ensure that it is built successfully. Simulate it and see if it works.
Next PROGRAM the PIC16F877 and ensure that the code works with the circuit.
Did the LED’s on the LED Bar light up with a bit pattern corresponding to test?
Yes or No (Circle your answer)
If it didn’t what was the problem? How did you  x it?
Use the MPLAB IDE and/or command line tools to determine:
i. The size (in bytes) of the source code  le: . 1 mark
ii. The number of instruction memory locations used by the code: . 1 mark
iii. The number of instruction cycles required to perform the loop: . 1 mark
Demonstrate to the lecturer/demonstrator/TA who will sign your script.
(d) Which of the three implementations do you prefer? Justify your answer. 1 mark
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 68
3. (a) Now we will attempt to make the 7 Segment LED active while making the LED Bar
inactive. Before we look at the code we need to know a few things. How are the
PIC16F877 pins connected to the Seven Segment LED?
RD7
RD2 RD6
RD1
RD3 RD5
RD4 RD0
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The patterns for displaying digits and characters were provided in the helpsheet on
page 62. Complete the table below which shows the logical values will let you display
hexadecimal digits between 0 and F, and the decimal point. 2 marks
RD7 RD6 RD5 RD4 RD3 RD2 RD1 RD0 Display digit Hex Value
1 1 1 1 1 1 0 0 0 0xFC
0 1 1 0 0 0 0 0 1 0x60
1 1 0 1 1 0 1 0 2
1 1 1 1 0 0 1 0 3 0xF2
4
1 0 1 1 0 1 1 0 5 0xB6
6
1 1 1 0 0 0 0 0 7 0xE0
1 1 1 1 1 1 1 0 8 0xFE
9 0xE6
1 1 1 0 1 1 1 0 A 0xEE
b 0x3E
0 0 0 1 1 0 1 0 c 0x1A
0 1 1 1 1 0 1 0 d 0x7A
1 1 0 1 1 1 1 0 e 0xDE
1 0 0 0 1 1 1 0 f 0x8E
0 0 0 0 0 0 0 1 . 0x01
(b) Use the values from the table, and Complete the code shown on the following page. 2 marks
Create a new assembly language project for the PIC16F877 in the MPLAB IDE. Compile
the code and ensure that it is built successfully. Simulate it and see if it works, then
PROGRAM the PIC16F877 and ensure that the code works with the circuit.
Did Seven Segment LED light up with a number? Yes or No (Circle your answer)
If it didn’t what was the problem? How did you  x it?
What value is displayed on the Seven Segment LCD?
Change the digit displayed on the Seven Segment LCD. Which line of code did you alter?
Demonstrate to the lecturer/demonstrator/TA who will sign your script. 1 mark
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 69
LIST p=16f877
INCLUDE <p16F877.inc>
ORG 0x00
nop
goto main
ORG 0x20
main
BANKSEL TRISD ; Select Bank 1
clrf TRISD ; Configure PORTD and PORTC pins
clrf TRISC ; all as outputs
BANKSEL PORTC ; Return to bank 0
movlw ________ ; Switch on the 7-segment display
MOVWF PORTC
loop movlw 0x08
_____ lookup ; convert digit to the bit pattern
BANKSEL PORTD
_____ PORTD ; put bit pattern on PORTD
goto loop
done sleep
; Data EEPROM lookup table
lookup andlw 0x0F ; Mask to ensure this is a hex digit
BANKSEL EEADR ;
movwf EEADR ; Specify lookup address
BANKSEL EECON1
bcf EECON1, EEPGD ; Trigger lookup from Data Memory
bsf EECON1, RD ;
BANKSEL EEDATA ;
movf EEDATA, W ; Retrieve Data into W
return
ORG 0x2100
DE 0xFC , 0x60 , _____ , 0xF2 , _____ , 0xB6 , _____ , 0xE0
DE 0xFE , 0xE6 , 0xEE , 0x3E , 0x1A , 0x7A , 0xDE , 0x8E
END
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 70
(c) In Lab 2, you used a di erent kind of lookup table (using retlw) to check whether a
BCD value was valid. Re-write this code, using a retlw lookup table to convert numbers
to appropriate outputs for the seven segment display.
Create a new assembly language project for the PIC16F877 in the MPLAB IDE. Compile
the code and ensure that it is built successfully. Simulate it and see if it works, then
PROGRAM the PIC16F877 .
Did Seven Segment LED light up with a number? Yes or No (Circle your answer)
If it didn’t what was the problem? How did you  x it?
What value is displayed on the Seven Segment LCD?
Change the digit displayed on the Seven Segment LCD. Which line of code did you alter?
Use the MPLAB IDE and/or command line tools to:
i. Print your new program, and attach the printout to this script. 2 marks
ii. Determine the number of instruction memory locations used by the code: . 1 mark
iii. Determine the number of instruction cycles required to perform the loop: . 1 mark
iv. Did the lookup change make the code either shorter or faster?
Shorter: Yes or No (Circle your answer)
Faster: Yes or No (Circle your answer)
v. Which lookup table method do you prefer? Explain your answer. 1 mark
Demonstrate to the demonstrator/TA who will sign your script & printout.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 71
(d) Challenge (Bonus 5 marks): Write a PICC program which will perform the same task
using a pre-de ned array. Write down the  nal program here:
Create a new PICC project for the PIC16F877 in the MPLAB IDE Compile the code
and ensure that it is built successfully. Simulate it and see if it works. Next PROGRAM
the PIC16F877 and ensure that the code works with the circuit.
Did Seven Segment LED light up with a number? Yes or No (Circle your answer)
If it didn’t what was the problem? How did you  x it?
What value is displayed on the Seven Segment LCD?
Change the digit displayed on the Seven Segment LCD. Which line of code did you alter?
If it didn’t what was the problem? How did you  x it?
Use the MPLAB IDE and/or command line tools to determine:
i. The size (in bytes) of the source code  le: .
ii. The number of instruction memory locations used by the code: .
iii. The number of instruction cycles required to perform the loop: .
Demonstrate to the lecturer/demonstrator/TA who will sign your script.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 72
4. (a) So far we have looked at outputting data (i.e. data moves from the PIC16F877 to the
LED’s). Let us try it the other way around (i.e. data moves from outside to inside the
PIC16F877). Take a look at your circuit.
The table below shows the relationship between the switches and the PIC Pins.
Switch PIC Pin
1 RB5
2 RB4
3 RB2
4 RB1
The following program is going to use the LED Bar to display the value read in from
the inputs. We will be able to see how the inputs are changing. Echoing inputs to a
simple display can sometimes be a useful technique when troubleshooting a  P-based
application.
LIST p=16F877
INCLUDE <p16F877.inc>
ORG 0x00
nop
goto main
main
BANKSEL TRISB ; Switch to bank 1
movlw 0xFF ; Set PORTB to
movwf TRISB ; an input port
bcf OPTION_REG,7 ; enable pull-ups
clrf TRISD ; PortD all outputs
bcf TRISC,2 ; Port C transistor is an output
BANKSEL PORTB ; Move back to bank 0
bsf PORTC,2 ; turn on transistor
loop movf PORTB,W
andlw B’00110110’ ; bit mask out only bits 5,4,2,1
movwf PORTD
goto loop ; Create a loop effect
sleep
END
Compile and check your program in the simulator. PROGRAM the PIC16F877 and
RUN the program. Play with the switches.
Describe what this program does, in your own words. 2 marks
Demonstrate to the lecturer/demonstrator/TA who will sign your script.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 73
(b) Let us change the program so that Switches 1-4 control the digit displayed on the 7-
Segment display.
You will need one of the lookup functions used earlier. Paste it in the appropriate place
in the code.
Because the switches are not connected to consecutive bits on PortB, we will need to
use a  le register to temporarily store the input, and make up the corrected output in
the working register. Fill in the missing mask values in the code.
LIST p=16F877
INCLUDE <p16F877.inc>
Inp EQU 0x20
ORG 0x00
nop
goto main
main
BANKSEL TRISB ; Switch to bank 1
movlw 0xFF ; Set PORTB to
movwf TRISB ; an input port
bcf OPTION_REG,7 ; enable pull-ups
clrf TRISD ; PortD all outputs
bcf TRISC,2 ; Port C transistor is an output
BANKSEL PORTB ; Move back to bank 0
bsf PORTC,2 ; turn on transistor
loop movf PORTB,W
movwf Inp
clrw
btfsc Inp,1 ; test bit 1, switch 4
iorlw ___________ ; if set, then set bit 3
btfsc Inp,1 ; test bit 2, switch 3
iorlw ___________ ; if set, then set bit 2
btfsc Inp,1 ; test bit 4, switch 2
iorlw ___________ ; if set, then set bit 1
btfsc Inp,1 ; test bit 5, switch 1
iorlw ___________ ; if set, then set bit 0
call lookup
movwf PORTD
goto loop ; Create a loop effect
sleep
; place either lookup function here!
END
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 74
Compile and check your program in the simulator. PROGRAM the PIC16F877 and
RUN the program. Play with the switches.
Does it work as you expected? Yes or No (Circle your answer)
If it didn’t what was the problem? How did you  x it?
i. Describe what this program does, in your own words. 2 marks
ii. Is it possible to use a mathematical algorithm, to convert the bits from PORTB? If
so, would it be faster/shorter than the lookup table? Explain your answer(s). 1 mark
End of CORE material. ALL students expected to reach here within 3 hours.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 75
Timers
5. (a) If the pre-scaler is used with Timer0, with a rate of 1:128, and T0CS is clear, what
frequency will the timer over ow at (assume 4MHz clock signal)? 2 marks
(b) Let’s use Timer0 to slowly count upwards on the 7-segment LED display.
Use the code on the next page. You will need one of the lookup functions used earlier.
Paste it in the appropriate place in the code. Verify that the code is consistent with the
comments. Check the program in the simulator, then program the PIC16F877 and run
the program.
Does it work as you expected? Yes or No (Circle your answer)
If it didn’t what was the problem? How did you  x it?
Show your results to the TA/lecturer who will sign your script. 4 marks
(c) i. What frequency does the digit change at? 2 marks
Is this frequency what you expected? Yes or No (Circle your answer)
If it isn’t, what was the problem? Use the space below to clarify your thoughts.
ii. What type(s) of delay did we use in the program: busy-wait, instruction-based, or
timer based delay(s)? Explain your answer. 2 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 76
LIST p=16F877
INCLUDE <p16F877.inc>
Cnt EQU 0x22
Dlys EQU 0x23
ORG 0x00
nop
goto main
main bsf STATUS,RP0 ; bank 1
movlw ___________ ; set T0CS=1, prescaler = 128
movwf OPTION_REG
clrf TRISD ; PortD all ouputs
bcf TRISC,2 ; Port C transistor for 7-segment display is an output
BANKSEL PORTB ; Move back to bank 0
bsf PORTC,2 ; turn on transistor for 7-segment display
clrf Cnt
mloop movlw .250 ; set the number of Delays we want
movwf Dlys
movf Cnt,W ; display the current count
call lookup
movwf PORTD
incf Cnt,F ; increment the Count
iloop call Delay ; call 250 Delay’s
decfsz Dlys
goto iloop
goto mloop
sleep ; in case we run past the goto, stop here
; Now for the delay routine
Delay bcf INTCON,T0IF ; clear to be sure
BANKSEL TMR0 ;
clrf TMR0
bsf STATUS,RP0 ; switch to bank 1 to access OPTION_REG
bcf OPTION_REG,T0CS ; start timing
bcf STATUS,RP0
Loop btfss INTCON,T0IF ; keep checking for overflow
goto Loop
bsf STATUS,RP0 ; switch to bank 1 to access OPTION_REG
bsf OPTION_REG,T0CS ; stop timing
bcf STATUS,RP0
return
; place either lookup function here!
END
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 77
6. We can change the rate at which the digits change, by modifying the pre-scaler used by
Timer0.
Re-write the program so that the initial switch settings (RB5,RB4,RB2,RB1) are used to
select the pre-scaler setting for Timer0 (PSA,PS2,PS1,PS0). The pre-scaler value should
remain constant until the PIC16F877 is reset.
Create a new assembly language project for the PIC16F877 in the MPLAB IDE. Compile the
code and ensure that it is built successfully. Simulate it and see if it works, then PROGRAM
the PIC16F877 .
Does it work as you expected? Yes or No (Circle your answer)
If it didn’t what was the problem? How did you  x it?
Use the MPLAB IDE and/or command line tools to:
(a) Print your new program, and attach the printout to this script. 4 marks
(b) Record the frequency at which the digit changes for: 4 marks
i. one switch on,
ii. two switches on,
iii. three switches on,
iv. all switches on.
(c) Does the pre-scaler perform as you expected? Comment on the accuracy and resolution
of your results. 2 marks
Demonstrate to the demonstrator/TA who will sign your script & printout.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 78
7. Challenge (Bonus 10 marks): We can display a message on the Seven segment LED display,
by displaying successive characters at regular intervals.
Write a program (either assembly or C) which will display a message which is stored in
memory (a lookup table (assembly), or an array (C)). The  rst byte stored in memory is the
number of characters in the message. Each of the following bytes in memory is a byte which
must be sent directly to the Seven Segment LED display module.
You should write your program so that the message can be easily changed for demonstration.
You should assume that messages will always be less than 180 characters long. Test your
program with the following data:
d’63’, 0x1E, 0x2E, 0xDE, 0x00, 0xDE, 0x8E, 0x8E, 0xDE, 0x1A, 0x1E, 0x22, 0x56, 0xDE, 0x00,
0x22, 0x2A, 0x26, 0x1E, 0x0A, 0x38, 0x1A, 0x1E, 0x22, 0x3A, 0x2A, 0x00, 0x1A, 0x5E, 0x1A,
0x18, 0xDE, 0x00, 0xB8, 0x22, 0x1E, 0x2E, 0x00, 0xFA, 0x00, 0x66, 0xAA, 0x2E, 0x4A, 0x00,
0x3A,0x26, 0x1A, 0x22, 0x18, 0x18, 0xFA, 0x1E, 0x3A, 0x0A, 0x00, 0x22, 0x26, 0x00, 0x60,
0x4E, 0x26, 0x01, 0x00
Create a new project for the PIC16F877 in the MPLAB IDE. Compile the code and ensure
that it is built successfully. Simulate it and see if it works, then PROGRAM the PIC16F877
.
Does it work as you expected? Yes or No (Circle your answer)
If it didn’t what was the problem? How did you  x it?
Attach a printout of the program to the script.
Did this code work? Yes or No (Circle your answer)
Demonstrate to the demonstrator/TA who will sign your script & printout.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 79
8. This question will allow the student to investigate and develop code that converts Analog
Data into Digital Data using the A/D peripheral. Con gure ADCON0, ADCON1 so that the A/D
meets the following speci cations:
A/D Conversion Clock Fosc8
Analog Channel Channel 0 selected
State A/D Conversion not in progress, A/D On
Result rightt justi ed
Reference voltages Vdd, Vss
Port Con guration AN0 as the ONLY analog input
Bit <7> <6> <5> <4> <3> <2> <1> <0>
ADCON0
ADCON1 1 mark
The result produced by the A/D module is 10 bits long and not 8 bits nor 16 bits. This
means that the answer is put into two registers named ADRESH and ADRESL. When the answer
is left justi ed, valid bits are in ADRESH<7:0>, ADRESL<7:6>. When the answer is right
justi ed, valid bits are in ADRESH<1:0>, ADRESL<7:0>. We will only be using ADRESH. The
code appears on the following page. Fill in the values from the table above.
You will need one of the lookup functions used earlier. Paste it in the appropriate place in
the code.
Check that the code is consistent with the comments before moving on. If not, what changes
did you make?
In the code, Timer0 is used to set the sampling rate. If Timer0 has just over owed, how long
will it take to over ow again? Show your working. 2 marks
Compile the code and ensure that it builds successfully.
Did your code compile? Yes or No (Circle your answer)
If not, what changes did you make?
If you have problems ask the TA for assistance.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 80
LIST p=16f877
INCLUDE "p16f877.inc"
ORG 0x00
nop
goto Start ; Start at the reset vector + 1 for ICD
ORG 0x20
Start
BANKSEL PORTD
clrf PORTD ;Clear PORTD
movlw ___________ ;Fosc/8, A/D enabled, Sample Channel 0
movwf ADCON0
BANKSEL OPTION_REG
movlw B’10000111’ ;TMR0 prescaler, 1:256
movwf OPTION_REG
clrf TRISD ;PORTD all outputs
clrf TRISC ;PORTC all outputs
movlw ___________ ;Right justify, 1 analog channel
movwf ADCON1 ;VDD and VSS references
BANKSEL PORTD ;Turn on 7-segment display
movlw _______
MOVWF PORTC
Main bcf INTCON,T0IF
Loop btfss INTCON,T0IF ;Wait for Timer0 to timeout
goto Loop
bsf ADCON0,GO ;Start A/D conversion for channel 0
Wait btfss PIR1,ADIF ;Wait for conversion to complete
goto Wait
movf ADRESH,W ;Write A/D result to PORTD
call lookup
movwf PORTD ;LEDs
bcf PIR1,ADIF ;Clear the completion flag
goto Main ;Do it again
sleep
; place either lookup function here!
END ;this is the last line in the file
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 81
Does the program work as you expected? Yes or No (Circle your answer)
If it didn’t what was the problem? How did you  x it?
Demonstrate to the demonstrator/TA who will sign your script.
Program the 16F877, run the program and use a voltmeter to read the actual input voltage
to the PIC from the potentiometer. Note the voltage and the value displayed by the 7-
segment display in the chart below. Take 4 readings between 0 and 5 Volts, then work out
the equivalent voltages for the readings you observed on the 7-segment display.
Input Voltage PIC LED’s Equivalent Voltage
Comment on the accuracy and resolution of your results. 1 mark
Would accuracy and resolution improve if we used more bits of the A/D result? How could
we modify the code to achieve this? Try it! Explain what you changed, and take new readings. 1 mark
Input Voltage PIC LED’s Equivalent Voltage
Demonstrate to the demonstrator/TA who will sign your script.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 82
Pulse-Width Modulation
9. We shall now investigate Pulse Width Modulation (PWM). Compile the code below and
PROGRAM the PIC16F877. Then vary the POT.
If the circuit is working properly the LED’s in the LED Bar vary their brightness. Answer
the following questions based on the code on the following page, and your observations.
(a) Fill in appropriate comments at the places indicated. 2 marks
(b) What is the PWM frequency? Show your working. 2 marks
(c) Explain in your own words why the LEDS vary their brightness. 2 marks
(d) What happens if you change the PWM frequency speci ed by the Timer2 pre-scaler and
re-compile and re-run the program? Your answer should state how you changed the
Timer2 pre-scaler. 2 marks
(e) Based on your observations, is PWM an appropriate technique for D/A conversion? Ex-
plain your answer. 2 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 83
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 84
LIST p=16F877
INCLUDE <p16F877.inc>
ORG 0x00
goto main
ORG 0x20
main
BANKSEL PR2 ; __________________________________
movlw 0xFF ; __________________________________
movwf PR2 ; __________________________________
BANKSEL T2CON ; __________________________________
movlw B’00000110’ ; __________________________________
movwf T2CON ; __________________________________
movlw B’00001100’ ; __________________________________
movwf CCP1CON ; __________________________________
BANKSEL TRISD ;
clrf TRISD ;Configure PORTD -- all output pins.
bcf TRISC,2 ;Configure PORTC<2> as an output pin.
BANKSEL ADCON0
movlw B’01000001’ ;Fosc/8, A/D enabled, Sample Channel 0
movwf ADCON0
BANKSEL OPTION_REG
movlw B’10000111’ ;TMR0 prescaler, 1:256
movwf OPTION_REG
movlw B’00001110’ ;Left justify, 1 analog channel
movwf ADCON1 ;VDD and VSS references
BANKSEL PORTD
movlw 0xFF ;put an indicator value on the LED’s
movwf PORTD
Loop bcf INTCON,T0IF
Here btfss INTCON,T0IF ;Wait for Timer0 to timeout
goto Here
bcf PIR1,ADIF ;Clear the completion flag
bsf ADCON0,GO ;Start A/D conversion for channel 0
Wait btfss PIR1,ADIF ;Wait for conversion to complete
goto Wait
movf ADRESH,W ;_______________________________________
movwf CCPR1L
goto Loop ;Do it again
sleep
END
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 85
Doing it yourself
10. Challenge (Bonus 10 marks): Attempt one of the following challenges in either assembly
language or C:
(a) Write a program which will light up the segments on the LED bar, in sequence, at
regular intervals. The interval is determined using the switches, and the brightness of
the display is determined by the pot.
OR
(b) Write a program which will simultaneously display a count which increments/decrements
on the hex digit, as the segments on the LED bar light up in sequence.
Create a new project for the PIC16F877 in the MPLAB IDE. Compile the code and ensure
that it is built successfully. Simulate it and see if it works, then PROGRAM the PIC16F877.
Does it work as you expected? Yes or No (Circle your answer)
If your program didn’t work on the  rst try what were the problems? How did you  x it?
Attach a printout of the program to the script.
Did this code work? Yes or No (Circle your answer)
If so, Show the demonstrator/TA who will sign your script & printout.
Total marks 100.
This exercise is worth 5% of your ECNG2005 lab mark.
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
 assistance in troubleshooting circuitry and/or using/operating the IDE/debugger
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 86
L
A
B
3Unit 22 Write the letter you have been assigned here .
Letters refer to the following peripherals located on the PIC16F877:
A: A/D, B: PWM, C: PSP, D: Data EEPROM
1. Which datasheet page(s) have information related to this peripheral? .
2. Identify and classify the registers associated with this peripheral:
Peripheral Con guration Control State Data
3. Identify ONE electrical and/or timing constraints associated with this peripheral.
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this lab exercise.
{ utilize the MPLAB IDE and CCS compiler, to develop software for the MicroChip
PIC16Cxxx series of microcontroller, in C, C++ and assembly language.
{ implement basic programming operations (loops, swaps, lookups), integer arithmetic,
and other computation/numerical methods, in assembly language on the PIC16Cxxx
series of microcontrollers.
{ utilize built-in peripherals of the PIC16F877 microcontroller in simple applications.
 Which aspect of this lab exercise did you have the most di culty understanding?
 Which aspect of this lab exercise did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this lab exercise.
 Identify one way in which this lab exercise could be improved.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VI 87
Interfacing Peripherals
An understanding of peripheral operation is necessary in order to e ectively interface the micro-
processor to the external environment. This section has reviewed peripherals in general and the
PIC peripherals in particular, as well as the basics of interrupts, and reading peripheral registers.
It has also looked at some of the problems that may arise in protoyping/testind a system involving
peripherals. As such it has contributed to the third course objective. You should now be able
to \design and implement an appropriate interface between a microprocessor-based device and a
peripheral device".
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) Introduction to microprocessors
c DECE, UWI, St. Augustine, Trinidad
original author: Feisal Mohammed; updated: March 11, 2008{ CLR
Part VII
System Issues
The previous part of the course looked at basic operation of the microprocessor in an applied
context. This requires correctness of not just the program, but also the interface and circuitry.
In this part of the course, we look at both hardware and software issues which, while they do
not directly a ect system design, can in uence system performance. These issues are related to
interrupts, timing, signals and microprocessor choice. Design guidelines are presented on the basis
of some consideration of these issues.
23 Interrupt issues
At the end of this unit the student will be able to:
understand the implications of (nested/multiple) interrupts for execution times, and the
need for context saving.
It is possible to predict the execution time of code, on the basis of the instructions which make up
the program, and knowledge of how the pipeline (if it exists) performs. This was the basis on which
instruction generated delays were constructed. For the PIC16F877 in particular, all instructions
take 1 machine cycle, except for branches (goto, call, return) which take 2 cycles, conditional
skips (bit test, increment/decrement) which take 2 cycles if the skip is performed, and the initial
instruction executed on reset. The 2 cycle duration for certain instructions is due to the use of the
pipeline. For example, the following code takes 3 (1+2) cycles for each iteration, and 4 (2+2) for
the  nal stage:
Loop decfsz count,F
goto Loop
goto Done
There are many other considerations when attempting to predict the performance of code on a
microprocessor. These include:
 overhead due to memory error detection/correction
 overhead due to memory cache
 overhead due to bus timing
 overhead due to writing peripheral/special function processor registers
 overhead due to interrupts
The last of these will be considered in the next section. The discussion of performance is followed
by a review of two applications of the PIC.
ECNG2006 (EE25M) March 11, 2008 VII 2
Timing considerations with interrupts
When an interrupt source sends an interrupt signal to the CPU, it is signaling that it is ready to
be serviced. Along with the signal, is the implication that this servicing needs to occur in a timely
manner otherwise a failure may occur. For example, if characters are being received by a serial
port, the UART will signal the CPU that a character has arrived and needs to be removed from
its bu er. If the UART has a two byte FIFO bu er then the CPU must respond to the interrupt
before two more characters arrive and overwrite the  rst character received. If the characters arrive
at 9600 baud then the CPU must respond within roughly 2 ms. In order to design interrupt service
routines that can service interrupts in a timely manner, the length of time that the ISR takes must
be determined by counting the number of cycles the ISR takes.
Entering/leaving the ISR
The PIC inserts two additional cycles between the execution of the main instruction and the start
of execution of the ISR. This is shown in Figure (14). When the retfie instruction is executed, the
extra cycles
Cycle
Cycle
Cycle
Cycle
Cycle
Interrupt
fetch of last main instructionexecution of last main instruction(fetch of next main instruction ignored)dummy cyclefetch of instruction at 0x004execution of instruction at 0x004
Figure 14: Timing for start of interrupt
CPU does not insert any extra cycles as it returns to execution of the main program (Figure (15)).
In addition, there are two more cycles to be taken into account since the actual ISR does not startretfie takes two cyclesCycle
Cycle
Cycle
execution of the retfie instructionfetch of the next main instructionexecution of the next main instruction
Figure 15: Timing at end of interrupt
at 0x004 but is located elsewhere and must be reached by a goto ISR located at the 0x0004.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 3
The ISR
Using the sample ISR (given previously in Part V) as a base, the number of cycles for execution can
be determined. If there are N interrupt sources and the ith source takes Ti time for the execution
of its interrupt handler then the execution time for each event is
Event Cycles
Extra cycles inserted after execution of last main instruction 2
goto ISR instruction 2
Save W and STATUS registers 3
Polling routine 2N - 1
Time for the ith handler Ti
Restore W and STATUS 4
retfie instruction 2
Total 12 + 2N + Ti
These numbers can be represented as an overhead, (On), where the subscript represents the number
of interrupt sources plus the time for one handler or On +Ti. The following table gives a listing of
the overhead as the number of interrupt sources increases.
N interrupt sources Overhead (On)
(with only one pending)
1 14
2 16
3 18
4 20
. .
. .
Another time, Pi, can also be de ned as the time it takes from the start of the ISR to the test of
the ith interrupt source. The following table shows the values of Pi for various values of i.
N i Pi
1 8
2 10
3 12
4 14
. .
. .
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 4
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 5
Multiple interrupts
When multiple interrupts are pending at the same time, there are two things that are of interest,
how long does the CPU take from the time it leaves the main code to when it returns and how
long is the delay between the occurrence of an interrupt to when it is actually serviced. If the test
for an interrupt source is higher up in the polling sequence then it is said to be of higher priority
than one whose test is lower. Under normal circumstances a high priority interrupt will be handled
before one of low priority but there are instances where this is not the case.
Consider a case where there are two interrupt sources, 1 and 2, where 1 is of a higher priority than
2 and that T1 = T2 = 30 cycles. The interrupts overlap with 2 occurring before 1. The time that
the CPU will stop the running of the main program to when it continues execution depends upon
whether source 1 or source 2 is serviced  rst. If source 2 is serviced  rst then the time will be
(O2 +T1) + (O2 +T2) = (16 +T1) + (16 +T2) = 92 cycles
On the other hand, if source 1 is serviced  rst then the time will be
(O2 +T1) + (O2 +T2) O2 = (16 +T1) + (16 +T2) 16 = 76 cycles
The di erence occurs because after servicing source 1, the ISR will then be able to poll source 2
and carry out servicing without having to return to the main program.
P1
Main programInterrupt source 1Interrupt source 2
T2
T1
16 +
16 +
T1
cycles
T2
cycles
Figure 16: Interrupt 1 arriving too late to be serviced
It is instructive to determine the worst case time for the request for service, servicing of the interrupt
and return to the main program ready to service a request by the same source. Figure (16) shows
the sequence when the interrupt from source 1 arrives too late to be serviced, this gives the worst
case time for source 1. Therefore the worst case time for source 1, Tw1 is
Tw1 = (O2 +T2) + (O2 +T1) P1
If from before T1 = T2 = 30 cycles then Tw1 is 84 cycles.
The worst case time for source 2, Tw2, is then
(O2 +T2) + (O2 +T1) O2
Using the same assumptions from before Tw2 = 76 cycles. Which gives the paradoxical result of the
lower priority interrupt servicing actually completing quicker than the higher priority interrupt’s
servicing.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 6
P1
Main programInterrupt source 1Interrupt source 2
T1
16 +
T2
cycles
T2
Figure 17: Interrupt 1 arriving early enough to be serviced
If now the situation is expanded to three interrupt sources with each handler taking 30 cycles, and
the priorities are in the order 1, 2, and 3. Then the worst case timing for each of the three can be
illustrated in Figures (18), (19), and (20). In Figure (18) the worst time, Tw1 is
Tw1 = (O3 +T2) + (O3 +T3) + (O3 +T1) P1 O3 = 118 cycles
Similarly, the worst case time for source 2, Tw2, is
Tw2 = (O3 +T3) + (O3 +T1) + (O3 +T2) P2 O3 = 116 cycles
and  nally
Tw3 = (O3 +T1) + (O3 +T2) + (O3 +T3) O3 O3 = 108 cycles
P1
Main programInterrupt source 1Interrupt source 2Interrupt source 3
T3
T2
T1
Figure 18: Worst case timing for source 1
This results in the following inequality
T1 +T2 +   +Tn +On worst case time <T1 +T2 +   +Tn + 2On (1)
Equation (1) shows that in the worst case, the priority of the interrupt source does not matter.
The problem lies in the fact that on completion of the handler for a speci c interrupt source, the
polling routine is continued instead of restarted. Because of this continuation, all other pending
interrupt sources are tested and serviced before attending to the higher priority interrupt source.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 7
T1
Main programInterrupt source 1Interrupt source 2Interrupt source 3
T2
T3
P2
Figure 19: Worst case timing for source 2
T1
Main programInterrupt source 1Interrupt source 2Interrupt source 3
P1
T3
T2
Figure 20: Worst case timing for source 3
If instead the ISR is written as follows, where at the end of each handler the poll is done again,
then the interrupt from the higher priority gets the opportunity to be tested and serviced.
; save W and STATUS registers
; Polling sequence
Poll
btfsc INTCON,... ; test if interrupt 1 occurred
goto Hnd1 ; jump to handler 1
btfsc INTCON,... ; test if interrupt 2 occurred
goto Hnd2 ; jump to handler 2
Hnd1 ; code for handler 1
goto Poll ; back to polling sequence
Hnd2 ; code for handler 2
goto Poll ; back to polling sequence
; restore W and STATUS registers
retfie ; return from interrupt
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 8
The worst times for the modi ed ISR is shown in Figures (21), (22) and (23). The worst case time
T1
Main programInterrupt source 1
P1
Longer of 2 and 3
T2
or
T3
Figure 21: Worst case timing for source 1 (new ISR)
for source 1 is
Tw1 = O3 + (2 3 1 +T3) + (2 1 1 +T1) P1 = 78 cycles
Here it is assumed that T3 is the longer of T2 and T3. The (2 3 1) term is the time taken by
the polling sequence to test all sources until it gets to source 3, whose  ag is set. The 1 cycle in
the (2 1 1) term is the time taken to test for source 1 when its  ag is set.
T2
Main programInterrupt source 1Interrupt source 2Interrupt source 3
T3
P2
T1
Figure 22: Worst case timing for source 2 (new ISR)
The worst case time for source 2 is
Tw2 = O3 + (2 3 1 +T3) + (2 1 1 +T1) + (2 2 1 +T2) P2 = 109 cycles
The worst case time for source 3 is
Tw2 = O3 + (2 2 1 +T2) + (2 1 1 +T1) + (2 3 1 +T3) = 117 cycles
Comparing the results with those of the previous ISR, it can be seen that there is signi cant
improvement for the highest priority interrupt, source 1. The next highest, source 2, is slightly
improved also and the lowest priority, source 3 is slightly worsened. Now the Tw1 depends only
upon the longest handler of low priority, T3 and, of course, T1 instead of the times of all the handlers
as before.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 9
Interrupt source 1Main program Interrupt source 3
P2
T3
T2
T1
Interrupt source 2
Figure 23: Worst case timing for source 3 (new ISR)
Race conditions
Whenever there are interrupts the occasion may arise where there is data that is shared between
the main program and an interrupt handler. If the data (register(s)) is modi ed by either the main
program or the handler then a race condition exists and the  nal result depends upon which code
runs and precisely when it runs. In order to avoid this condition is to ensure that reading or writing
to the shared data is done by only one part of code at any time. In other words, mutual exclusion
must exist. This is normally accomplished by identifying critical sections, which are sections of code
where reading or writing takes place of the shared data. When the main program is in a critical
section it must not be interrupted. As an example, assume that the main program decrements a
16-bit variable,count = (counth, countl) while the interrupt handler increments it. When the
code is being executed the variable is being decremented from 0x0100 to 0x00 .
movf countl,F ; line 1
btfsc STATUS,Z ; " 2
decf counth,F ; " 3
decf countl,F ; " 4
If an interrupt occurs between lines 3 and 4, the interrupt handler will  nd count equal to 0x0000
and it will return to the main program after changed it 0x0001. The main program will then
execute line 4 and count will have the  nal value of 0x0000 instead of the correct value of 0x00 .
The critical section can be protected in the following manner.
Again bcf INTCON,GIE ; disable interrupts
btfsc INTCON,GIE ; check that an interrupt did not occur
goto Again ; ...in the meanwhile and set GIE back to 1
; Start Critical region
; code
; End Critical region
bsf INTCON,GIE ; re-enable interrupts
After the instruction to disable interrupts has executed it is necessary to check that the GIE  ag
is clear because an interrupt could have occurred just as the bcf instruction was executed. If this
happened then the interrupt would be serviced before the next instruction leaving the GIE bit set.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 10
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 11
Rotary Pulse Generator (RPG)
One of the biggest uses of microcontrollers such as the PIC chips is in embedded control and the
range of applications is quite extensive. One such use is in digital instruments. In contrast to fully
digital systems, analog systems would use potentiometers to vary values e.g. voltage output from
a power supply.
In digital systems, use of a potentiometer would require an A/D to convert the signal from the
potentiometer. It would be better to have the control to be fully digital. One such method is
through the use of a Rotary Pulse Generator (RPG).
A rotary pulse generator is an optical rotary incremental encoder. It is commonly used by in-
struments that require the ability to vary setup parameters, such as voltage, frequency, etc. The
RPG has two outputs, channel A and channel B and by comparing the both outputs, it can be
determined if the RPG is being turned in a clockwise or anticlockwise direction. Figure (24) shows
the waveforms when it is turned in either direction. If the rising edge of channel A is compared
with the signal level on channel B, it can be seen that for clockwise motion, the rising edge of A
coincides with a low on B. For counter-clockwise motion, the rising edge of A coincides with a
high on B. Thus the number of pulses can be determined in addition to the direction of motion.
CW
Channel A Channel B
CCW
Figure 24: Encoder output
RPG
A B
RB0/IntRB1PIC
Figure 25: Connection between the PIC and RPG
Suppose the RPG is connected to the PIC as shown in Figure (25). Channel A is used to generate
external interrupts at each rising edge and an thus an Interrupt Service Routine (ISR) can be used
to determine the direction of rotation. The use of a RPG makes it possible to have additional
features such as coarse/ ne control of parameters using a single control knob, something which is
not possible if a potentiometer is used. Assume that the RPG is being used to control the voltage
of a voltage supply. When the control knob is turned quickly, the voltage is to be changed in 1.0V
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 12
16 int/s
80 int/s
0 int/s
FAST
SLOW
Figure 26: Di erentiation between fast and slow speeds
steps and when the control knob is turned slowly the voltage steps are 0.1V. Therefore in addition
to knowing whether to increase or decrease the voltage, the speed of rotation must also be known.
The maximum rate of rotation of the RPG is 2.5 rev/sec and if it generates 32 cycles per revolution,
there will be 80 interrupts generated in one second or one every 12.5 ms. This represents the high
limit of rotation with the lower limit being 0. A reasonable threshold between fast and slow is
0.5 rev/sec, which translates to 16 interrupts/sec or one every 62.5 ms (Figure (26). The built-in
timer of the PIC can be used to detect whether the RPG is turning fast or slow by having the timer
timing out when turning slowly and not timing out when turning fast. The test that determines
this is as follows:
; This is called at every interrupt
if ( TMR0 has timed out )
then RPG is turning slowly
else
RPG is turning fast
Reset TMRO
It is important to reset TMR0 back to zero at every interrupt because it is the time between
interrupts that is of interest. Now it is necessary to setup the timer and prescaler such that it times
out in approximately 62.5 ms. A setting of 1:256 in the prescaler, when the PIC is driven by a
4MHz clock signal, will cause timeouts every 65.5 ms which is reasonably close enough to 62.5 ms.
The main program now looks like the following:
ORG 0x000
goto Init
ORG 0x004
goto ISR
; The Init section configures TMR0, the prescaler and PORTB
Init
bsf STATUS,RP0 ; switch to Bank 1
movlw B’11000111’ ; bit 7 pull up disable
; bit 6 interrupt on rising edge
; bit 5 use internal clock for TMR0
; bit 3 assign prescaler to TMR0
; bits 2,1,0 prescaler scaler set at 1:256
movwf OPTION_REG
bsf TRISB,1 ; Line 1 of PORTB set as input
bcf STATUS,RP0 ; back to Bank 0
bsf INTCON,INTE ; enable external interrupt
bsf INTCON,GIE ; enable Global interrupt
main ....
goto main
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 13
The ISR now implements the algorithm
ISR movwf w_temp ; save W and STATUS
movf STATUS,W ;
movwf status_temp ;
Poll btfsc INTCON,INTF
goto RPG ; test for only one interrupt source
movf status_temp,W ; restore W and STATUS
movwf STATUS ;
swapf w_temp,F ;
swapf w_temp,W ;
retfie ; return from interrupt
RPG bcf INTCON,INTF ; clear the extern interrupt flag
clrf TMR0 ; reset the timer
btfsc INTCON,T0IF ; check if timer overflowed
goto update_small ; overflowed... turning slowly
goto update_large ; turning fast
update_small
bcf INTCON,T0IF ; reset the overflow flag
btfss PORTB,1 ; check channel B of RPG
goto inc_small ; low? clockwise
goto dec_small : high? counterclockwise
update_large
btfss PORTB,1 ; check channel B
goto inc_large
goto dec_large
inc_small
; increase the voltage by 0.1 V
call increase_voltage_small
goto Poll
dec_small
; decrease the voltage by 0.1 V
call decrease_voltage_small
goto Poll
inc_large
; increase the voltage by 1.0 V
call increase_voltage_large
goto Poll
dec_large
; decrease the voltage by 1.0 V
call decrease_voltage_large
goto Poll
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 14
While the system as shown will work, much of the processing is being done in the ISR which
can cause the ISR take too long to complete its processing. A better solution is to have the ISR
determine what change in voltage is needed and have the main program implement the change of
voltage. What is required now is a means of informing the main routine that a change is required.
This can be accomplished through the use of  ags encoded as bit positions in a  le register e.g.
upd_m. The meaning of the various bits are
Bit Action
0 decrease small
1 decrease large
2 increase small
3 decrease large
The ISR would then determine what change is required, set the appropriate  ag and exit. The
main routine polls the variable, upd_m, for the requested change. The main routine then becomes
decS equ 0
decL equ 1
incS equ 2
incL equ 3
org 0x000
goto Init
org 0x004
goto ISR
Init
; Initialize TMR0, the prescaler and PORTB
; ... as before
main movf upd_m,F
btfsc STATUS,Z
goto main ; if zero keep polling
btfsc upd_m,decS
call decrease_voltage_small
btfsc upd_m,decL
call decrease_voltage_large
btfsc upd_m,incS
call increase_voltage_small
btfsc upd_m,incL
call increase_voltage_large
goto main
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 15
The ISR can now execute much faster with the removal of the subroutine calls to change the voltage.
ISR ; save W and STATUS etc
Poll btfsc INTCON,INTF
goto RPG : test for only one interrupt source
; restore W and STATUS
retfie ; return from interrupt
RPG bcf INTCON,INTF ; clear the extern interrupt flag
clrf TMR0 ; reset the timer
clrf upd_m ; clear prior to setting flag
btfsc INTCON,T0IF ; check if timer overflowed
goto update_small ; overflowed... turning slowly
goto update_large ; turning fast
update_small
bcf INTCON,T0IF ; reset the overflow flag
btfss PORTB,1 ; check channel B of RPG
goto inc_small ; low? clockwise
goto dec_small : high? counterclockwise
update_large
btfss PORTB,1 ; check channel B
goto inc_large
goto dec_large
inc_small
; increase the voltage by 0.1 V
bsf upd_m,incS
goto Poll
dec_small
; decrease the voltage by 0.1 V
bsf upd_m,decS
goto Poll
inc_large
; increase the voltage by 1.0 V
bsf upd_m,incL
goto Poll
dec_large
; decrease the voltage by 1.0 V
bsf upd_m,decL
goto Poll
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 16
Frequency measurement
Another use of the timer/counter is to measure frequencies. For this purpose, a number of cycles
must be counted over a known time. The timer/counter cannot be used to generate this time
reference, so an external time reference must be used. For simplicity it can be assumed that an
external time reference exists with a base of 1 sec, this can be fed into the external interrupt line
so that interrupts will be generated every second. At the  rst interrupt, the counter will be cleared
and at the next interrupt the value in the counter will be the frequency. If the counter is used
without the prescaler, the range of frequencies would be from 0 to 255 Hz. If the prescaler, with a
scale of 1:256, is used, the range of frequencies is now 0 to 65280 Hz, but since the prescaler cannot
be read there can be an error in that measurement which can be as much as 255 cycles. This error
can be signi cant if the same 1:256 scale is used for low values of frequency. Therefore it is better
to use an additional  le register to count the number of times the counter has over owed and keep
the prescaler at low values e.g. 1:2 where the error is low or even not to use the prescaler at all.
The ISR responds to two sources, counter over ow and time reference rising clock edge. When the
counter over ows the  le register, say ncycles is incremented by one. The actions for the time
reference interrupt were given above. For  exibility, the measuring system can be assigned four
states, so that measurements can be taken at speci ed intervals or on a continuous basis.
 Idle
 Measurement requested
 Measurement in progress
 Measurement complete
These four states can be encoded as bits within a  le register as
00 Idle
01 Measurement requested
11 Measurement in progress
10 Measurement complete
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 17
The main routine as show below requests a measurement and loops until it is done and can be
easily changed to take continuous measurements. At the end of the measurement the frequency is
in the 16-bit variable (Freq hi, Freq lo).
DONE equ B’00000010’
org 0x000
goto Init
org 0x004
goto ISR
Init bsf STATUS,RP0 ; switch to Bank 1
movlw B’11100000’ ; bit 7 pull up disable
; bit 6 interrupt on rising edge
; bit 5 use external clock for TMR0
; bit 4 increment on rising edge
; bit 3 assign prescaler to TMR0
; bits 2,1,0 prescaler scaler set at 1:2
movwf OPTION_REG
bcf STATUS,RP0 ; back to Bank 0
bsf INTCON,INTE ; enable external interrupt
bsf INTCON,T0IE ; enable overflow interrupt
bsf INTCON,GIE ; enable Global interrupt
main bsf Flags,0 ; request a single measurement
Wait movlw DONE ; pattern to check for in W
xorwf Flags,W ; xor will give 0 if pattern found
btfss STATUS,Z
goto Wait ; not found? loop
clrf Flags ; go to idle mode, then show measurement
; the frequency = Ncycles*256*prescale + TMR0*prescale
clrf Freq_hi
rlf Freq_lo,F ; shift left by 1 i.e. mult by 2 (prescale)
rlf Freq_hi,F ; excess into hi byte
movf Ncycles,W ; add the rest
addwf Freq_hi,F
rlf Freq_hi,F ; times 2 again (prescale)
call Display_frequency
end
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 18
The ISR has to respond to two sources, the external interrupt and the over ow interrupt from the
counter.
INPROG equ B’00000011’
ISR ; save W and STATUS
Poll btfsc INTCON,INTF
goto Measure
btfsc INTCON,T0IF
goto Counter
; restore W and STATUS
retfie
Measure bcf INTCON,INTF
btfss Flags,0
goto Poll
btfss Flags,1
goto Start_read
goto Stop_read
Start_read
clrf TMR0 ; initialize counter and Ncycles
clrf Ncycles
bsf Flags,1 ; set to 11
goto Poll
Stop_read
movf TMR0,W ; save contents
movwf Freq_lo
bcf Flags,0 ; set to 10
goto Poll
Counter bcf INTCON,T0IF ; clear interrupt flag
movlw INPROG
xorwf Flags,W ; check if measurement in progress
btfsc STATUS,Z
incf NCycles,F ; yes? increment
goto Poll
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 19
Review Exercises
1. A student writes a PIC16F877 ISR in which he checks for the TMR0 interrupt, the external
interrupt, and the Serial RX interrupt in that order. He observes that if the TMR0 interrupt
and the Serial RX interrupt occur at the same time, sometimes the Serial RX interrupt gets
processed  rst. This is probably because:
(a) Sometimes the interrupt signals occur when the ISR is already handling an external
interrupt.
(b) Sometimes the interrupt handlers, interrupt each other.
(c) Sometimes TMR0 malfunctions and over ows.
(d) Sometimes the Serial peripheral receives alot of data.
(e) Sometimes the PIC16F877 malfunctions and resets itself.
2. A rotary pulse generator is connected to the INT line of a PIC16F877. If the generator
produces pulses such that the INTF  ag goes high multiple times while executing the handler,
we can ensure that the program does not get stuck in the ISR by any of the following means
EXCEPT:
(a) moving lengthy sections of code from the handler to the main program.
(b) clearing the INTF bit just before leaving the handler
(c) clearing the INTE bit before leaving the handler
(d) clearing the GIE bit before leaving the handler
(e) none of the above
3. You are attempting to stop ALL interrupts from interfering with a section of your code.
Which ONE of the following code wrapper snippets will do the job:
(a) redo bsf INTCON, GIE
btfsc INTCON, GIE
goto redo
......
bcf INTCON, GIE
(b) redo bcf INTCON, GIE
btfsc INTCON, GIE
goto redo
......
bsf INTCON, GIE
(c) redo bcf INTCON, PEIE
btfsc INTCON, PEIE
goto redo
......
bsf INTCON, PEIE
(d) redo bcf INTCON, GIE
btfss INTCON, GIE
goto redo
......
bsf INTCON, GIE
(e) redo bsf INTCON, GIE
btfss INTCON, GIE
goto redo
......
bcf INTCON, GIE
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 20
4. The Working register and status register must be stored/restored in any ISR. Which of the
following code fragments, demonstrate how this is correctly done from ANY data bank?
(a)
_W EQU 0x21
_STATUS EQU 0x22
isr movwf _W
movf STATUS,W
clrf STATUS
movwf _STATUS
.
.
movf _STATUS,W
movwf STATUS
movf _W
(b)
_W EQU 0x71
_STATUS EQU 0x22
isr movwf _W
movf STATUS,W
clrf STATUS
movwf _STATUS
.
.
movf _STATUS,W
movwf STATUS
swapf _W,F
swapf _W,W
(c)
_W EQU 0x71
_STATUS EQU 0x72
isr movwf _W
movf STATUS,W
clrf STATUS
movwf _STATUS
.
.
movf _STATUS,W
movwf STATUS
movf _W
(d)
_W EQU 0x21
_STATUS EQU 0x72
isr movwf _W
movf STATUS,W
clrf STATUS
movwf _STATUS
.
.
movf _STATUS,W
swapf _W,F
swapf _W,W
(e)
_W EQU 0x21
_STATUS EQU 0x22
isr movwf _W
movf STATUS,W
clrf STATUS
movwf _STATUS
.
.
movf _STATUS,W
movwf STATUS
swapf _W,F
swapf _W,W
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 21
5. Consider a PIC application that has four interrupt sources, with T1 =10 cycles, T2 =20 cycles,
T3 =40 cycles and, T4 =80 cycles
(a) Using the inequality
T1 +T2 +   +Tn +On worst case time <T1 +T2 +   +Tn + 2On; (2)
determine the worst-case response of the CPU to any of these interrupts given the polling
scheme where subroutine calls are used for each handler.
(b) What does this mean for the minimum period between any of these interrupts?
6. Consider the timing diagrams from Figures (8), (9) and (10) in the notes titled Input/Output.
(a) Why does the worst-case time for source 1 begin at the end of the time labeled P1 instead
at the time before this when the CPU actually stops execution of the main program?
(b) Why does the worst-case time for source 2 begin at the end of the time labeled P2 instead
at the time before this when the CPU actually stops execution of the main program?
(c) Why does the worst-case time for source 3, the lowest priority interrupt, begin when the
CPU actually stops execution of the main program rather at the end of P3 or P1?
7. Using the new polling structure for ISRs i.e. using goto for the handlers instead of call, con-
sider an application with four interrupt sources having times exactly like those in Problem 5
and priority T1, T2, T3, and T4.
Draw the worst-case timing diagram for the response of the CPU to interrupts from source 1.
Also determine the time TP1, the minimum time between source 1 interrupts such that the
CPU will assuredly execute source 1’s handler and be ready to handle another source 1
interrupt. Repeat this exercise for source 3.
8. When an interrupt occurs, the global interrupt enable bit, GIE, is automatically cleared by
the CPU. Sometimes it is necessary to disable interrupts for a short time in the main program
in order to execute a short sequence that can give an incorrect result if interrupted and then
re-enable the interrupt after the sequence.
A problem could arise if the instruction to disable the interrupts is being executed at the
exact moment that an interrupt occurs. Describe the e ect if the instruction to disable the
interrupts is indeed executed, but the interrupt is acknowledged and handled by the CPU
before the program instruction is able to take e ect.
9. The code presented for frequency measurement should produce a 16-bit estimate for the
switching frequency of the signal connected to Timer0. Comment on the accuracy of this
measurement. Hints: Check the minimum duration change detected on the Timer0 input,
and the ISR execution time.
10. Challenge:
(a) Using the code developed for the Rotary Pulse Generator, modify it to increment a 2-
byte variable, Amplitude if the RPG has been rotated CW and to decrement it if the
rotation is CCW. Do not increment past 65,535 or decrement past zero.
(b) Modify the solution to Problem (10a) to change the variable Amplitude by one-eighth of
its value or by one count, whichever is larger when fast turning of the RPG is detected.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 22
Assignment D 39 ID#
The notes contain two sets of code for the RPG (Rotary Pulse Generator). In the  rst set (page
13) the subroutines are called from within the ISR, and in the second set (page 14) the subroutines
are called from the main program. You should presume that EACH subroutine: takes n instruction
cycles to execute, requires p instructions, and uses f  le registers.
1. Determine how many program instructions and  le registers are required for each version of
the code. Show all reasoning. 6 marks
2. In each case, presuming that the ISR completes before the next interrupt occurs, determine
the time it takes to from the occurrence of an interrupt event to: 12 marks
(a) the completion of the ISR
(b) the completion of the relevant subroutine
3. In each case, determine the largest value of n which will allow: 6 marks
(a) the ISR to complete BEFORE the next interrupt event occurs.
(b) the relevant subroutine to complete BEFORE the next interrupt occurs
4. Explain (for each case), whether the RPG will continue to function correctly if another inter-
rupt event occurs before:
(a) the previous interrupt event has been handled.
(b) the previous interrupt event has exited the ISR.
Where the RPG may malfunction, suggest amendment(s) which will correct the problem. 6 marks
5. In the second case, a variable called updm is used to communicate the ISR result to the main
routine. There is a  aw in the main routine, in that updm needs to be cleared before the last
line of the main routine i.e. goto main. In your own words, explain why this is so. 2 marks
6. The correction introduced in the last question solves one problem, and introduces another
according to where the main routine is interrupted. Find, and explain the problem, and
suggest a solution. 4 marks
7. Based on the above, which RPG program would you prefer to use? In your own words, justify
your decision. 4 marks
This assignment is worth 2% of your ECNG2006 mark. It contains a total of 40 marks.
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
39Students are advised that there is no online version of this assignment, and that electronic submissions will not
be accepted without the explicit permission of the course lecturer.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 23
A
S
S
T
DUnit 23 A PIC16F877 program samples the value on the upper bits of PORTB<7:4> whenever a change in those
values is detected. The most recently sampled value is output to PORTD whenever a button press is detected at
PORTB<0>. Interrupts are ONLY generated by activity on PORTB. The main loop polls the variable MyFlags and
updates PORTD as required.
Loop btfss MyFlags,0 ; If the button was pressed
goto Loop
bcf MyFlags,0
movf SampleA,W ; Copy the new value to PORTD
movwf PORTD
goto Loop
The interrupt handler fragments are executed by the main ISR routine, and return to the main ISR routine to retest
all interrupt  ags, on completion. RBIF is tested for before INTF.
RBIFInt bcf INTCON,RBIF; if input value changed, store it
movf PORTB,W
andlw 0xF0
movwf SampleA
swapf SampleA,F ; place input bits PortB<7:4> in SampleA<3:0>
goto Poll ; back to polling sequence
INTFInt bcf INTCON,INTF; if button pressed set the flag
bsf MyFlags,0
goto Poll ; back to polling sequence
Write the name of the ISR handler you have been assigned here . The handler takes
cycles, and has a worst case response time of cycles.
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ understand the implications of (nested/multiple) interrupts for execution times, and the
need for context saving.
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
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ECNG2006 (EE25M) March 11, 2008 VII 24
24  P interface/timing issues
At the end of this unit the student will be able to:
 interpret and recognize the implications of, microprocessor timing and interfacing
requirements.
 determine the noise, voltage, and current considerations due to microprocessors
and peripheral fan-in/out.
In the world of the ideal  P, inputs and outputs switch instantly; outputs can source/sink in nite
amounts of current, and inputs never sink/source current. Unfortunately not one of these state-
ments is true. When designing a  P-based system, it is therefore important that we not only check
that our design are functionally correct, we must also check that the components are capable of
achieving the demands being made, and will do so reliably. In practice this means reading electrical
characteristics, and timing diagrams, and performing worst case analyses. (Arnold 2001; Ch. 3)
Timing diagrams
\ The most important timing speci cations for interfacing components to a bus-oriented
design are:
 Rise/fall time
 Propagation delay time
 Setup time
 Hold time
 Tri-state enable and disable delays
 Pulse width
 Clock frequency
" { (Arnold 2001; Ch.3)
Rise and fall times refer to the time it takes for a signal to move from logic high to logic low,
and vice versa. These have maximum acceptable values (inputs) and minimum achievable values
(outputs).
Propagation delay time refers to the time it takes between a signal changing on one line, to a
response being observed on another line. This may not necessarily be the same for high-low and
low-high transitions.
Setup and hold times refer to the times that the signal must be stable, pre/post a transition on
another signal line (generally clock).
Tri-state circuitry has an inherent delay between the change of input signal, and the change of
output voltage.
Some signals must be held for a time within a  xed range, in order to communicate a particular
value. The time from leading to falling edge is the pulse width.
Clock frequency determines the rate at which micro-operations are performed (Q cycles), and hence
determines how long the signal must be held in order to ensure that the relevant micro-operation
was performed.
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ECNG2006 (EE25M) March 11, 2008 VII 25
It is important to ascertain whether any of the timing parameters are being violated, as transient
\glitches" can result if timing is violated.
Electrical characteristics
The device pin may be modelled as a load with input and output resistance and capacitance. It
may also be modelled as a voltage source with a current limitation. It follows that any signals
passing between devices will be a ected by the electrical characteristics of the pins at either side
of the connection.
Noise margin analysis { input voltage limits should not lie \outside" of output voltage limits.
AC load analysis { the sum of the input capacitances should not exceed the load capacitance.
DC load analysis { current sourced/sunk should not exceed the speci ed maximum values.
Varied Behaviour
Temperature, pressure and operating frequency can a ect the electrical characteristics and the
operating voltage ranges. Curves are normally provided to allow you to predict how the device
behaviour will change as these deviate from nominal values.
Compensating Strategies
If your analysis reveals a problem, a number of strategies can be employed to  x it.
 Signals which do not rise/fall fast enough may be pulled high/low using a resistor { this will
reduce the transition times to this default state.
 Clock frequencies (generally externally supplied) can be reduced/increased in order to allow
more time between events.
 Handshaking or other arbitration protocols may be used to ensure that no contention occurs
on signal lines.
 Loading/noise margin issues can generally be  xed by using appropriate logic bu ers.
Additional strategies appear below.
\ Forms of digital signal degradation.
Signal degradation Possible solution at input
Slow edge Schmitt trigger
Voltage attenuation Schmitt trigger
EM interference Analogue  lter + Schmitt trigger,
{ data error digital  lter
EM interference Current limiting resistor + diode
{ ... equipment damage opto-isolate, other voltage clamp
Switch bounce Debounce in hardware/software
Earth di erential Opto-isolate
"
{ (Wilmhurst 2001; Table 9.1, pg 253)
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ECNG2006 (EE25M) March 11, 2008 VII 26
Reference Notes for the PIC16F877
Output Pin Load resistance RL 464 
Output Pin Load capacitance CL 50 pF
Input Pin capacitance Cin 5 pF
Max. External Clock rise/fall time Tos 25ns
Max. Port output rise/fall time Tio 40ns
Max. Delay port output TosH2io 255ns
Min. Input Hold time TosH2ioI 100 ns
Max. current sunk/sourced by output (ignoring totals) 20mA
Max. input leakage current IIL  1 A
Output Low Voltage VOL 0.6V
Output High Voltage VOH VDD 0:7 Volts
Max. Input Low Voltage VIL 0:15VDD Volts
Min. Input High Voltage VIH 0:25VDD + 0:8 Volts
Machine cycle (4Q) Q1 Q2 Q3 Q4
(in 1 instruction cycle)
Pipeline { Fetch stage PC on bus Check Interrupts PC=0x04 IR=0x00
(Interrupt) Disable GIE Latch Stack
Pipeline { Fetch stage PC on bus Check interrupts Increment PC Latch IR
(No Interrupt) Trigger Program memory
Pipeline { Execute stage Decode IR Register Read Execute/Process Register Write
Peripheral actions Output signal Input signal Set  ags (interrupt/status)
All voltages must lie between Vdd and Vss. All ratings taken at 25 .(PIC 2001)
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Review Exercises
Use the parameters for the PIC16F877 supplied in the notes, to answer the following questions.
1. A PIC16F877 powered at 4.9V is required to switch a digital signal line. The load at the end
of the line sinks 40mA, and has a load capacitance of 0.1 pF. The load reads a logic high at
V=2.5V and a logic low at 1.8V. Required rise/fall times for the signal are comparable to the
nominal values for the PIC16F877 I/O pins. Label the following statements as True/False:
(a) A pull-up resistor may be used to maintain the high bus voltage at appropriate levels.
(b) The PIC16F877 pin will be able to supply the load current.
(c) The interface can be implemented if we use a bu er, or switch a transistor in order to
drive the load.
(d) The rise and fall times for the signal voltage will be greater than those cited in the
data-sheet, because the load capacitance is too great.
(e) The signal voltage will be less than the nominal values, and therefore may not meet the
load requirements.
2. A PIC16F877 with a 4MHz oscillator is used with an external A/D converter which responds
with the latest digital reading on PORTD<3:0> 1 s after the write line on PORTD<4> is raised.
Assuming that PORTD has already been appropriately con gured, which piece of code will
successfully read the converter in the least instruction cycles?
(a) bsf PORTD,4
movf PORTD,W
andlw 0x0F
movwf VAL_IN
(b) bsf PORTD,4
nop
movf PORTD,W
andlw 0x0F
movwf VAL_IN
(c) bsf PORTD,4
movf PORTD,W
andlw 0x0F
movf VAL_IN
(d) bsf PORTD,4
movlw 0x0F
andwf PORTD,W
movwf VAL_IN
(e) bsf PORTD,4
movf PORTD,W
nop
andlw 0x0F
movwf VAL_IN
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ECNG2006 (EE25M) March 11, 2008 VII 28
3. The timing diagram is for the PIC16F877 Parallel Slave Port.
Which time re ects the setup time for data which needs to be written to the PIC16F877?
(a) a
(b) b
(c) c
(d) d
(e) none of the above
4. A PIC16F877 with a 4MHZ oscillator is used to communicate with a device which requires:
 enable line is held high for at least 3:5 s,
 draws 10mA from the enable line, and has an input capacitance of 3pF
 interprets 0.5 V as input-low, and 2.1 volts as input-high
Which piece(s) of code (if any) will achieve the appropriate timing for the enable line?
(I) bsf PORTA,E
nop
bcf PORTA,E
(II) bsf PORTA,E
nop
nop
bcf PORTA,E
(III) bsf PORTA,E
nop
nop
nop
bcf PORTA,E
(a) none of these will achieve the required timing
(b) (III) only
(c) (III) and (II) only
(d) (III), (II) and (I)
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ECNG2006 (EE25M) March 11, 2008 VII 29
5. A PIC16F877 with a 4MHZ oscillator is used to communicate with a device which requires:
 enable line is held high for at least 3:5 s,
 draws 10mA from the enable line, and has an input capacitance of 3pF
 interprets 0.5 V as input-low, and 2.1 volts as input-high
Once the timing is appropriate, will the communication be successful, and if not why not?
(a) Yes, communication will be successful.
(b) No, we have violated the voltage restrictions.
(c) No, we have violated the current restrictions.
(d) No, we have violated the capacitance restrictions.
(e) No, we have violated two or more of the restrictions.
6. What happens when the load capacitance of an IC signal pin is exceeded?
(a) The signal voltage will never get to the expected maximum/minimum values.
(b) On transition, the signal will exceed the speci ed rise/fall times.
(c) The signal pin will not be able to deliver the speci ed currents.
(d) The IC will stop working, and there will be no signal at all.
(e) None of the above.
7. A PIC16F877 is to be interfaced to a device which has the following electrical characteristics:
Max. current sunk/sourced by output (ignoring totals)  10 A / 5 A
Max. input leakage current  10 A
Output Pin Load capacitance 25 pF
Input Pin capacitance 10 pF
Max. Output Low Voltage 1V
Min. Output High Voltage 3:5 Volts
Max. Input Low Voltage 2:5 Volts
Min. Input High Voltage 4 Volts
All ratings taken at 25 .
Which one of the following statements (if any) is NOT a correct analysis:
(a) DC analysis PIC16F877 output to device input: OK  10 A < 20mA
(b) DC analysis device output to PIC16F877 input: OK  1 A < 10=+ 5 A
(c) AC analysis PIC16F877 output to device input: OK 10pF < 50pF
(d) AC analysis device output to PIC16F877 input: OK 5pF < 25pF
(e) none of the above
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ECNG2006 (EE25M) March 11, 2008 VII 30
8. Two PIC16F877 microcontrollers (A and B) are to be interfaced with each other.
(a) What is the maximum number of inputs on B, which can be driven from a single output
on A? Show your working. Hint: Consider current load and load capacitance on A’s
output pin.
(b) In your own words, explain what happens if the load capacitance on A’s output pin is
exceeded.
(c) A and B communicate across a single signal line. A needs to output a high for 10 time
units, and a low for 20 time units. We write a subroutine Output which we call with the
appropriate value (high/low) already in W. The sequence of instruction operations is as
follows:
...
Output movf Port ;A places a value on the signal line (output)
btfss Port, SigBit ;A tests the value it just put out
call delay10 ; and delays for an appropriate time
call delay10
return
....
When this code is executed on the simulator it works. In each of the following scenarios,
identify the cause of this problem, and suggest a solution.
i. When this code is executed on the PIC with the pin open, the oscilloscope shows
that it stays high for 10 time units and low for 20 time units. When B is connected,
the oscilloscope shows that it stays high for 10 time units and low for 10 time units.
ii. When this code is executed on the PIC with the pin open, the oscilloscope shows
that it stays high for 10 time units and low for 10 time units.
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9. You are given a device whose I/O pins have the following characteristics:
Max. current sunk/sourced by output (ignoring totals)  10 A / 5 A
Max. input leakage current  10 A
Input Pin capacitance 10 pF
Output Low Voltage 1V
Max. Input Low Voltage 2:5 Volts
Min. Output High Voltage 3:5 Volts
Input signal hold time (after Write Line) 4 s
Input signal setup time (before Write Line) 2 s
Output signal response time (after Write Line) 2 s
Your task is to interface this device to a PIC16F877 with a 4MHz clock. The pins from the
device are connected directly to those of PortD on the PIC16F877, and the code to access
the device is as follows:
Read_in clrf PORTD ; set the Write Line Low
movf PORTD,W
andlw 0b00001100 ; mask out all but the recv bits
movwf VAL_IN
return
Send_out movf VAL_OUT,W
andlw 0b00000011 ; mask out all but the send bits
iorlw 0b00010000 ; set the Write Line high
movwf PORTD
nop
clrf PORTD
return
Perform timing, noise, AC and DC analyses, and suggest corrective actions where appropriate.
10. Challenge: The speci cations for the PIC16F877 given in the notes are for a nominal temper-
ature of 25 . Explain (in your own words) how you would cater for operation at 40 . (Hint:
you may wish to check the data-sheet for temperature curves.)
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ECNG2006 (EE25M) March 11, 2008 VII 32
Tutorial Exercise 9AO ine Version40 ID#
1. You are asked to interface an HD77380-compatible LCD display (controller) with a PIC16F877
which is powered at 5V, and uses a 20MHz oscillator. The electrical and timing characteristics
for the PIC16F877 as well as the bus timing characteristics for the HD77380 are given in your
"green" databook. In addition you are given the following electrical information for the
HD44780:
Item Symbol Min Typ Max Unit
Input high voltage VIH 2.2 VCC V
Input low voltage VIL 0.3 0.6 V
Output high voltage VOH 0:9VCC - V
Output low voltage VOL 0:1VCC V
Output Current Sunk/Sourced IOH - - 0.04 mA
Input leakage current ILI 1 1  A
Input Pin capacitance CIN - 10 - pF
Capacitive Load on Output Pins CIO - - 25 pF
and the code snippet used to "trigger" an LCD character send is
bcf LCD_CNTL, LCD_RW
bcf LCD_CNTL, LCD_RS
movwf LCD_DATA
bsf LCD_CNTL, LCD_E
bcf LCD_CNTL, LCD_E
Perform timing, noise, AC and DC analyses. State any assumptions that you make, note any
missing information required, and suggest corrective actions if appropriate. 16 marks
40Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
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PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
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ECNG2006 (EE25M) March 11, 2008 VII 34
T
U
T
9
AUnit 24 Write the letter you have been assigned here .
A Timing analysis
B AC analysis
C DC analysis
D Noise analysis
Perform the analysis you have been assigned for the system described in Review Question 9.
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ interpret and recognize the implications of, microprocessor timing and interfacing re-
quirements.
{ determine the noise, voltage, and current considerations due to microprocessors and
peripheral fan-in/out
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
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ECNG2006 (EE25M) March 11, 2008 VII 35
25 Power/Signal issues
At the end of this unit the student will be able to:
 determine the noise, power considerations due to microprocessors and peripheral
fan-in/out
 determine how di erent logic families (as well as inverted voltage logic) may be
used in conjunction in a design.
The previous unit looked at some of the problems which arise when the functional design falls
outside of voltage, current, load and timing speci cations for the components. This unit will
investigate factors which are speci c to a particular implementation of the design.
Power source
All of the preceding discussions assumed an ideal power source, which provides a constant supply of
the ideal voltage level, and can source an in nite amount of current. In practice, these assumptions
are not true.
Power conditioning refers to production/maintenance of the required voltage. It incorporates volt-
age regulation (maintaining output DC voltage as long as the input voltage lies within a speci ed
range), voltage conversion (stepping a DC voltage up/down), and voltage inversion (changing po-
larity of DC signal). Special circuitry may be used to achieve these functions (Horowitz and Hill
1989), prior to feeding the microprocessor based system. It is advisable to check which methods
are being used to generate the supplied voltages, and whether they will meet load requirements,
and noise tolerances.
Power supply supervision refers to the pro-active monitoring and/or control of the power supply for
the microprocessor based system. Typical functions include control of battery charging, switching
between power supplies, monitoring of the supply voltage, switching power on/o .(Wilmhurst 2001;
Ch. 10) These functions may be directly carried out by the microprocessor (watchdog timer,
brown-out detection), facilitated by a microprocessor controlled peripheral, or may be done entirely
independently of the microprocessor-based system. The decision about which method to use (if
any) will depend on the application requirements and the consequences of power failure.
Low power circuits, are those explicitly designed to minimise the amount of current drawn (thereby
prolonging the run time on a battery-supply, or reducing the current requirements for a mains-
converter).(Horowitz and Hill 1989)
\ the guidelines for minimising power consumption in CMOS circuits, loosely in order
of descending importance, are as follows:
 de ne all inputs clearly as logic 0 or 1
 minimise clock frequencies
 minimise the power supply voltage
 ensure fast logic transitions
 minimise load and interconnection capacitances
"{(Wilmhurst 2001; pp 312{313)
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ECNG2006 (EE25M) March 11, 2008 VII 36
This can be achieved through selection of components for their low power characteristics, switching
o unused circuitry, and designing interfaces to use the widest possible noise/load margins.
Routing Wires/Tracks
Load capacitance and resistance are not just determined by the device at the end of the line, the
wire connecting the devices also has an e ect on the signal behaviour.
Ground bounce, occurs when large current spikes are returned through the common ground line.
The e ects can be reduced by using a larger conductor or track to reduce the resitance/inductance
of the ground line, and by the use of decoupling/bypass capacitors.(Horowitz and Hill 1989; Ch.9).
Where a mechanical connection needs to be made between two conductors, it should have as much
surface exposed to contact as possible, in order to reduce the contact impedance to a minimum.
All e orts should be made to keep signal wires away from \known" noise sources, either by passing
them elsewhere, or by shielding the noise source/signal wire. A single shield may be used for
multiple signal wires.
When multiple signals physically run along parallel (proximate) wires/tracks (which happens in
interfacing many devices), it is likely that they will be exposed to the same EM noise, therefore a
di erential signal will always be received correctly; the disadvantage is that more connections will
be needed to send a single signal.
Interfacing di erent logic families
\The three things that can keep you from connecting any pair of logic chips together are
(a) input logic-level incompatibility (b) output drive capability, and (c) supply voltages
"{(Horowitz and Hill 1989; Section 9.03)
Where logic families use the same supply voltages, and the input logic levels are compatible (or
can be made so with a pull-up resistor) there may be reduced fan-in/out for the devices. Further,
while the logic levels are ok, the fast edge-triggered devices may not respond (or may respond
multiple times { ringing) to slow-moving edges. Where they cannot be made compatible, level
translators/shifters (Wakerly; Ch. 3) must be used. These are specialised IC’s which perform level
conversion. (Horowitz and Hill 1989; see Table 9.2 Logic Family Connections)
Overall voltage/current requirements
The naive way to determine the maximum current and voltage requirements for the system is to  nd
the sum/max of the requirements for all components. This does not take transient requirements
into account. Be sure to verify that the power supply can handle sudden demands if your system
requires them.
Review Exercises
1. Write de nitions for all the italicised terms in this unit.
2. (a) Question from (Wilmhurst 2001; 10.5) \A microcontroller can operate from 3V to 6V.
Under a particular set of operating conditions it acts to the power supply as a load of 1k .
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ECNG2006 (EE25M) March 11, 2008 VII 37
It is powered from a 5V supply and the total circuit decoupling capacitance is 220 F. If
the supply is disconnected, approximately how much time does the microcontroller have
to perform a controlled system shutdown if it draws its power during this time only from
the decoupling capacitance? " Hint: Draw the circuit model, look at the stored charge,
and check to see how long the voltage takes to drop below 3V.
(b) For 2a, what is the approximate current drawn from the power supply during normal
operation? Estimate the lifetime of an \ideal" 1Ah (amp-hour) 5V battery.
(c) For 2a, what is the maximum current drawn from the power supply during normal
operation? Suggest a way to ensure that the power supply can provide the required
maximum current.
3. Question from (Katzen 2003; 10.3) \The current consumption of a PIC operating at 4MHz
and a VDD of 5V is measured as 550 A with no loading at the port pins. What will be
the current consumption if the device were to be clocked at 100 kHz and powered by a 4V
supply?"(Hint: current consumption is proportional to operating frequency, and the square
of the operating voltage. You may ignore the quiescent current consumed by the PIC when
operating frequency is 0.)
4. Question from (Wilmhurst 2001; 10.6) \Each line of an 8-bit data bus is connected to four
ICs, each input of which has an estimated capacitance to ground of 6pF. Each line of the
bus is switching between 0V and 3.3.V at a frequency of 2MHz. Estimate the power drain
due to the activity of the bus alone."
5. (a) Question from (Katzen 2003; 10.2) \In an attempt to reduce the current consumption of
the circuit when reset, a student has used a 1M pull-up resistor in the Manual Reset
circuit ... Why does the PIC not come out of reset?" What is the maximum value of
pull-up resistor that should be used?
Based on (Mitra 1997): Manual Reset Modi ed Manual Reset (slow rise time)
(b) In your own words, explain why the modi ed reset circuitry is needed for slow-rise time
power supplies, and how it’s component values are chosen.
6. (a) Explain in your own words, the term \brown-out" as it relates to microprocessors/digital
circuitry.
(b) Identify 1 possible consequence of brown-out occurring in microprocessors/digital cir-
cuitry.
(c) Question based on (Katzen 2003; 10.4, Fig 10.8). The circuits shown have been proposed
as brown-out protection circuits. Discuss their operation, and how the component values
are chosen.
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ECNG2006 (EE25M) March 11, 2008 VII 38
Taken from (Mitra 1997)
(d) Can the brown-out protection circuitry be combined with the reset circuitry? Explain
your answer.
7. (a) Can the PIC16F877 operate with all pins simultaneously sinking/sourcing their individ-
ual maximum currents? Explain your answer.
(b) Suggest one way in which we can ensure that the current drawn by the PIC16F877 never
exceeds speci cation.
8. A PIC16F877 is powered at 5V and connected to a 4MHz clock. A N.O. push-button switch
changes an input (port pin 0) voltage from high to low when pressed. A counter connected
to an output (port pin 4), will increment on the rising edge of the signal. The code for the
PIC16F877 is as follows:
; ... appropriate configuration ...
clrw
main btfsc PORTB,0
goto main
xorlw 0b00010000
movwf PORTB
goto main
(a) Describe what you would expect this code to do, and recommend any changes to make
this more e ective.
(b) What is the maximum frequency at which the output pin will change?
(c) For the circuit and code described/shown, predict anomalous behaviour you would ex-
pect (and suggest a solution) if:
i. the pin-output capacitances (device) are large
ii. the pin-input voltage (switch) varies between 0.7V and 1V
iii. the PIC16F877 outputs are TTL level and the counter requires CMOS level inputs
iv. the power supply voltage is disturbed by a motor connected to the same source
causing a  0:5V swing
v. the tracks for the signal and power lines are not EM-isolated from each other
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9. Challenge: Question based on (Wilmhurst 2001; worked example 10.3): A PIC16F877 micro-
controller is used as part of a data logger system, which makes a measurement every second
and then records two bytes of data in external RAM. It takes 50 instruction cycles to make
this measurement. It requires 5 instruction cycles to write the data to the external RAM,
and enables the external RAM chip for only 1 of these instruction cycles. When the system
is powered at Vdd = 5V, with a 4MHz oscillator,
 the PIC16F877 microcontroller draws 4mA when active, and 42 A when asleep.
 the external memory draws 20mA when enabled, and 150 A otherwise.
No other tasks are required of the microcontroller when logging. You may assume that the
Watchdog Timer can be utilised without additional current consumption.
(a) Describe how the sleep mode, and the Watchdog timer, could be used to minimize the
current consumption of the system.
(b) What would be the resulting current consumption?
10. Challenge: Question from (Wilmhurst 2001; 10.2) \What are the relative advantages of lin-
ear and switching voltage regulators/converters, particularly when applied to lower power
applications?"
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ECNG2006 (EE25M) March 11, 2008 VII 40
Tutorial Exercise 9BO ine Version41 ID#
1. In your own words, describe the phenomenon known as brown-out, and di erentiate between
brown-out, and power-on-reset. 6 marks
2. An active low signal is connected to a PIC16F877 input pin, PORTx < n >. The input
pin is read as ’1’ when a logic-high is on the input, and ’0’ when a logic-low is on the input.
During operation, the signal is active approximately 30% of the time.
Indicate whether the following statements are True or False. 4 marks
(a) According to the following snippet, the PIC16F877 will execute CRoutine when the
signal is active.
btfss PORTx,n
call CRoutine
(b) The device sending the signal will use logic-low when the signal condition is true.
(c) Using an active-low signal will lower overall power consumption of the system.
(d) Using an active-low signal will make the system less susceptible to input-noise.
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
41Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
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ECNG2006 (EE25M) March 11, 2008 VII 41
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BUnit 25 Write the letter you have been assigned here .
A Power Conditioning
B Power Supply Supervision
C EM noise reduction
D Power budget
Your lecturer will propose a microprocessor-based system: .
Discuss the requirements of the system for your assigned topic.
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ determine the noise and power considerations due to microprocessors and peripheral
fan-in/out
{ determine how di erent logic families (as well as inverted voltage logic) may be used in
conjunction in a design
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
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ECNG2006 (EE25M) March 11, 2008 VII 42
26  P choice/comparison
At the end of this unit the student will be able to:
compare alternate microprocessors in terms of:
 the instruction set,
 available enhancement features (such as cache, pipelining etc.)
 best performance, which may be facilitated by the instruction set and features.
Unit 8 looked at some criteria for deciding between di erent microprocessor families and forms.This
unit will look at a procedure for quantifying the importance of con icting criterion, balancing
tradeo s and documenting the decision process.
\Device Selection Process
In selecting a device from a  eld of several devices, there is more to be considered than just the speed
of the processor. Some factors, such as the availability of secondary suppliers may be an absolute
requirement in some applications. In order to make a systematic evaluation and selection of the best
alternative, the following method has proved to be valuable, particularly when the selection process must
be documented and justi ed. The process consists of three major steps: eliminating the alternatives that
are completely inappropriate, ranking the remaining options, and evaluating the adverse consequences
of a catastrophic event. The three decision matrices are:
1) Pass/fail criteria for elimination of non-conforming alternative.
2) Weighted scoring of parametric values to rank options.
3) Consideration of adverse consequences, including their probability and severity.
The  rst matrix consists of a table with all the options on one axis and all the \must have" criteria on
the other axis. Each criterion is checked o for each option. The second matrix consists of the surviving
options from the  rst matrix on one axis of a table, and a list of quantitative measures on the other axis,
along with a weighting factor for each measure, indicating its relative importance. Each option receives
a weighted score allowing them to be ranked. Finally, each of the top ranking options is evaluated with
respect to probability of occurrence" { (Arnold 2001; p. 207{208).
When ranking, items pay careful attention to make sure that items are ranked in the same order
(i.e. high score == better ranking for all items OR low score == better ranking for all items).
It is possible to use scales for non-numeric or range-numeric data e.g. the price of a processor may
be expressed using the scale (0- cheap 1- average 2- expensive), or as it’s $ value.
Weights are assigned on a subjective basis according to the perceived relative importance of the
ranking metrics. The weights do not necessarily need to sum to 1.
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ECNG2006 (EE25M) March 11, 2008 VII 43
An example follows for a decision amongst 4 processors. They must have a particular instruction
in their instruction set, and two other features (e.g. clock speed or a certain #/type of in-built
peripherals). Only two processors meet these criteria. The remaining processors are then ranked
in terms of cost (using a de ned scale), performance, code size, and the presence of a non-essential
peripheral (using a binary yes/no). In this example, a lower total re ects a higher rank.
Processor InstructionA FeatureB FeatureC
i p p p
j p   
k p p p
l  p p
Criterion Weighting Processor i Processor k
Cost (ranked 0(cheap), 1(ok), 2(expensive) 0.2 2 0
Performance (benchmark results/s) 0.6 4 2
Code Size (benchmark results/bytes) 0.1 2 4
Built-in A/D converter (0, present; 1: not present) 0.1 0 1
Total 3 1.7
Ranking # 2 #1
Processor k is preferred unless there are extenuating circumstances such as supply problems.
Review Exercises
1. You are told to choose a microcontroller based on it’s ability to perform 16 bit multiplication.
The PIC16F877 is under consideration. You could rule out the PIC16F877 because of which
of the following (if any):
(a) The PIC16F877 has 14 bit instructions
(b) The PIC16F877 does not have a multiply instruction
(c) The PIC16F877 instructions operate on 8 bit data
(d) The PIC16F877 has Harvard architecture
(e) none of the above
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ECNG2006 (EE25M) March 11, 2008 VII 44
2. The 80C51 supports register-register addressing. This means that compared to the PIC16F877:
(a) It will always run faster when moving data from one register to another.
(b) It will take less instruction cycles to move data from one register to another.
(c) It will take less machine cycles to move data from one register to another.
(d) It will take less clock cycles to move data from one register to another.
(e) All of the above statements are correct.
3. You are told that the 80C51 supports register-register instructions, while the PIC16F877
only supports load/store to accumulator. Based on this information, which of the following
statements is correct:
(a) The 80C51 does not have an accumulator.
(b) We can move information between registers using 1 instruction on the 80C51.
(c) We can move information between registers using 1 instruction on the PIC16F877.
(d) We can move information between registers in 1 machine cycle on the 80C51.
(e) We can move information between registers in 1 machine cycle on the PIC16F877.
4. You are told that the 80C51 and PIC16F877 both have data transfer instructions.The 80C51
can move a literal to any register using one instruction: MOV Reg, #6.
The PIC16F877 can move a literal to the working register, and subsequently to any register
using two instructions: movlw 6 followed by movwf Reg.
Based on this information ONLY, we can assume that an 80C51 program involving several
literal-to-register data transfers will probably:
4-I have less lines of code
4-II require less program storage space
4-III run faster
Which combination of stated assumptions is appropriate?
(a) none of the above assumptions are appropriate.
(b) 4-I only
(c) 4-I and 4-II only
(d) 4-I and 4-III only
(e) 4-I, 4-II and 4-III
5. Five microprocessor devices are being compared on the basis of cost, and execution speed.
The device selection matrix is shown below. Which device should we choose?
Name Scaled Cost Scaled exec’n speed Rating = 1*cost + 2*speed
A 1 1 3
B 2 5 12
C 0.2 1.5 3.2
D 0.1 0.6 1.3
E 5 0.1 5.2
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ECNG2006 (EE25M) March 11, 2008 VII 45
6. From: (Wilmhurst 2001; 1.5) \Three microprocessors, A, B, C have maximum clock speeds
respectively of 10MHz, 24 MHz and 20 MHz. Processor A divides its clock by 4 to give one
machine cycle, processor B by 12, and processor C by 8. A and B take two machine cycles to
perform an ’add accumulator to immediate data’ instruction, while C takes 3 [machine] cycles.
Place the processors in order of the speed with which they can perform this instruction."
7. Due to lack of availability, we need to replace the PIC16C5x in our embedded system, with
one of the alternatives investigated in AN520(Palmer 1997a). Presuming that:
 the loop and shift benchmarks are relevant for our application, and
 that loops currently account for 20% of our execution time and 5% of the code space
 that shifts currently account for 60% of our execution time and 10% of the code space
Use the reported relative code size, and relative execution times to develop a ranking scheme,
and draw a ranking table for the COP800, ST62, and 8051 processors, where
(a) code e ciency is weighted at 0.8, and speed e ciency is weighted at 0.2
(b) code e ciency is weighted at 0.2, and speed e ciency is weighted at 0.8
In each case, explain your chosen ranking scheme, and identify the best-ranked processor.
8. We need to rank the processors A and B to make a decision, where code size is weighted twice
as heavily as cost. On Processor A, an algorithm requires 80 instructions, and on Processor
B the same algorithm takes 40 instructions; however Processor A costs 20 money units, and
Processor B costs 300 money units. Select the table which correctly ranks the processors.
(a)
Cost Relative cost Weighted Cost Code Size Relative Code Size Weighted Size Total
A 20 1 2 80 1 1 3
B 300 15 30 40 0.5 0.5 30.5
Weighting factors: 2 (cost), 1(code size). Choice Processor B.
(b)
Cost Relative cost Weighted Cost Code Size Relative Code Size Weighted Size Total
A 20 1 1 80 1 2 3
B 300 15 15 40 0.5 1 16
Weighting factors: 1 (cost), 2(code size). Choice Processor A.
(c)
Cost Relative cost Weighted Cost Code Size Relative Code Size Weighted Size Total
A 20 1 2 80 1 1 3
B 300 15 30 40 0.5 0.5 30.5
Weighting factors: 2 (cost), 1(code size). Choice Processor A.
(d)
Cost Relative cost Weighted Cost Code Size Relative Code Size Weighted Size Total
A 20 1 1 80 1 2 3
B 300 15 15 40 0.5 1 16
Weighting factors: 1 (cost), 2(code size). Choice Processor B.
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9. Based on (Wilmhurst 2001; 1.9) \How long do the longest and the shortest instructions take
to execute for each of the 16F877, .... 68HC05[, AT90S8535], when each is operating at
 it’s fastest clock frequency?
 and external clock frequency of 2MHz?
"
10. Challenge: Find data-sheets/speci cations for a microcontroller based on a 16 or 32-bit data-
word processor.
(a) Identify features on your chosen microcontroller which are NOT available on the 8-bit
data-word processors we have been studying.
(b) Identify a task which your chosen microcontroller would be better suited for, and perform
appropriate ranking to justify your choice.
(c) Identify a real-world product in which your chosen microcontroller is used.
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ECNG2006 (EE25M) March 11, 2008 VII 47
Tutorial Exercise 10AO ine Version42 ID#
Your team is developing an embedded system. You have been asked to choose between the
PIC16F877 and the 80C51, each clocked at 4MHz.
1. Write a program for each of the microcontrollers, which matches the following pseudo-code:
loop read switch
if switch off
turn on light
else
turn off light
goto loop
How many instruction cycles, program locations and data storage locations are required in
each case? 8 marks
42Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
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ECNG2006 (EE25M) March 11, 2008 VII 48
2. On the basis of how well the microcontroller performs light-switching (weight 0.2 program
space; weight 0.2 data space; 0.6 execution speed) choose a microcontroller. 2 marks
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 49
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AUnit 26 Write the letter you have been assigned here .
A instruction set size, # operands supported, architecture
B built-in peripherals
C electrical and frequency speci cations
D support for debugging/development & packaging options
Compare the PIC16F877 and the 80C51 in your assigned area.
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
Compare alternate microprocessors in terms of:
{ the instruction set,
{ available enhancement features (such as cache, pipelining etc.)
{ best performance, which may be facilitated by the instruction set and features.
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
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ECNG2006 (EE25M) March 11, 2008 VII 50
27 Case study
At the end of this unit the student will be able to:
 recognize the issues and tradeo s involved in the design of a microprocessor based
(embedded) system, and
 justify the role of a microprocessor(-based system) in an applied context.
\Design is the task of de ning a system’s functionality and converting that functionality into a physical implemen-
tation, while satisfying certain constrained design metrics, and optimizing other design metrics. Design is hard
...Not only is getting the functionality right hard, but creating a physical implementation that satis es constraints
is also very di cult, because there are so many competing, tightly constrained metrics ...."(Vahid and Givargis
2002; pp 281, Fig 11.1). \ A design metric is a measurable feature of a system’s implementation. Commonly
used metrics include:
 NRE cost (nonrecurring engineering cost) ...
 Unit cost ...
 Size ...
 Performance ...
 Power ...
 Flexibility ...
 Time-to-prototype ...
 Time-to-market ...
 Maintainability ...
 Correctness ...
 Safety ...
Metrics typically compete with one another: Improving one often leads to worsening of another. " (Vahid and
Givargis 2002; pp4-5)
The design process may be subdivided into four main stages: clari cation of need, conceptual
design, embodiment design and detail design (Wilmhurst 2001; Figure 12.1, Table 12.1).
The  rst stage begins with a \single-phrase function statement { a statement which summarises
the main function of the product without ... saying how that function is to be achieved. Following
this a Requirements List should be drawn up, showing all the required features and functions of the
product. These can be divided into Demands (i.e. essential requirements) and Wishes (i.e. those
that would enhance the product but are not essential)."(Wilmhurst 2001; p.354) To complete the
 rst design stage, a speci cation is drawn up. The speci cation should address all of the above
design metrics, as well as: functionality, user interaction, physical form, operating environment,
signals, and quali cation test(s) (Wilmhurst 2001; Table 12.3) (Predko 2000b). The speci cation
may be enhanced as the design progresses.
The second stage: conceptual design involves the generation of alternate functional solutions which
will meet the speci cation and requirements from the  rst design stage. One means of doing this
is to identify alternate ways of achieving each requirement function, and then combine the blocks
to give alternate solutions, and identify the preferred combination. (Wilmhurst 2001; p. 357)
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ECNG2006 (EE25M) March 11, 2008 VII 51
The third stage, embodiment design is where we will make the decisions regarding hardware
parts/software breakdown, and the processes which will be used to manufacture and test the pro-
totype/product. It will include consideration of several issues including:
Do functions in hardware or software? \The choice of hardware versus software for a partic-
ular function is simply a trade-o among various design metrics, like performance, power,
size, NRE cost, and especially  exibility; there is no fundamental di erence between what
hardware or software can implement" (Vahid and Givargis 2002; p21)
Choice of microprocessor/microcontroller; allocation of resources The microprocessor is
chosen so that it meets the conceptual design function requirements, and is ranked according
to the design metrics. Once the microprocessor or microcontroller is chosen, we must decide
on peripheral locations in the memory map, and pin allocations to speci c functions.
Layout and communication Identi cation of the way in which di erent modules will commu-
nicate with each other, and their relative positions within the system (if applicable).
Choice of programming language; tool-chain; libraries These will be constrained by prior
experience, and the hardware chosen, as well as the design metrics.
Identi cation of test strategies These will be used to test the  nal system. By considering
them at this stage, we can tailor the design to facilitate testings e.g. placing LED’s/other
circuitry on the board to facilitate BIST.
The  nal stage detail design involves the production of software  owcharts/code, and hardware
layouts. A bill of materials and a work-plan should also be generated. This design stage is often
the most time-consuming. \ In response to low production rates, the design community has focused
much e ort and resources to developing design technologies that improve productivity ...
1. Automation is the task of using a computer program to replace a manual design e ort.
2. Reuse is the process of using predesigned components (whether designed by humans or com-
puters) rather than designing those components oneself.
3. Veri cation is the task of ensuring the correctness and completeness of each design step
" (Vahid and Givargis 2002; pp 282, Fig 11.1). The prototype can be implemented directly from
the results of this  nal design stage.
Do we really need a microprocessor? In closing we should warn against the typical engineer’s
tendency to use technology unnecessarily. In many instances a microprocessor-based system simply
performs a task at similar, or not much better levels than a non-microprocessor-based solution. For
example while we can design a system for maintaining water levels in a tank ( P, level/pressure
sensor, relay { valve control), the task can be done just as e ectively (probably cheaper and more
robust) by a  oat attached to a mechanical lever.
Review Exercises
1. The design of an embedded system starts with the 1-I , which is followed by the 1-II
and then the 1-III .
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ECNG2006 (EE25M) March 11, 2008 VII 52
(a) 1-I: conceptual design; 1-II: function statement; 1-III: requirements list
(b) 1-I: conceptual design; 1-II: requirements list; 1-III: function statement
(c) 1-I: function statement; 1-II: conceptual design; 1-III: requirements list
(d) 1-I: function statement; 1-II: requirements list; 1-III: conceptual design
(e) 1-I: requirements list; 1-II: function statement; 1-III: conceptual design
2. From (Vahid and Givargis 2002; 1.4): \List a pair of design metrics that may compete with
one another, providing an intuitive explanation of the reason behind the competition."
3. From (Wilmhurst 2001; 12.5) Write the function statement for a microprocessor-based system
\that would genuinely improve the quality of life for a person or group of people. Consider
devices which would give independence to the disabled or vulnerable, enhance education or
save energy."
(a) For your idea, list one alternative method of accomplishing the task without using a
microprocessor-based system.
(b) Identify one bene t gained by using a microprocessor-based system in this application.
(c) Identify one disadvantage of using a microprocessor-based system in this application.
(d) Write the requirements list, and speci cation for your proposed system.
4. Which ONE of the following statements (if any) about the microprocessor-based system design
process is NOT correct?
(a) The single phrase function statement establishes the product function.
(b) The requirements list, contains two types of information, those requirements which are
necessary, and those which are desirable.
(c) A design metric is some property of the system implementation which can be measured,
and used for comparison. with other implementations.
(d) During the design process, decisions about hardware and software can be independently
made.
(e) All of the above statements are correct.
5. Your team is developing an embedded system which requires a digital reading of a potentiome-
ter value. The design requires an 8 bit RISC microcontroller in a 40 pin DIP, which accepts a
4MHz clock signal. You have been asked to choose between the PIC16F877 and the ATMEL
AVR AT90S8535. Apply your knowledge of CPU architecture, and use the data-sheets to
determine at least  ve functional di erences between the two micro-controllers.
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6. Your company is designing an independent device for monitoring RS232 tra c. The device
should passively monitor electrical signals on the TX/RX lines of an RS232 serial cable, and
display current and cumulative estimates of communication tra c. You have been asked to
choose between the PIC16F877 and the 80C51.
(a) Use your knowledge of the PIC16F877 characteristics and the data-sheets provided to
identify at least 3 functional di erences which would be relevant for this application.
Based on the functional di erences which you have previously identi ed, recommend
one of the microcontrollers to your team.
(b) After selecting the processor, the team discusses the support tools which will be needed
for this project. Identify at least  ve items which you believe are necessary in order to
get this project working. Justify your answers.
7. You are a member of a development team which is developing a battery-powered refrigerator
monitoring system. The system checks the actual current being drawn by the refrigerator
twice an hour, and keeps an estimate of the kWh drawn to date. The system has a reset
button and a display button. The system is connected to 7-segment LED display digits which
show the current kWh estimate when the display button is pressed. The estimate is reset to
0 when the reset button is pressed. Your group has decided to use a 4MHz oscillator and a
40-pin DIP package.
(a) You have been asked to choose between the PIC16F877 and the Motorola 68000. Use
your knowledge of the PIC16F877 characteristics and the data-sheets provided to identify
at least 3 functional di erences which would be relevant for this application.
(b) Based on the functional di erences which you have previously identi ed, recommend one
of the microcontrollers to your team, and justify your recommendation (presume that
cost, availability, and prior experience are not of concern).
(c) The reading from the current probe, needs to be scaled and added to the previous
kWh estimate. The kWh estimate must then be displayed on the 7 segment display.
Presuming that you are using the PIC16F877, suggest an appropriate representation for
the estimate and justify your answer.
8. You are a member of a development team which is working on a new microprocessor-controlled
refrigerator, using a PIC16F877 with a 700 Hz clock. The refrigerator consists of a cooling
unit, a defrost unit, and a thermostat, which are connected to PORTD pins <2>, <1> and <0>
respectively.
 The cooling unit is switched on when PORTD<2> is high.; the defrost unit is switched on
when PORTD<1> is low.
 PORTD<0> has an external pull-up resistor. The thermostat pulls PORTD<0> low when the
fridge temperature is above c+ 2 degrees Celsius, and keeps the line low until the fridge
temperature drops below c 2 degrees Celsius; where c is the temperature setting for
the fridge.
 The defrost unit is activated for approximately 6 minutes each day.
(a) Write the pseudo-code for your proposed system.
(b) Suggest an appropriate test strategy for your proposed system.
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ECNG2006 (EE25M) March 11, 2008 VII 54
9. You have been asked to design a gate opener for a new apartment complex which has 6
apartments. Each apartment owner, and the security guard, are given a unique entry code.
(a) Write the function statement, the requirements list, and speci cation for your proposed
system.
(b) Write the pseudo-code for your proposed system.
(c) Suggest an appropriate test strategy for your proposed system.
10. You wish to make a microprocessor-based coin-sorter which can accommodate multiple cur-
rencies.
(a) List alternative method(s) of accomplishing the task without using a microprocessor-
based system.
(b) Identify bene t(s) gained by using a microprocessor-based system in this application.
(c) Identify one disadvantage of using a microprocessor-based system in this application.
(d) Write the function statement, the requirements list, and speci cation for your proposed
system.
(e) Write the pseudo-code for your proposed system.
(f) Suggest an appropriate test strategy for your proposed system.
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ECNG2006 (EE25M) March 11, 2008 VII 55
Tutorial Exercise 10BO ine Version43 ID#
Your task is to design a controller, for a mobile robot which driven by two DC motors. The
motors will be driven by a PWM signal. The controller will receive (EIA)RS-232 compatible serial
commands at 9600 baud. The three commands are: start/stop, left speed and right speed.
1. Write the function statement, the requirements list, and speci cation for your system. 6 marks
2. Draw a block diagram to illustrate how your controller will function. 4 marks
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
43Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 56
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BUnit 27 Write the letter you have been assigned here .
A function statement
B requirements list
C speci cation
D conceptual design
1. Write a sentence which explains your assigned term in the context of microprocessor-based
system design.
2. Produce an example of your assigned term, for a refrigerator temperature controller.
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ recognize the issues and tradeo s involved in the design of a microprocessor based (em-
bedded) system, and
{ justify the role of a microprocessor(-based system) in an applied context.
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 57
28 Design guidelines
At the end of this unit the student will be able to:
remember and follow a checklist of design guidelines, in order to perform a simple design
for a microprocessor based system.
Processes in Embedded system Design (Wilmhurst 2001; Table 12.1)
A design model for embedded systems (Wilmhurst 2001; Figure 12.2)
Checklist for embedded system speci cation preparation (Wilmhurst 2001; Table 12.3)
Functionality All functions the product is meant to perform
User interaction Controls, displays
Physical form Dimensions, weight, materials, aesthetics
Safety Legal requirements, special considerations, results of product failure
Operating Environment Target environment, e.g. temperature and humidity ranges, EMC
Signals Inputs, Outputs, response times
Power Supply Source of power, power consumption
Maintenance Reliability, time between failures
Costs Development costs, product cost
Schedules Product time scales, delivery times
Hardware Design Checklist (Arnold 2001; Appendix A, Hardware Design checklist)
1. De ne Power Supply Requirements
2. Verify Voltage Level Compatibility
3. Check DC Fan-out: Output Current Drive vs. Loading
4. AC (capacitive) Output Drive vs. Capacitive Load and De-rating
5. Verify Worst case timing conditions
6. Determine if transmission line termination is required
7. Clock Distribution
8. Power and Ground Distribution
9. Asynchronous inputs
10. Guarantee Power-On Reset State
11. Programmable Logic Devices
12. Deactivate Interrupt and other requests on power-up
13. Electromagnetic compatibility issues
14. Manufacturing and test issues
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 58
A Baker’s Dozen Rules to Help Avoid Application Software Problems (Predko 2000d;
pp 520{521,Ch. 9 PICmicro R MCU Assembly-Language Software-Development Techniques)
1. Always initialize your variables
2. Indent conditionally executing code after a skip instruction
3. Let the compiler/assembler do the calculations for you
4. Use Microchip’s register de nition  les without modi cation in all your applications
5. Keep your code as simple as possible
6. Develop your application in terms of functional blocks and interfaces
7. Establish a plan to test and con rm your code is correct
8. Use CBLOCK or other built-in tools to allocate variables instead of de ning them manually
9. Avoid changing the register bank unless it is absolutely necessary
10. Don’t allow code to go over page boundaries
11. Use the __CONFIG statement always in your source code and use a programmer that programs
the con guration information automatically (not one that requires manual intervention)
12. Simulate as much of your application as possible
13. Keep subroutine calling to a minimum
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 59
Review Exercises
1. Write one sentence to justify each of the listed design guidelines.
2. A PWM motor controller is under development. We need to choose between a microcontroller
with a PWM peripheral, and a microcontroller with a timer. Both microcontrollers are based
around the same CPU. Which of the following issues(if any) are appropriate for consideration?
(a) the PWM frequency and resolution requirements
(b) the program and data memory requirements for bit-banging PWM using the timer
(c) cost/availability of the microcontrollers
(d) clock speed/packaging for the microcontrollers
3. When writing routines for a microprocessor, you have been advised to always initialize your
registers, even if the default value is the value you want. Label the following statements as
valid/invalid reasons for this guideline:
(a) The microprocessor may malfunction and/or reset without powering up
(b) The microprocessor may not power-up correctly
(c) The routine may attempt to access more memory than originally intended/allocated
(d) The routine may be called after the value was changed subsequent to power-up
(e) The routine may be interrupted while it is running.
4. When designing a microprocessor based system for the PIC16F877, you have been advised
to simulate as much of your application as possible. Which of the following statements is an
INVALID reason for this guideline?
(a) Flash EEPROM has a  nite number of write cycles; simulation avoids using them up
needlessly.
(b) Simulation is faster for debugging, as it avoids download time.
(c) Software testing can begin before hardware is available.
(d) We can avoid inadvertently starting up harmful equipment.
(e) We can use the simulator to troubleshoot noise problems.
5. Based on your experiences in the lab, review the lists of design guidelines, and add/remove
guidelines, to form your own compiled list.
6. Challenge: Investigate the operation of the IPod Music Player. Can you design a similar
product?
7. Challenge/Role Play: Find/read stories about the Therac-25 accidents
(try: http://sunnyday.mit.edu/therac-25.html).
 Identify the design/testing guidelines that could have averted these problems.
 Identify the key persons whose decisions may have contributed to these problems. What
would they have said in their defence?
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 60
8. You have been asked to design a microprocessor controlled cooling system for equipment
mounted in an industry standard 19 inch rack.
 The rack is  tted with a power supply which delivers 30V for fans and equipment.
 The rack is  tted with DC cooling fans which require 30V.
 At 20 degrees Celsius the fans should be o , and at 50 degrees Celsius they should be
running at full speed.
 There is a thermocouple mounted in the equipment rack which delivers a signal be-
tween 0 and 2V for temperatures between 20 and 60 degrees Celsius. The thermocouple
impedance is 200 ohms.
 When the rack temperature rises above 60 degrees Celsius, it automatically shuts o all
power. The power supply must subsequently be manually reset.
 The microprocessor should report the current rack temperature on an Hitachi 44780
LCD display mounted at the front.
 The microprocessor should asynchronously report (via RS232 cable) the present tem-
perature to a remote PC at 9600 baud, no parity, no stop bit.
Your boss wants you to use the PIC16F877 with an 8MHz crystal oscillator because he
happened to have them left over from a previous project. 44
(a) What voltage will you operate your system at and how will you deliver power to the
microprocessor/oscillator? (Keep in mind that the fans or other equipment need to be
isolated for noise)
(b) The thermocouple voltage will be sampled using the A/D converter. Determine an
appropriate setting for the A/D conversion clock when the chip oscillation frequency is
8MHz, and the minimum acquisition time for this thermocouple. What is the overall
sampling time?
(c) Temperature change in a rack is generally a slow process, so it does not matter in this
case; but what is the maximum frequency we should attempt to sample with the above
sampling time?
(d) Choose reference voltages for the A/D conversion. What range of digital values will be
obtained from the thermocouple?
(e) The fan motors will be driven using the built-in PWM module and a transistor. The
duty cycle will be determined from the range of digital values from the thermocouple.
Choose an appropriate PWM frequency, and a method of calculating duty cycle such
that each 10-bit A/D value obtained for the thermocouple generates a di erent duty
cycle.
(f) The LCD display may be interfaced to the PIC by using the I/O port pins to make a
parallel bus. Read the timing diagram for the LCD panel, and design subroutines which
will communicate with the LCD. (Note: you do not need to write any code!!)
(g) With no parity, the remote PC has no means of verifying if individual data bytes have
been corrupted. Suggest a possible scheme for checking whether data has been corrupted
en-route.
44Yes I do realize this is a very far-fetched/unrealistic scenario but it serves our purposes.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 61
(h) At 9600 baud, is it possible to send every reading to the remote PC? If not, suggest one
method of reducing the data to be sent.
(i) There are many available software communication protocols for sending data from the
PIC to the remote PC. Suggest an alternative which might be better suited to an indus-
trial environment than RS232, and justify your choice.
(j) Your boss runs into your o ce while you are still designing, and says he wants to have
one microcontroller system do temperature control for 8 racks simultaneously i.e. 8
PWM fan outputs; 8 LCD displays; 8 thermo-couples. Suggest one way in which you
could do it.
(k) Draw  ow-chart; block diagrams; and write algorithms to illustrate your  nal design.
(l) Run through the checklists; which issues still need to be addressed?
(m) Draw up a testing procedure for the  nal system.
9. You have been asked to design a gate opener for a new apartment complex which has 6
apartments. Each apartment owner, and the security guard, are given a unique 8-bit entry
code. Each valid entry attempt is logged by incrementing the 16-bit count associated with
that entry code. There is a separate count for invalid entry attempts.
(a) 16-bit counts are stored in memory at consecutive locations, hi-byte  rst, starting at
location CtrArray. Write a routine called UpdArray which uses indirect addressing to
increment the counter speci ed by the value in the working register.
(b) Write a routine which will compare an 8 bit code stored in CodeRead to the entries in a
\regular" lookup table CodeTbl and return with either
 the index of the code in the table, or
 the value 7 if the code is not found in the table
in the working register.
(c) Challenge: modify your routine so that the data values are stored in the data EEPROM.
(d) Write a routine which reads 4 switches connected to PORTD<3:0> and displays the ap-
propriate histogram bin value on the LED bar connected to PORTC<7:0>.
(e) Challenge: modify your routine so that it displays a level and not just the binary number.
(f) Identify an appropriate means for the user to enter the code, and justify your choice.
Design circuitry, and code such that the entry code is stored in CodeRead when the user
attempts to enter the complex.
(g) Challenge: modify your routine to use interrupts.
(h) Write a main loop, add all your routines, compile and verify that your system works in
simulation.
(i) Criticise the system you have just produced, suggest at least two improvements.
(j) Outline how you would verify that the  nal system was functioning as required.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 62
10. You have been asked to produce a 5 words-per-minute (= 250 counts-per-minute) Morse Code
Generator, using a PIC16F877 with an 8MHz clock, which will accept a string of characters
from a keyboard, and produce the corresponding morse code tones using a speaker which is
switched on/o by circuitry connected to PORTB pin 5.
(a) Write a timing routine which will delay for 1 Morse code count.
(b) Using your timing routine, write routines which will:
i. play a short tone
ii. play a long tone
(c) Using your wait routine, write routines which will wait for 3 and 7 counts respectively
(Suggestion: Keep a counter and check when the appropriate bit goes high).
(d) Decide on an appropriate means of representing morse code in a register in the PIC16F877.
(Hint: Is it su cient to use 0’s for dots, and 1’s for dashes? How will we know how
many codes are used to represent a character?)
(e) Using your representation for morse codes, write a routine and a lookup table, such that
the routine will translate the ASCII value in the working register into the the appropriate
code, and return the code in the working register.
(f) Write a routine which will \play" the morse character tones using your tone routines,
based on the representation returned from the lookup table routine.
(g) Write a routine which will \play" an indirectly addressed null-terminated string. (the
address of the start of the string is either passed in the working register, or set up prior
to calling the routine).
(h) Assemble and test all your routines. You should specify how you tested your routines,
and what modi cations you subsequently made to the routines/representation (if any).
Morse code/ASCII Summary:
(dot, di) short tone: 1 count
(dash, da )long tone: 3 count
space between tones in a character: 1 count
space between characters in a word: 3 counts
space between words in a message (ASCII code 0x20): 7 counts
Char ASCII Morse Char ASCII Morse Char ASCII Morse
A 0x41 (0x61) . - B 0x42 (0x62) - . . . C 0x43 (0x63) - . - .
D 0x44 (0x64) - . . E 0x45 (0x65) . F 0x46 (0x66) . . - .
G 0x47 (0x67) - -. H 0x48 (0x68) . . . . I 0x49 (0x69) . .
J 0x4A (0x6A) .- - - K 0x4B (0x6D) - . - L 0x4C (0x6C) . - . .
M 0x4D (0x6D) - - N 0x4E (0x6E) - . O 0x4F (0x6F) - - -
P 0x50 (0x70) . - - . Q 0x51 (0x71) - - . - R 0x52 (0x72) . - .
S 0x53 (0x73) . . . T 0x54 (0x74) - U 0x55 (0x75) . . -
V 0x56 (0x76) . . . - W 0x57 (0x77) . - - X 0x58 (0x78) - . . -
Y 0x59 (0x79) - . - - Z 0x5A (0x7A) - - . .
0 0x30 - - - - - 1 0x31 . - - - - 2 0x32 . . - - -
3 0x33 . . . - - 4 0x34 . . . . - 5 0x35 . . . . .
6 0x36 - . . . . 7 0x37 - - . . . 8 0x38 - - - . .
9 0x39 - - - -.
. Fullstop 0x2E . - . - . - , Comma 0x2C - - . . - - ? Query 0x3F . . - - . .
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 63
Assignment E ID#
You are a member of a team of engineers working on a wired sensor array to monitor the  uid  ow
distribution in a natural gas pipeline.
 The sensor array consists of several nodes which can each communicate with a base station.
 Each node consists of:
{ a PIC16F877, using a 4MHz external clock signal, powered at 5V.
{ a pyroelectric anemometer, to sense  ow rate.
{ 7 switches which specify the node identi er.
 A pyroelectric anemometer is a sensor which detects  uid  ow rate at a location in the pipeline.
It requires an AC input signal. It generates an AC output signal of the same frequency as
the input signal, but whose amplitude is proportional to the rate of  uid  ow at the sensor.
 The input of the pyroelectric anemometer is connected to digital output RD0 of the PIC16F877.
A software generated 3Hz; 0 to 5Volt; square wave is output from the PIC16F877 on digital
output RD0. The 3Hz signal is generated using Timer1, using a 1:8 pre-scaler, and initialised
with an appropriate value p.
 The output of the pyroelectric anemometer is an AC signal varying between 0 and 5V. It
is connected to analog input 0 (AN0) of the PIC16F877. The resulting 10-bit readings are
stored for later reporting to the base-station.
 For communication, the base station acts as the master and polls each (slave) node period-
ically. Upon receiving a request from the (master) base station, the (slave) node responds
with the most recent 10-bit output of the ampli er software.
 Nodes & base station are attached to an I2C bus, consisting of two lines (SCL,SDA) with
external pull-ups.
 Each node should respond to a unique 7-bit I2C address. This address is con gured at node
startup. The node I2C address should be set by the 7 switches connected to RD1-RD7.
System Overview
Node
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 64
Pseudocode
main: call Cfg
enable global interrupts
goto loop
loop: if Timer1 has overflowed
toggle RD0 pin state
place initial value p in Timer1
goto loop
Cfg: configure pins, Timer1, A-D & I2C
enable A-D & I2C interrupts
initialize all variables
place initial value p in Timer1
start A-D and Timer1
MainISR: store context
if I2C interrupt; do I2C Handler
if A-D interrupt; do A-D Handler
restore context
I2C Handler: if request for this node
report upper 8 bits of Amplifier output
clear I2C flag
A-D Handler: retrieve 10 bit A-D output
reading = (upper 4 A-D bits) - 8
call Amplifier
clear A-D flag
start next A-D sample
1. Write an assembly language routine named average for the PIC16F877 which will return the
average value of two 16-bit signed two’s complement representation numbers (contained in
registers Ahi:Alo and Bhi:Blo ) in the registers Chi:Clo. 12 marks
2. Write an assembly language routine named multiply for the PIC16F877 which will return the
product of two 8-bit signed two’s complement representation numbers (contained in registers
M1, and M2) in the registers Qhi:Qlo. Please state the mathematical principle on which your
routine is based. 12 marks
3. Use the psuedo-code provided to write a main loop, a main interrupt-subroutine, add all your
routines, compile and verify that your system works in simulation. Explain how you tested
your code. 8 marks
4. Criticise the system as speci ed, and suggest at least two improvements. 4 marks
5. Outline how you would verify that the  nal system was functioning as required. 4 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 65
Submission Guidelines
You will be penalised (up to a maximum of 4) marks if you violate ANY of the program formatting
guidelines. All submitted programs MUST follow the speci ed format i.e.
 labels in  rst column;
 code in second column;
 operands (where appropriate) in 3rd column;
 comments (where appropriate) in the 4th column
 directives in capital letters;
 assembly language in common letters;
 variable names/labels lower case (optional  rst capital letter).
Your  les should be named Eqnnnnnnnn.* where nnnnnnnn is your ID Number and q is the question
number.
Please type your answers into plain ASCII text  les (no Word documents please!) with the same
name(s) as your other  les, and the extension .asc . Alternatively you may write your answers in
the *.asm  les as comments.
The *.asc, *.hex, *.asm and *.lst  les for all questions should be zipped into a single archive named
Ennnnnnnn.zip where nnnnnnnn is your ID Number.
The archive should be uploaded to myELearning. Please remember to electronically submit AHEAD
of the deadline in order to avoid problems with system overload.
This assignment is worth 2% of your ECNG2006 mark. It contains a total of 40 marks.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 66
A
S
S
T
EUnit 28 Write the letter you have been assigned here .
The guideline is:
What is the worst thing that could happen if we ignore this guideline:
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ remember and follow a checklist of design guidelines, in order to perform a simple design
for a microprocessor based system.
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VII 67
System Issues
In this section we have looked at issues which a ect overall system performance. These include
the e ects of integrating hardware and software modules (which may have been developed and
tested independently), a comparison of components for selection and a methodology for system
design. In doing so we have ful lled the fourth learning objective \select (and critique the selection
of) a microprocessor-based system for an application, given relevant datasheets and application
requirements."
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) Introduction to microprocessors
c DECE, UWI, St. Augustine, Trinidad
original author: Feisal Mohammed; updated: March 11, 2008{ CLR
Part VIII
Communication
In Part VI, we examined ways in which peripherals could be interfaced with the microprocessor.
In this section, we will elaborate on peripherals whose function is to facilitate communication
between the microprocessor and other devices;  rstly we will look at standard communication
protocols/interfaces which such peripherals may support.
29 Communication protocols
At the end of this unit the student will be able to:
 understand commonly used parallel and serial communication protocols (I2C, CAN,
RS232, USB, Firewire)
 interpret descriptions of proprietary protocols (Dallas 1-wire)
In the notes so far, we have used the term bus to describe the physical means by which:
 the CU communicates with other parts of the CPU,
 the CPU communicates with other parts of the microcontroller,
 the microprocessor/microcontroller communicates with other parts of the microprocessor-based sys-
tem.
In e ect a bus is any transport mechanism which moves information between devices. The term
can also be used to describe the physical means by which two independent microprocessor-based
systems communicate with each other.
Communication between two microprocessor-based systems can be achieved in many di erent ways.
We have already noted that internal bus transactions may be either synchronous or asynchronous.
In order to demarcate the bounds of an asynchronously transmitted message speci c message
formats are used: e.g. start/stop bits, frames with Header and EOF.
Another distinction which we can make is between bit-serial (only one bit is transmitted at a time)
and bit-parallel (more than 1 bit transmitted at a time, may also be called byte-serial) transfers. We
can also di erentiate between simplex (where a signal is only sent in one direction on a particular
signal line) half-duplex (where a signal is sent in both directions (at di erent times) along the same
signal line) and full-duplex (where signals are sent in both directions along di erent lines). Signals
sent along the bus may be referenced to the system ground (unbalanced or single-ended), a single
reference line (signal ground), or individual return lines (di erential or balanced).
We can look at whether the bus connects only 2 devices together (point-to-point) or if several
devices have independent physical access to the bus(network). \Virtual" buses (as opposed to
physical buses) can be created by relaying information from one physical bus to another. Where
ECNG2006 (EE25M) March 11, 2008 VIII 2
several devices are communicating on the bus, the message must contain information which indicates
who the sender/recipient of the information is (internal to the microprocessor-based system { this
is addressing).
Information may be transmitted across any type of bus using di erent line coding techniques. So
far, we have been using two voltage levels to represent the state of the bit, and the same voltage
levels always represent the same state. We may also use the transition (from hi to lo or lo to hi), the
presence or absence of a pulse, and the pulse width to represent the state of a bit. Furthermore,
consecutive bits of the same value need not necessarily be represented by the same signal, and
multi-bit patterns may be represented using 3 or more voltage levels.
The use of such line coding techniques improves the robustness of the bus communication. Where
additional surety is required, some form of error detection/correction (e.g. parity, checksums,
message length) may be employed. This requires additional information to be passed across the
bus. The receiver can check that the additional information is consistent with the message, and
thereby determine that the information is the same as that which left the source. The rate at which
information is passed across a bus will depend not only on the signal transaction speed of the bus,
but also on the line coding technique employed, and the amount of additional information that is
transmitted (i.e. destination, source, error check).
Communication protocols de ne the means in which communication will take place, and generally
de ne:
 the number of signal wires, and the voltage tolerances
 transaction timing
 message format and the form of error detection/correction to be used (if any).
 the form of synchronization to be used (if any)
 standard interface connections (if any)
The standard methods of communication which we will examine in this unit, all utilise di erent
ways of dealing with these issues and are summarised in the following tables.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 3
I2C(I2C 2000)
Operating mode Synchronous, bit-serial, (multi-)mastered
Signal Level unbalanced signal; bus lines  oats high by default; low voltage 0V, high voltage
 Vdd (di erent voltage devices can communicate)
Line coding NRZ level code (hi - 1; lo - 0)
Speed 100kHz (400kHz { fast mode)
Max. # devices \The number of ICs that can be connected to the same bus is limited only by a maximum
bus capacitance of 400 pF."(I2C 2000)
Transmission distance This is limited by the maximum bus capacitance. This can be overcome
by reducing the frequency of transactions, using I2C drivers, or adding pull-up resistors.
\wiring must be so chosen that crosstalk and interference to/from the bus lines is minimized. The bus
lines are most susceptible to crosstalk and interference at the HIGH level because of the relatively high
impedance of the pull-up devices.
If the length of the bus lines on a PCB or ribbon cable exceeds 10 cm and includes the VDD and VSS lines,
the wiring pattern must be: SDA VDD VSS SCL
If only the VSS line is included, the wiring pattern must be: SDA VSS SCL
These wiring patterns also result in identical capacitive loads for the SDA and SCL lines. The VSS and
VDD lines can be omitted if a PCB with a VSS and/or VDD layer is used.
If the bus lines are twisted-pairs, each bus line must be twisted with a VSS return. Alternatively, the SCL
line can be twisted with a VSS return, and the SDA line twisted with a VDD return. In the latter case,
capacitors must be used to decouple the VDD line to the VSS line at both ends of the twisted pairs.
If the bus lines are shielded (shield connected to VSS), interference will be minimized. However, the shielded
cable must have low capacitive coupling between the SDA and SCL lines to minimize crosstalk." (I2C
2000)
# Communication Lines 2
# Signal Lines 1 (SDA)
# Handshaking / Clock lines 1 (SCL)
Arbitration Distributed { monitor line { 0 bit has priority { drop o and wait for stop.
Message/Transaction Format MSB  rst;
 (Master) Start Condition,
 (Master) Receiver Address (7 bits) ,
 (Master) Read/Write Request (1 bit),
 (Device) Acknowledge (1 bit),
 (Master - Write; Device - Read) Data Byte (8 bits),
 (Device) Acknowledge
 (Master) Optional: additional Start (read/write), or Data Byte (write only)
 (Master) Stop Condition
Standard Connector(s)/Cable(s) { none {
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 4
Controller Area Network (CAN) ISO 11898
Operating mode asynchronous
Signal Level 0 - 16 V; \The bus can have one of two complementary logical values: ‘dominant’ or
’recesive’. During simultaneous transmission of ‘dominant’ and ‘recessive’ bits, the resulting
bus value will be ’dominant’."(CAN 1991)
Line coding NRZ
Speed 200 kbps (nominal) { 1Mbps
Max. # devices \The CAN serial communication link is a bus to which a number of units may
be connected. This number has no theoretical limit. Practically, the total number of units
will be limited by delay times and/or electrical loads on the bus line."(CAN 1991)
Transmission distance dependent on speed 40m @1Mbps. Point-to point cabling.
# Communication lines 2 lines
# Signal lines 1 di erential pair
# Handshaking/Clock lines { none {
Arbitration Distributed { monitor line { 0 bit has priority { drop o and wait for stop.
Message/Transaction Format \
SOF Start of Frame, A single Dominant Bit
Identi er 11 or 19 Bit Message Identi er
RTR This Bit is set if the transmitter is also TXing Data
r1/r0 Reserved Bits, Should always be Dominant
DLC Four Bits indicating the number of bytes that follow
Data Zero to 8 Bytes of Data, Sent MSB First
CRC 15 bits pf CRC data followed by a recessive bit
Ack Two Bit  eld, Dominant/ Recessive Bits
EOF End of Frame, at least 7 Recessive Bits
"
(Predko 2000c)
Standard Connector(s)/Cable(s) 9 pin D-type connector, STP, UTP or ribbon.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 5
GPIB/HPIB (IEEE488,IEC60625)
Operating mode Asynchronous (handshaking)
Signal Level TTL level; active low
Line coding NRZ
Speed limited to speed of slowest device on bus (daisy-chain); 1Mbps
Max. # devices 20
Transmission distance 20m total { maximum separation 2m (point-to-point)
# Communication lines 24 lines
# Signal lines 8 data lines + 5 interface lines
# Handshaking/Clock lines 3 handshaking lines
Arbitration Central { all devices must request permission from the controller
Message/Transaction Format \When the Controller or a Talker wishes to transmit data
on the bus, it sets the DAV line high (data not valid), and checks to see that the NRFD
and NDAC lines are both low, and then it puts the data on the data lines. When all
the devices that can receive the data are ready, each releases its NRFD (not ready for
data) line. When the last receiver releases NRFD, and it goes high, the Controller or
Talker takes DAV low indicating that valid data is now on the bus.
In response each receiver takes NRFD low again to indicate it is busy and releases
NDAC (not data accepted) when it has received the data. When the last receiver has
accepted the data, NDAC will go high and the Controller or Talker can set DAV high
again to transmit the next byte of data. "
{ from HTBasic GPIB Tutorial, http://www.techsoft.de/htbasic/tutgpib.htm
Standard Connector(s)/Cable(s) Centronics-type 24 pin connectors, shielded 24 conductor
cable.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 6
Review Exercises
1. The signal shown in the  gure appears in a communication scheme that uses NRZ coding, a
logic ’1’ appears as a negative voltage, and a logic ’0’ appears as a positive voltage. What
7-bit pattern is being transmitted?
(a) 0110010
(b) 0110011
(c) 1001100
(d) 1001101
(e) none of the above
2. Which of the following statements does NOT describe the RS232 protocol:
(a) Signal voltages:  12Volts
(b) Hardware, software or no handshaking
(c) Error correction through the use of parity bits
(d) Supported speeds: 1200/second, 9600 bits/second
(e) Start and Stop bits with each byte; LSB  rst transmission
3. Serial communication is being carried out across a channel which can support a maximum of
n transitions per second. The line coding scheme used has not more than 2 transitions per
bit, and 7 bit data-words are transmitted with an additional 3 bits of overhead. The e ective
bit-rate at which data can be transferred across this channel is:
(a) n bits/second
(b) n 7(3+7) bits/second
(c) n2  7(3+7) bits/second
(d) n2  37 bits/second
(e) n2  3(3+7) bits/second
4. The PC has standard ports which can be used for interfacing di erent devices. What is the
advantage of having standard (as opposed to proprietary ports/interface cards) for external
devices?
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 7
5. Identify one serial/parallel interface standard for PC’s which was not identi ed in the unit.
6. Di erentiate between simplex and half-duplex communication.
7. What would happen if two senders simultaneously tried to transmit over a half-duplex line?
8. NRZ line coding involves maintaining an active high or low for the entire bit-period. Manch-
ester line coding involves a signal transition during each bit period (hi >lo) for 1 and
(lo >hi) for 0. In the Dallas 1 wire protocol, a 1 is encoded by pulling the line low for
a short time, and a 0, by pulling the line low for a longer time. Draw the line signals for
the nibble 0101 transmitted using NRZ, Manchester and Dallas 1-wire encoding schemes
respectively.
9. A particular serial interface has the following message format: 2 bits header, 8 bits information
(LSB  rst), 1 bit checksum, 1 bit end-of frame. The channel supports n transitions/second.
What is the true rate at which information is transferred across the channel if we use each of
the following line coding schemes?
(a) NRZ coding
(b) Manchester encoding
10. Challenge: Research and draw up similar tables for one of the following communication stan-
dards: RS232, USB, Firewire, IEEE1294, ModBUS RTU, DNP3, Ethernet, AS-I (Siemens)
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 8
Tutorial Exercise 11AO ine Version45 ID#
You have been asked to design an RFID (Radio Frequency ID) stock tracking system which
can be integrated into the shelves of a retail store. You are given the PIC16F877 with a 16
MHz clock and the Texas Instruments Series 2000 Micro-reader module (RI-STU-MRD1). (see
http://www.ti.com/tiris/docs/manuals/refmanuals/micro 8.pdf) Describe the communica-
tion protocol for communication between the module and the PIC16F877 in the space below. 12 marks
Operating mode .
Signal Level .
Line coding .
Speed .
Max. # devices .
Transmission distance .
# Communication lines .
# Signal lines .
# Handshaking/Clock lines .
Arbitration .
Message/Transaction Format .
Standard Connector(s) .
45Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 9
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 10
T
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1
A
Unit 29
Dallas 1-wire (Micro-LAN)
Operating mode .
Signal Level .
Line coding .
Speed .
Max. # devices .
Transmission distance .
# Communication lines .
# Signal lines .
# Handshaking/Clock lines .
Arbitration .
Message/Transaction Format .
Standard Connector(s) .
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 11
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ understand commonly used parallel and serial communication protocols (I2C, CAN,
RS232, USB, Firewire)
{ interpret descriptions of proprietary protocols (Dallas 1-wire)
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 12
30 Bit-banging vs. hardware
At the end of this unit the student will be able to:
di erentiate between bit-banging and dedicated hardware interfaces for serial/parallel
communication (PPI/PIA vs ACIA/UART, PPC)
In the previous unit we talked about serial/parallel communication protocols. Software implemen-
tations of standard communication protocols are often described as bit-banging i.e. they bang out
bits repeatedly.
\The bit banging method is a way that does not require assistance from any external
circuitry to send or receive data stream. At the sending or receiving device, normally
there is a controller handles everything during the communication period. The task of
sending/checking on the clock and sending data/waiting for response from the other end
on a bit-by-bit basis requires constant attention during the communication period that
leaves heavy burden on the controller on both parties. There is little room and time for
the controller to perform tasks during the sending/receiving data period. It has little error
detecting that the controller can perform since it is constantly busy in dealing with data
transmission on every single bit."
{ from http://www.21centuryengineer.com/issues/fall2002/siung.htm
Previous units have shown how dedicated peripherals can be used to o -load work from the mi-
croprocessor. This can also be done for communications. However because of the continuous
and potentially complex nature of communication, the peripherals must be particularly \smart".
Features which appear in communication peripherals include:
 peripheral bu ering (TX and/or RX) w/programmable interrupt levels, and transmission
speeds
 checksum/parity generation/checking
 transmission/recieve error detection/noti cation
 automatic hardware/software handshaking
 automatic message re-transmission/re-request
 automatic reset of interrupt  ags
 automatic RX  ltering: e.g. address/message type recognition
Ideally, the presence of any subset of these features on a peripheral will make it a more e cient
alternative to bit-banging software. In practice, peripherals tend to be customised to accommo-
date a certain protocol(s), and bit-banging software o ers more  exibility and control over the
communication process.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 13
Bit-serial communications, in particular, are very dependent on timing; in order to pass large
quantities of data, individual bits must be switched/detected on the signal lines at fast rates. This
means that either the microprocessor (bit-banging) or the peripheral must sample the line at a
higher speed than the signal transition rate. The sample bu er can then be examined for \glitches"
and \bounce", and these e ects ignored. The \true" bit can be used to synchronize the receive
and transmitter clocks. Certain line/message coding techniques (e.g. start/stop bits, Manchester
encoding, bit stu ng) facilitate easier synchronization. Because of these characteristics the receive
is very sensitive to anything which a ects rise/fall time on the signal line, and mismatched tx/rx
speeds while detecting data, will not be reconstructed properly.
One particular limitation of peripherals is their ability to accommodate only a  nite # of sampling
speeds. The sampling clock is generally derived from the microprocessor clock, and therefore must
have a sampling period which is an integer multiple of the clock period. Where the desired sampling
period is not an exact multiple, synchronisation must be used to correct for any error.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 14
Review Exercises
1. Which of the following (if any) are advantages of using bit-banging software to perform
communication, rather than a dedicated peripheral device?
(a) ability to perform communication in parallel with other tasks
(b) ability to perform error detection
(c) ability to service communications on demand
(d) ability to support/adapt to multiple protocols
2. You are given the choice of using bit-banging or a dedicated peripheral to communicate with
the device. Label the following statements True/False:
(a) If we use a peripheral, then our program will need to provide the peripheral with com-
mands about the required function.
(b) If we use bit-banging, then our program will need to explicitly set and clear bits at the
correct times.
(c) Monitoring communications for bit-banging may occupy most of the processor time.
(d) Peripherals are always faster than bit-banging.
(e) We may use built-in timers and timer interrupts to perform bit-banging.
3. A key di erence between the use of bit banging and dedicated communication peripherals is:
(a) the peripheral can perform bu ering, and communication handshaking independently of
the processor.
(b) the processor cannot communicate as fast as a dedicated peripheral can.
(c) the processor cannot use interrupts to do bit-banging.
(d) the processor has less work to do with bit-banging than with a dedicated communication
peripheral.
4. \Why are transmit and receive clocks usually a factor of 16 or 64 times the data rate?" (Cady
1997; 10.3)
5. Draw a diagram to illustrate the problems which may arise when the clock is not properly
recovered from an asynchronously transmitted signal.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 15
Tutorial Exercise 11BO ine Version46 ID#
You have been asked to design an RFID (Radio Frequency ID) stock tracking system which
can be integrated into the shelves of a retail store. You are given the PIC16F877 with a 16
MHz clock and the Texas Instruments Series 2000 Micro-reader module (RI-STU-MRD1). (see
http://www.ti.com/tiris/docs/manuals/refmanuals/micro 8.pdf)
In order to communicate with this module, you decide to use the built-in serial communications
peripheral. Write appropriate initialisation and termination routines for the peripheral, presuming
that you will be using TX and RX interrupts. You should note any requirements/presumptions
you make concerning the PIC16F877 support circuitry. 16 marks
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
46Students are advised that O ine versions are to be used either with the explicit permission of the course lecturer
OR at times when the electronic course support site is unavailable for more than 48 hours.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 16
T
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1
BUnit 30 Write the letter you have been assigned here .
Serial Parallel
Bit-banging A B
Peripheral C D
1. Describe how your assigned combination operates in the performing communication tasks.
2. Identify one advantage of your combination over bit-banging/peripheral use for communica-
tion.
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this unit.
{ di erentiate between bit-banging and dedicated hardware interfaces for serial/parallel
communication (PPI/PIA vs ACIA/UART, PPC)
 Which aspect of this unit did you have the most di culty understanding?
 Which aspect of this unit did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this unit/tutorial.
 Identify one way in which this unit/tutorial could be improved.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 17
31 I/O on the PC
At the end of this unit the student will be able to:
perform I/O programming on the PC in C/Assembly using a C/C++ compiler
The modern-day Intel Pentium(or compatible) PC is based on the original 8086/88. In order to
appreciate how instructions work in the 8086-type architecture, we need to understand a bit about
the architecture. \There are four general purpose registers: AX, BX, CX, and DX, each of which
can be used to manipulate a whole 16-bit word .... Each of these registers can be used as two
8-bit registers, for example, AL represents an 8-bit register that is the lower half of AX, and AH
... the upper half of AX. Each of the ... registers has one or more implied functions ... AX is the
accumulator, and is used for all input/output operations ... DX is the data register, and is used
for some input/output ..." (Buchanan and Wilson 2001; p.97)
\The move [assembly language] instruction (mov) moves either a byte (8 bits) or a word (16 bits)
from one place to another. There are three possible methods:
 moving data from a register to a memory location. [mov [ax],bx]
 moving data from a memory location to a register. [mov ax,[bx]]
 moving data from one register to another. [mov ax,bx]
....
mov cx,20 ; moves decimal 20 into cx
mov ax,10h ; moves 10 hex into ax
mov ax,01110110b ; moves binary 01110110 into ax
mov bx,200h
mov [bx],50h ;load 50 hex into memory location 200h
" (Buchanan and Wilson 2001; p.105)
I/O devices on the PC are mapped into a separate 16-bit address space. \I/O address are called
ports. ... The I/O address space is from 0000h to FFFFh ... split into two main areas: 00h {
FFh: embedded I/O address ... 100h{FFFFh: expansion I/O addresses ... Some I/O addresses
are index or data register addresses. The data written to an index register indicates to the device
which internal register is to be accessed. The data written to or read from a data register is the
actual data value to be placed in the register or the data contained in the register, indicated by
the preceding write to the index register. "(Buchanan and Wilson 2001; p. 181)
The in and out assembly language instructions are used to access ports. The address to be accessed
can be speci ed in the instruction, or may be loaded into the DX register, as shown below
in al,03f8h
in al,dx
out 03f8h, al
out dx,al
\Most modern C++ development systems use an inline assembler that allows assembly language
code to be embedded with C++ code. This code can use any C variable or function name that is
in scope. The __asm keyword invokes the inline assembler, and can appear wherever a C statement
is legal. "
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 18
void main()
{
unsigned int i=5;
__asm mov ax,[i];
__asm
{
inc ax
i=ax;
}
}
In C/C++ development tool-chains, frequently used assembly language instructions are often
wrapped into C-style functions provided in header/library  les. For example the _inp and _outp
C-functions may be used instead of assembly language calls to in and out.
A couple of provisos to the preceding notes:
 When programming to access a device in the I/O space, it is necessary to ascertain whether
the device actually exists at the speci ed location. The result of accessing an unspeci ed
device may vary from machine to machine, and compiler to compiler.
 This technique should not be viewed as an alternative to having a separately assembled
assembly language module which is linked with the compiled code. It is best used when the
assembly language functionality required is limited to simple operations.
 Variations exist between compilers: the actual C functions may be named di erently e.g.
_asm or inportb; and pre-processors may not correctly interpret assembly language syntax.
Review Exercises
1. Which of the following is NOT a valid comparison between the PIC16F877 microcontroller
and Intel x86 processors?
(a) Intel x86: CISC PIC16F877: RISC
(b) Intel x86: Harvard PIC16F877: Von Neumann
(c) Intel x86: Isolated I/O PIC16F877: Memory Mapped I/O
(d) Intel x86: Vectored Interrupts PIC16F877: Single Interrupt
(e) Intel x86: Identical data/address widths PIC16F877: Di erent data/address widths
2. If we need to communicate between a PC and this device, we could interface individual lines
to the Parallel Port. To access the various lines of the Parallel Port, we use the inp and outp
instructions. These instructions refer to AL (Register A). However, when moving information
between register A and other registers we refer to AX. This is because of the following reasons
EXCEPT:
(a) AX is a 16 bit register, and AL is the lower byte.
(b) The parallel port peripheral registers are 8 bits wide.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 19
(c) The outp instruction format only has room for 8 bits of data.
(d) Of the bus lines used to communicate with the parallel port peripheral, only 8 are data
lines.
(e) The PC processor is backwards compatible with earlier members of the x86 family.
3. An assembly language program which accesses peripheral registers on the PC, uses the inp
and outp assembly language instructions to access them. Data registers on the PC are
accessed using the mov instructions. The PIC16F877 uses the same mov instructions to access
both data and peripheral registers. This is because:
(a) The PIC16F877 is a RISC processor and the PC is a CISC processor.
(b) The PIC16F877 has Harvard architecture and the PC has Von Neumann architecture.
(c) The PIC16F877 has a memory-mapped I/O system and the PC has an isolated I/O
system.
(d) all of the above
(e) none of the above
4. PC’s maintain a system variable containing the number of seconds which have elapsed since
the PC was switched on. Outline how this could be implemented using the clock signal and
a digital counter. Is the counter really necessary?
5. Choose a PC C/C++ compiler, and determine
(a) the name/format of the keyword for including assembly language statements within the
C program
(b) the name of the wrapper functions for the I/O instructions in and out
6. Choose one of the following devices (Serial Port COM3, Programmable Interrupt Controller,
Programmable System Timer, Lan-on-Motherboard Controller), and  nd out
(a) where it is located in the PC I/O memory map (i.e. base address)
(b) the number of registers associated with the device,their functions, and their respective
o set addresses.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 20
32 PC to  P serial link
At the end of this unit the student will be able to:
produce assembly language programs for communication between a microprocessor and
a PC using the parallel and/or serial port, given UART and PPC datasheets/details,
and respective instruction sets.
The PIC16F877 does not have dedicated peripherals which perform parallel communication. The
8259 parallel port controller performs this function on the PC, and was originally intended for
communication with a printer. We used this peripheral in the previous unit’s lab exercise.
The PIC16F877 includes peripherals which will independently perform serial communications: The
Master Synchronous Serial Port (MSSP) for speci c synchronous communication protocols (i.e.
I2C and SPI), and the Universal Synchronous Asynchronous Receiver Transmitter (USART) for
general synchronous/asynchronous serial communications.
\An example of a late 1980s UART was the Intel 8450. In the 1990s, newer UARTs were developed with
on-chip bu er space for data. This allowed higher transmission speed without data loss and without
requiring such frequent attention from the computer. For example, the Intel 16550 has a 16 byte FIFO."
(FOL 1993)
Like all other peripherals, communication peripherals have 4 types of peripheral registers:
data generally gives access to the RX and TX bu ers
state report errors, medium state, completion of TX/RX, state of TX/RX bu ers
con guration/control protocol settings; interrupt masks; TX/RX enabled/disabled
Because serial communications must be processed in a relatively quick manner, there are generally
multiple interrupt triggers associated with a serial communication peripheral. Further, certain
interrupt  ags are automatically cleared (in order to save machine cycles) for e.g. the RX  ag
might be cleared as soon as the received data is accessed. A U(S)ART may be used to implement
RS232 communications between microprocessors (when combined with appropriate level shifter(s)).
\ (UART) An integrated circuit used for serial communications, containing a transmitter (parallel-to-serial con-
verter) and a receiver (serial-to-parallel converter), each clocked separately.
The parallel side of a UART is usually connected to the bus of a computer. When the computer writes a byte
to the UART’s transmit data register (TDR), the UART will start to transmit it on the serial line. The UART’s
status register contains a  ag bit which the computer can read to see if the UART is ready to transmit another
byte. Another status register bit says whether the UART has received a byte from the serial line, in which case
the computer should read it from the receive data register (RDR). If another byte is received before the previous
one is read, the UART will signal an "overrun" error via another status bit.
The UART may be set up to interrupt the computer when data is received or when ready to transmit more data.
The UART’s serial connections usually go via separate line driver and line receiver integrated circuits which provide
the power and voltages required to drive the serial line and give some protection against noise on the line.
Data on the serial line is formatted by the UART according to the setting of the UART’s control register. This
may also determine the transmit and receive baud rates if the UART contains its own clock circuits or "baud rate
generators". If incorrectly formatted data is received the UART may signal a "framing error" or "parity error".
Often the clock will run at 16 times the baud rate (bits per second) to allow the receiver to do centre sampling -
i.e. to read each bit in the middle of its allotted time period. This makes the UART more tolerant to variations
in the clock rate ("jitter") of the incoming data. "(FOL 1993)
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 21
In RS232 communication, 9 way D-type connectors and \25-way D-type connectors are common but
often only three wires are connected - one ground (pin 7) and one for data in each direction. The other pins
are primarily related to hardware handshaking between sender and receiver and to carrier detection on modems,
inoperative circuits, busy conditions etc.
The standard classi es equipment as either Data Communications Equipment (DCE) or Data Terminal Equipment
(DTE). DTE receives data on pin 3 and transmits on pin 2 (TD). A DCE EIA-232 interface has a female connector.
DCE receives data from DTE on pin 2 (TD) and sends that data out the analog line. Data received from the
analog line is sent by the DCE on pin 3(RD).
Originally DCE was a modem and DTE was a computer or terminal. The terminal or computer was connected
(via EIA-232) to two modems, which were connected via a telephone line.
The above arrangement allows a computer or terminal to be connected to a modem with a straight-through (2-2,
3-3) cable. It is common, however, to  nd equipment with the wrong sex connector or with pins two and three
reversed, requiring the insertion of a cable or adaptor wired as a gender mender or null modem. Such an adaptor
is also required when connecting a computer directly to a terminal or to another computer without the use of
modems. "(FOL 1993)
\ A logic ’1’ ranges from -3V to -25V, but will typically be around -12V. A logical ’0’ ranges from 3V to 25V,
but will typically be around +12V. Any voltage between -3V and +3V has an indeterminate logical state. If no
pulses are present on the line the voltage level is equivalent to a high level, that is -12V. A voltage level of 0V at
the receiver is interpreted as a line break or a short circuit. ... In the transmission of data there can either be no
handshaking, hardware handshaking, or software handshaking. If no handshaking is used, then the receiver must
be able to read the received characters before the transmitter sends another. ... Hardware handshaking involves
the transmitter asking the receiver if it is ready to receive data. If the receive bu er is empty it will inform the
transmitter that it is ready to receive data. Once the data is transmitted .... the transmitter is informed not to
transmit any more characters .... The main hardware handshaking lines used for this purpose are:
 CTS - Clear to Send
 RTS - Ready to Send
 DTR - Data terminal ready
 DSR - Data set ready
... Software handshaking involves sending special control characters. ... There are two ASCII characters that
start and stop communications. These are X-ON (^S, Cntrl-S or ASCII 11) and X-OFF (^Q, Cntrl-Q or ASCII
13). When the transmitter receives an X-OFF character it ceases communications until an X-ON character is
sent. "(Buchanan and Wilson 2001)
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 22
Review Exercises
1. The parity bit in RS232 serial communications may be used for error detection. When Even
parity is selected, the parity bit is set/cleared so that when combined with the data bits, there
is an even number of ’1’ bits. In 7 bit, even parity communication, which of the following
should have the parity bit set?
(a) 1111 101
(b) 1010 100
(c) 1010 101
(d) 1100 110
(e) 1011 111
2. A device uses a proprietary synchronous communication protocol that requires 3 lines, Signal,
Ground, and an externally controlled clock. The maximum rate at which data can be transmitted is
2400 bits/s. A message transaction involves 8 data bits and 3 overhead bits (start, stop and parity).
We need to communicate between the PIC16F877 and this device. It has been suggested that
we could use the UART to do so. Which of the following is a valid con guration value for
the TXSTA register?
(a) 1010 1111
(b) 1100 1111
(c) 1101 1110
(d) 1010 1110
(e) 0001 1111
3. We wish to con gure the PC UART for 9600 baud communication. The crystal frequency
typically used with these UART’s is is 1.8432 MHz. Given that the UART samples the serial
line 16 times per bit period, the divisor value must be set to:
(a) 0x0C
(b) 0x0F
(c) 0x06
(d) 0x40
(e) 0x01
4. The PIC16F877 has built-in peripherals which support synchronous and asynchronous com-
munication. The asynchronous communication peripheral can be used to interface with an
RS232 signal (presuming appropriate signal level conversion). The peripheral does not sup-
port automatic parity generation/checking or  ow control.
For an application that needs to communicate using even parity, and software  ow control,
choose between bit-banging and using this peripheral, and justify your choice.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 23
5. Your company is designing an independent device for monitoring RS232 tra c. The device
should passively monitor electrical signals on the TX/RX lines of an RS232 serial cable, and
display current and cumulative estimates of communication tra c. The team has decided to
use the UART on a PIC16F877 to passively monitor the lines.
Your job is to specify the electrical connection, and then write an appropriately commented
code module which will allow the information transmitted in both directions to be stored in
a circular bu er.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 24
Lab 4 ID#
You should complete the pre-lab before coming to your lab session. Your Teaching Assistant/Demonstrator
may refuse to allow you into your lab session if your pre-lab is incomplete, or if you are unduly
late.
You will have 12 hours in the lab to complete the exercises. All answers should be written on this
lab-sheet in PEN. Please do not attach any extraneous pieces of paper unless SPECIFICALLY asked
to do so.
Please bring your PIC16F877 datasheet book (provided in ECNG2006) to the lab with
you. You may need to refer to it in order to complete the pre-lab and lab exercises.
Your lab submissions will be checked for unwarranted collusion, and unreferenced use
of Intenet-available/other resources.
At the end of this unit the student will be able to:
 produce programs to interface external peripherals/devices to the PIC16F877 mi-
crocontroller.
 produce programs for communication between a microprocessor and a PC using
the parallel and/or serial port, given UART and/or PPC datasheets/details, and
respective language/instruction sets.
This lab exercise will look at four tasks commonly performed during embedded application pro-
gramming: multiplexing, interrupts, master/slave communication, PC communication.
Pre-Lab
1. Let’s start with some general questions about parallel and serial communication.
(a) What is the di erence between parallel and serial communications. 3 marks
(b) What is a UART? In your own words, provide a brief description of it’s function. 2 marks
(c) Each UART has 3 pins: transmit, receive, and ground. Draw a diagram to show con-
nections between two UART’s so that they can exchange information. 3 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 25
(d) When establishing a connection across an RS232 link, we must set the baud rate,  ow
control setting, parity, # data bits and # stop bits for the UART. In your own
words (and using diagrams if needed), explain each of these terms. 5 marks
(e) What would you expect to happen if two UARTS were con gured di erently, and they
tried to communicate? Explain your answer. 2 marks
(f) \Serial peripherals can be polled by the CPU or they can interrupt the CPU when
they need service". Explain the highlighted terms in the context of the sentence, and
list the relative advantages/disadvantages of the two methods. 4 marks
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ECNG2006 (EE25M) March 11, 2008 VIII 26
2. Next, some questions speci c to the PIC16F877. The USART allows the PIC to make and an-
swer requests from another microprocessor. The Parallel Slave Port on the PIC16F877 allows
an external \master" to retrieve information from, or send information to the PIC16F877.
(a) Read the PIC16F877 datasheet (10.2.1, 10.2.2) and identify the data, con guration,
state and control bits/registers for the USART operating in asynchronous mode. 5 marks
Processor Peripheral Con guration Control State Data
PIC16F877 USART
(b) The PIC16F877 has several banks in the data memory map. The bottom bytes of each
bank are all mapped onto the same memory locations. Explain, in your own words, why
it is useful to have memory mapped into the same location of each bank. 2 marks
(c) Identify the pins associated with the PSP, and the PC connections (for these pins) which
exist in your circuit. 11 marks
There are switches connected to some of the pins. Choose two of these pins. What
voltage do you expect to be applied to pin - when:
 the switch is on? 1 mark
 the switch is o ? 1 mark
What voltage do you expect to be applied to pin - when:
 the switch is on? 1 mark
 the switch is o ? 1 mark
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 27
3. Let us start with a basic delay routine which we can use to build standard times. We will use
instruction based delays. We can write a general delay, which will call a 1 millisecond delay
routine Dly1ms as many times as speci ed in the working register W.
Dly call Dly1ms
addlw 0xFF
btfss STATUS,Z
goto Dly
return
4. We have used macros in the preceding code. The preprocessor replaces the macro name with
the code included between the MACRO and ENDM directives. What are the names of the
two macros? What does the code included in each macro do? 2 marks
5. In the second macro, what is dval? 1 mark
6. Identify at least two di erences between using a macro and using a subroutine. 2 marks
7. What possible advantage is there to using macros in an assembly language program? 1 mark
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ECNG2006 (EE25M) March 11, 2008 VIII 28
8. Finally let us write/consider some support routines/algorithms which you will use later in
the lab.
(a) Write a 1 millisecond instruction-based delay routine for the PIC16F877 named Dly1ms
whose last line will return on completion, which does not a ect the value of the working
register. You should assume a 4MHz clock. 6 marks
(b) Write a routine to con gure and start Timer0 on the PIC16F877 so that Timer0 will
count the number of falling edges it sees from an external input. 4 marks
Helpsheet: Connecting to Bluetooth Modules
What is Bluetooth?
Ensure board is powered. Drop pin in program so that it is ready to receive connections Check
Blue Soliel - make sure that connection is established to your module - check the ID number under
module if unsure. Use the default pin 0000. Open PC - start service for serial port - note COM
port Proceed to open Hyperterminal for that COM port.
HelpSheet: Hyperterminal and the PC serial port
Hyperterminal is the terminal emulation program supplied with the Windows Operating system.
It may be used to communicate across any serial channel, including the RS232 communications
ports (COMn:).
To start Hyperterminal:
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 29
 Click on the Start menu, and then select:
Programs>Accesories>Communications>Hyperterminal.
 Click cancel on the box that is presented. You should now have a window entitled "New
Connection".
 Choose File, Properties, this will bring up a communications box.
 Choose the relevant COM port from the connect using box.
 Click the Con gure button { this will bring up a dialogue box which will allow you to choose
the baud rate, data bits, parity and stop bits, and the  ow control. Once you have chosen
your settings, click OK twice. The settings should now appear in the status bar at the bottom
of the window.
 Click Call> Call to start communicating. (You’ll need to enter a name)
 Click Call> Disconnect to stop communicating.
 If you change the settings while connected, you must disconnect and reconnect for them to
take e ect.
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ECNG2006 (EE25M) March 11, 2008 VIII 30
HelpSheet: Frequently Asked Questions: Embedded Application Programming
multiplex ing parallel i/o devices
Q: Why do we use multiplexors when we can just connect each device to di erent pins?
A: Micro-controller (packages) have a limited number of i/o ports (pins). Multiplexing
allows us to control more devices with the same number of pins.
Q: There must be some tradeo ...
A: Yes - the e ective response/control time of the devices will be reduced.
interrupts for device i/o handling
Q: What is an interrupt?
A: An interrupt is a
Q: Interrupts! They are so confusing! Won’t polling be easier?
A: Polling only works well when events will ALWAYS occur in a known sequence. If the
sequence is not known, a polling program runs the risk of getting stuck polling for one
event while another one is occurring.
master /slave synchronous serial communications
Q: What does synchronous mean?
A: The word synchronous refers to things that happen at the same time. For communi-
cations that means that the sender and the receiver have an agreed clock signal, using
which they send/recieve at agreed times.
Q: So does the slave do all the work (sending and receiving) in master/slave communica-
tions?
A: No. The master controls/generates the clock signal which is used to synchronize com-
munication, however they are both sending and receiving - in fact the master may do
more sending/recieving than the slave.
Personal Computer (PC) asynchronous serial communications
Q: Why would anyone want to communicate between the PC and a microcontroller?
A: Lot’s of reasons ... you may want the PC to act as a storage device, or as a terminal
emulator, or the microcontroller may be providing updated values for a PC program
(e.g. MATLAB).
Q: Haven’t we been using a serial connection all along?
A: Yes there was a USB serial cable for communication between the ICD and the MPLAB
IDE.
Q: So what’s the di erence?
A: This time you will have two serial connection { one for MPLAB-ICD and a wireless Blue-
tooth serial connection between the PC and the PIC16F877 (without passing through
the ICD). In practice, once you have  nished developing your application, the ICD cable
will no longer be necessary.
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ECNG2006 (EE25M) March 11, 2008 VIII 31
In the Lab ID#
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ECNG2006 (EE25M) March 11, 2008 VIII 32
You are given the circuit diagram for a system board containing
 four 7-segment LED displays,
 four DIP switch package.
 an opto-switch,
 a joystick connected to an Analog-to-Digital converter,
 a Bluetooth module with antenna,
The board is connected via a ribbon cable to a board on which the PIC16F877 is mounted. The
pin connection from the PIC16F877 to the system board are as follows:
Check that the circuit matches the circuit diagram on page 51; if there are any di erences, make
appropriate corrections to the diagram and/or circuit.
Please let the lab technician check your circuit before you proceed.
7 segment output w/timer to scroll between di erent outputs
Switches input - parallel port
Optoswitch - interrupt source - count up
Joystick - serial SPI - using the interrupts to drive an o around your segments.
Bluetooth - serial async - info to PC.
Challenge - drive the o around from the PC -arrow keys.
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ECNG2006 (EE25M) March 11, 2008 VIII 33
The PIC16F877 can also control a parallel bus connection. We are going to use the LCD functions
we wrote in the pre-lab to write a simple program which will display a string, and a value.
1. Let us start with a basic program which will use the include  le, and simply initialise the
LCD. Make a new project, a new  le, and enter the following code (N.B. the LCD include
 le must be in the same directory).
INCLUDE <P16f877.inc>
LIST P=16F877
ORG 0x00
nop
goto main
ORG 0x10
main bsf STATUS,RP0
movlw B’00000110’ ; we need this for PORTA
movwf ADCON1 ; to operate properly as digital pins
bcf STATUS,RP0
call LCDSetup
loop goto loop
sleep
ORG 0x200 ; place this somewhere where it won’t clash with the program
LCD_DATA EQU PORTB
LCD_DATA_TRIS EQU TRISB
LCD_CNTL EQU PORTA
LCD_CNTL_TRIS EQU TRISA
LCD_E EQU 3
LCD_RW EQU 2
LCD_RS EQU 1
LCD_Vars EQU 0x20 ; we need 4 locations starting from LCD_Vars in Block 0
INCLUDE <LCDASM.INC>
END
Compile and run all code, and make sure it works in the simulator.
Program and run the code on the cirucit using the ICD. If the program is working your LCD
will initialise and display Ok. If it doesn’t work, apply your troubleshooting skills to locating
the problem. Use the space below to record your troubleshooting investigations.
Demonstrate working code to your TA/lecturer who will sign your script. 4 marks
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ECNG2006 (EE25M) March 11, 2008 VIII 34
2. Choose an appropriate place in your code to place the following lookup tables.
lookup_nibble addwf PCL,F
DT "0123456789ABCDEF\0"
lookup_string addwf PCL,F
DT "The value is: \0"
Where did you put them? Explain why you chose that position. 2 marks
Now, alter the main loop so that it uses a register named indx which starts from 0 and
increments until we get to the end of the string. On each iteration, fetch the character from
lookup_string associated with indx, and display it on the LCD using the LCD_SEND_CHAR
routine.
Compile and test your code. Does it work as you expected? Make changes until it does.
If your program didn’t work on the  rst try what were the problems? How did you  x it?
Write down the  nal version of your main loop here.
Did this code work? Yes or No (Circle your answer)
If so, demonstrate the working code to your TA/lecturer who will sign your script. 5 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 35
3. Strings can also be stored in RAM. Write a routine that will take the following 16-byte string:
MyStrInit addwf PCL,F
DT "The value is: ",0
and copy it using indirect addressing to locations starting from MyStr. You should de ne the
16 spaces for MyStr using the CBLOCK directive.
Alter the main loop so that it will convert the number stored in a register num into the ASCII
equivalent of the hexadecimal number, and store it in MyStr at positions 13 and 14, before
displaying MyStr on the LCD display.
(Hint: Use the lookup_nibble table to translate each nibble of num separately.)
Initialise num with a value.
Compile and test your code. Does it work as you expected? Make changes until it does.
If your program didn’t work on the  rst try what were the problems? How did you  x it?
Write down the  nal version of your main loop here.
Did this code work? Yes or No (Circle your answer)
If so, demonstrate the working code to your TA/lecturer who will sign your script. 10 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 36
4. Now alter the main loop so that:
 the motor runs at a speed determined by the value written to the PSP from the PC,
 Timer0 is con gured as a counter, whose value is continuously copied to num for display.
As the motor turns we should see the LCD displaying the number of encoder pulses counted
by Timer0.
Compile and test your code. Does it work as you expected? Make changes until it does.
If your program didn’t work on the  rst try what were the problems? How did you  x it?
Write down the  nal version of your main loop here.
Did this code work? Yes or No (Circle your answer)
If so, demonstrate the working code to your TA/lecturer who will sign your script. 16 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 37
We would like to write a program for the PIC16F877, which communicates with the PC over an
RS232 link, instead of across the PSP.
Disconnect the parallel cable and connect the serial cable.
Firstly, let us locate all the serial ports on the computer that you are using. Look at the back of
the computer. How many serial ports do you see?
(N.B. remember that the computer is DTE, so you are looking for male D-type connectors, either
9 pin or 25 pin.)
The computer \labels" it’s serial ports COM1, COM2, COM3, COM4 etc Which serial port are
you connected to?
The circuit you have been provided with has a PIC in a header, and an RS232 driver IC. What is
the IC part #?
You have already located the registers of the USART peripheral in the PIC (as part of your pre-lab
exercises). We will start with a simple program which polls for a character, and then echoes the
character received to the transmit register. We will use the following communication settings:
 2400 baud
 8 data bits
 no parity bits
 no  ow control
 1 stop bit
The structure of the program will be:
con gure and control
loop:
wait for state
read data
write data
wait for state
goto loop
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ECNG2006 (EE25M) March 11, 2008 VIII 38
Fill in appropriate settings, and register/ ag names in the code below. 7 marks
LIST P=16F877
INCLUDE <P16F877.inc>
ORG 0x00
nop
goto main
ORG 0x10
main call UART_Cnfg
goto UART_poll
UART_Cnfg bsf STATUS,RP0 ; Go to Bank1
movlw __________ ; Set Baud rate to 2400
movwf SPBRG
movlw __________ ; 8-bit transmit, transmitter enabled,
movwf TXSTA ; asynchronous mode, low speed mode
bcf STATUS,RP0 ; Go to Bank 0
movlw __________ ; 8-bit receive, receiver enabled,
movwf RCSTA ; serial port enabled
return
UART_poll clrw
loop btfss ______,RCIF; wait on char
goto loop
nchar movf _________,W; transmit what you rx (NB reading clears the flag)
movwf ___________
wait btfss ______,TXIF; flag clears when character is gone from buffer
goto wait
goto loop
sleep
END
Start up MPLAB, and create a new project and  le for the code. Compile your code, and make
sure there are no errors. Power up your circuit, and download the code.
Start Hyperterminal (as per helpsheet) and choose the required communication settings. Connect
to the communication channel, and then run the program on the PIC16F877.
What happens when you type in the Hyperterminal window? Is it what you expected? If not, why
not, and/or what did you do to  x it?
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ECNG2006 (EE25M) March 11, 2008 VIII 39
What happens if you disconnect, change the communication settings, and then reconnect and start
typing? Is that also what you expected? If not, why not, and/or what did you do to  x it?
Explain your observations. 4 marks
Change the settings in HyperTerminal back to their original values before proceeding.
We can modify the previous example, so that instead of just repeating what it received, the micro-
controller changes the content somehow.
Add the routine lookup_string (that you used earlier) between return and UART_poll.
Change the original program by inserting a call to your routine, on a new line just after the nchar
labelled line.
Compile your code, and make sure there are no errors. Download the code and test your program
by typing di erent characters in HyperTerminal. Does it work as you expected? Make changes
until it does.
If your program didn’t work on the  rst try what were the problems? How did you  x it?
Write down the  nal version of your routine here. 5 marks
Did this code work? Yes or No (Circle your answer)
If so, demonstrate the working code to your TA/lecturer who will sign your script.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 40
So far we have been polling. Let us convert the polling program to an interrupt driven one. We
can rewrite the main part of the code as:
con gure and control
loop:
{ do something else
goto loop
and then write an interrupt service routine, and handlers for the individual interrupts.
Let us start with the interrupt subroutine. Choose appropriate locations to store _W, _STATUS and
_PCLATH, so that the code will work in all banks. Justify your choices. 3 marks
_W EQU _______
_STATUS EQU _______
_PCLATH EQU _______
isr ; save all registers
movwf _W ; Copy W to TEMP register
swapf STATUS,W ; Swap status to be saved into W
clrf STATUS ; bank 0, clears IRP,RP1,RP0
movwf _STATUS ; Save status to bank zero STATUS_TEMP register
movf PCLATH, W ; Only required if using pages 1, 2 and/or 3
movwf _PCLATH ; Save PCLATH into W
clrf PCLATH ; Page zero, regardless of current page
; no need to clear flags as they change automatically for this device
btfsc PIR1, RCIF
goto rx_handler
rtn_isr movf _PCLATH, W ; Restore PCLATH
movwf PCLATH ; Move W into PCLATH
swapf _STATUS,W ; Swap STATUS_TEMP register into W
; (sets bank to original state)
movwf STATUS ; Move W into STATUS register
swapf _W,F ; Swap W_TEMP
swapf _W,W ; Swap W_TEMP into W
; all registers restored
retfie
Next we have the individual handlers. We would like to transmit as soon as we receive, so the
transmit handler can be empty for now.
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ECNG2006 (EE25M) March 11, 2008 VIII 41
rx_handler ; on rx, tx the next character
movf ________,W ; read the byte to clear the flag
movwf ___________
goto rtn_isr
Now for the associated main loop,  ll in the blanks in the following code . 2 marks
LIST P=16F877
INCLUDE <P16F877.inc>
ORG 0x00
nop
goto main
ORG 0x04
goto isr
ORG 0x10
main call UART_Cnfg
goto otherloop
UART_Cnfg bsf STATUS,RP0 ; Go to Bank1
movlw __________ ; Set Baud rate to 2400
movwf SPBRG
movlw __________ ; 8-bit transmit, transmitter enabled,
movwf TXSTA ; asynchronous mode, low speed mode
bsf PIE1, RCIE ; enable receive interrupts
bcf STATUS,RP0 ; Go to Bank 0
movlw __________ ; 8-bit receive, receiver enabled,
movwf RCSTA ; serial port enabled
bsf INTCON, ___; enable peripheral & general interrupts
bsf ______, PEIE
bsf RCSTA , CREN
return
otherloop goto otherloop
sleep
END
Type in all your code, compile and run it. Test your program by using HyperTerminal.
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ECNG2006 (EE25M) March 11, 2008 VIII 42
Does it work as you expected? If not, why not, and/or what did you do to  x it?
Demonstrate the working code to your TA/lecturer who will sign your script. 4 marks
Previously, we adjusted motor speed from the PC using the PSP. Write a program which will change
motor speed based on a single digit typed in HyperTerminal, where ’0’means stop, 9 means full
speed, and digits 1 through 8 adjust speed proportionally between no and full speed.
Compile and test your code. Does it work as you expected? Make changes until it does.
If your program didn’t work on the  rst try what were the problems? How did you  x it?
Write down the  nal version of your main loop here.
Did this code work? Yes or No (Circle your answer)
If so, demonstrate the working code to your TA/lecturer who will sign your script. 10 marks
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ECNG2006 (EE25M) March 11, 2008 VIII 43
Modify your program so that it in response to a single character, it sends the 3-character string
‘OK\n\0’. You should use TX interrupts, and indirect addressing in a way which will allow you to
send any null-terminated string. 15 marks
Compile and test your code. Does it work as you expected? Make changes until it does.
If your program didn’t work on the  rst try what were the problems? How did you  x it?
Write down the  nal version of your main loop here.
Did this code work? Yes or No (Circle your answer)
If so, demonstrate the working code to your TA/lecturer who will sign your script.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 44
Modfy your program to report the measured motor speed(Timer0) across the serial link when the
character S is typed in HyperTerminal. 20 marks
Compile and test your code. Does it work as you expected? Make changes until it does.
If your program didn’t work on the  rst try what were the problems? How did you  x it?
Write down the  nal version of your main loop here.
Did this code work? Yes or No (Circle your answer)
If so, demonstrate the working code to your TA/lecturer who will sign your script.
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ECNG2006 (EE25M) March 11, 2008 VIII 45
Challenge: The pre-lab asked you to explain an algorithm for manipulating regularly sampled data.
1. Use either instruction-based delays or Timer1 (with/without interrupts) to regularly sample
the distance turned by the motor (Timer0 counts). 20 marks
2. All results should be reported serially using RS232. 10 marks
3. Bonus(30 marks): For the sampled data, implement the algorithm you explained in the pre-
lab.
Compile and test your code. Does it work as you expected? Make changes until it does.
If your program didn’t work on the  rst try what were the problems? How did you  x it?
Attach a printout of your  nal code.
Did this code work? Yes or No (Circle your answer)
If so, demonstrate the working code to your TA/lecturer who will sign/stamp your script/printout.
Total marks 194.
This exercise is worth 15% of your ECNG2007 lab mark.
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
 assistance in troubleshooting circuitry and/or using/operating the IDE/debugger
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
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ECNG2006 (EE25M) March 11, 2008 VIII 46
Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this lab exercise.
{ produce programs for communication between a microprocessor and a PC using the
parallel and/or serial port, given UART and/or PPC datasheets/details, and respective
language/instruction sets.
 Which aspect of this lab exercise did you have the most di culty understanding?
 Which aspect of this lab exercise did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this lab exercise.
 Identify one way in which this lab exercise could be improved.
33  P Parallel Interface
At the end of this unit the student will be able to:
interface external peripherals/devices to the PIC16F877 microcontroller using a parallel
bus, and interface the PIC16F877 microcontroller as an external device on a parallel
bus (memory mapped).
A bus is a collection of lines used to transfer information between two devices. The bus master
is responsible for initiating bus transactions, and may also provide address, clock and/or enable
signals for the external (slave) device.
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ECNG2006 (EE25M) March 11, 2008 VIII 47
Typically a bit-parallel bus has selection/address lines, data lines, and read/write control lines.
While some bit-parallel bus devices support addressing, most devices do not include such function-
ality. Bit-parallel bus addressing can be implemented using logic circuitry and/or decoders with
inputs from the parallel bus address lines, and outputs tied to the enable lines (or clock signals/local
address lines) of the respective slave devices.
A bit-parallel bus, for which the PIC16F877 is the bus master/slave, may be simulated using the
I/O ports. There is no peripheral available in the PIC16F877 which supports parallel bus mastering,
however the Parallel Slave Port peripheral will independently handle receipt and transmission of
data, as demanded by an external master.
A bit-parallel bus may be synchronous or asynchronous. In either case, the software/peripheral on
the master must adhere to the read/write timing speci cations of the external (slave) device. The
circuit designer should verify that the required signal rise/fall/hold times of the slave can be met
by the master.
When the Parallel Slave Port is used, the master should observe the PIC’s response timing. The
CU responds to external signals of the PSP in Q2 of the instruction cycle. Therefore to ensure
that the signals are properly processed, the master must hold them for 5 clock oscillations (1.25
instruction cycles) to guarantee that a Q2 has passed. The PIC sets it’s internal event  ags for the
PSP at the start of Q4 following the Q2 in which the event occurred. The PSP  ags may either be
polled in software, or used as interrupt triggers.
A slave port may be simulated using a standard I/O port by either:
 polling the simulated enable/chip select/clock line at regular intervals (use a timer interrupt?)
 or interrupting on change on the enable/chip select/clock line
The PIC16F877 may also be used to master other devices across a bit-parallel bus. Where there are
insu cient port pins available on the PIC16F877 for all the bus lines, special-purpose interfaces,
or even general purpose serial-to-parallel converters may be employed. The PIC16F877 then pulses
out the bits one at a time, before triggering the bus operation. Some devices also operate at
di erent data-word widths.
One such device is the standard character display LCD panel. The controller for such panels are
all instruction-compatible with the Hitachi 44780 IC, however there may be some variations in the
timing speci cations. You should always check the timing speci cations for your particular LCD
controller. Operation of a typical panel is described in (Predko 2000c).
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ECNG2006 (EE25M) March 11, 2008 VIII 48
The minimum cycle time for a transaction across the bus is speci ed as 1000ns == 1 s for the
HD44780. Therefore a PIC with a 4MHz clock i.e. instruction cycle 1 s, can change the bus signals
with each instruction. Delays in the code will be necessary if a faster clock signal (e.g. 20MHz;
instruction cycle 250ns) is used.
Because there is no peripheral available to perform these operations, mastering a parallel bus on
the PIC16F877 can consume signi cant resources (and cannot be interrupted for fear of violating
timing).
An alternative to bit-banging in the main loop, is to model the system using state machines and use
the timer interrupts to generate the state transitions. This leads to rather complicated interrupt
software, but will allow the PIC to perform non-time critical tasks in the main loop.
The data transfer rate across a bit-parallel bus is generally faster than across a bit-serial bus. Use
of a reduced bit-width bus, or a serial-to-parallel converter, will reduce the data transfer rate.
With slaves which have signi cantly slower timing than the microprocessor however, there will be
little to no slow-down, at the cost of the additional circuitry, and more complicated software. One
way of countering increased software complexity, is to use a serial peripheral (e.g. USART) to
independently send out the bit-stream.
Review Exercises
1. A PIC16F877 is used as the controller on an interface card built for a personal computer. The
interface card has a 10MHz oscillator which generates the clock signal for the PIC16F877.
The computer processor communicates with the PIC16F877 using Parallel Slave Port. In
order to ensure that timing requirements are met, the computer processor must hold the
Chip Select signal line low for:
(a) 5 s
(b) 1:25 s
(c) 0:5 s
(d) 0:25 s
(e) none of the above
2. A device uses a proprietary synchronous communication protocol that requires 3 lines, Signal,
Ground, and an externally controlled clock. The maximum rate at which data can be transmitted is
2400 bits/s. A message transaction involves 8 data bits and 3 overhead bits (start, stop and parity).
Four of these devices are to be interfaced to the PIC16F877. It is suggested that a parallel bus
could be used to send/receive data from all devices simultaneously. Which of the following is
NOT a viable alternative?
(a) Clock lines: tied together; Ground lines: tied together; Data lines: tied together
(b) Clock lines: individual; Ground lines: tied together; Data lines: tied together
(c) Clock lines: tied together; Ground lines: tied together; Data lines: individual
(d) Clock lines: tied together; Ground lines: individual; Data lines: individual
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ECNG2006 (EE25M) March 11, 2008 VIII 49
(e) Clock lines: individual; Ground lines: individual; Data lines: tied together
3. Two 2-line LCD displays use di erent LCD controllers. The controllers have compatible
instructions, and the same transaction sequence across the parallel bus, but they have di erent
timing requirements.
Controller Rise/Fall Time Setup Time Hold Time Wait Time
A 40ns 100 ns 50 ns 40  s
B 20ns 80 ns 80 ns 20  s
What timing should you use in your system to ensure that it will work for either LCD display?
Rise/Fall Time Setup Time Hold Time Wait Time
(a) 20 ns 100 ns 80 ns 40  s
(b) 40 ns 80 ns 50 ns 20  s
(c) 20 ns 80 ns 80 ns 20  s
(d) 40 ns 100 ns 50 ns 20  s
(e) 20 ns 80 ns 80 ns 20  s
4. An external microprocessor is to be used to access the PIC16F877, through the Parallel Slave
Port.
(a) What connections need to be made between the PIC and the microprocessor bus, so
that the PIC16F877 will be mapped into the external microprocessor’s memory address
space?
(b) With reference to the timing diagrams, identify the timing constraints which the external
microprocessor should observe for reliable operation.
(c) The deadline for the project is so tight that you cannot wait for the hardware to be
completed before starting software development. Discuss the software tool-chain and
hardware tool requirements needed to facilitate this.
5. Your company is designing an independent device for monitoring RS232 tra c. The device
should passively monitor electrical signals on the TX/RX lines of an RS232 serial cable, and
display current and cumulative estimates of communication tra c. The team has decided to
use an LCD display module to show the tra c estimates. Your job is to write the module
which will send a single character to an LCD display which was initialised in 4-wire mode.
Write appropriately commented code for the PIC16F877.
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ECNG2006 (EE25M) March 11, 2008 VIII 50
Lab 4 Alternate{developed A. Sinanan, Mar. 03, modi ed Mar. 07
In this lab exercise we will look at performing serial communication between the PIC16F877 mi-
crocontroller and the PC using the RS232 serial port. We will also look at performing bit-parallel
communication between the PIC16F877 microcontroller and other devices. Let’s clear up some
questions you may have:
Q: Why would anyone want to communicate between the PC and a microcontroller?
A: Lot’s of reasons ... you may want the PC to act as a storage device, or as a terminal emulator,
or the microcontroller may be providing updated values for a PC program (e.g. MATLAB).
Q: Haven’t we been using a serial connection all along?
A: Yes there was a USB serial cable for communication between the ICD and the MPLAB IDE.
Q: So what’s the di erence?
A: This time you will have two serial cables { one for MPLAB-ICD and the other between the PC
and the PIC16F877 (without passing through the ICD). In practice, once you have  nished
developing your application, the ICD cable will no longer be necessary.
Q: What does parallel communication involve?
A: It involves using multiple signal lines simultaneously, some of the lines will carry data, and
others will carry additional information which will facilitate the transfer (like address, clock,
enable etc.).
Q: Which peripheral on the PIC16F877 does parallel communication?
A: The Parallel Slave Port can be used for parallel communication which is initiated by another
system. For parallel communication which is initiated by the PIC16F877 you must use one
of the standard I/O ports.
Q: The PIC16F877 and the Intel x86 PC both have UART peripherals. So why doesn’t the x86
PC have a slave port, or the PIC16F877 have a parallel port?
A: The PIC family of microcontrollers were originally designed to function as peripherals within
a PC. The slave port feature is so that they can respond to the PC processor. The PC
doesn’t need that feature. The PC parallel port, was intended for the original display device:
printer/tele-type. The PIC microcontrollers never needed that feature.
Q: So why do we have to consider parallel communication when serial communication can do
the same job with less pins over longer distances?
A: The primary reason is speed. Given a constant rise/fall time for signals on a line, data can
be transferred much more quickly over a parallel bus.
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ECNG2006 (EE25M) March 11, 2008 VIII 51
You are given the circuit diagram for a system containing
 a PIC16F877,
 a potentiometer,
 an LCD display,
 a DC motor  tted with switching transistor & incremental encoder disc + opto-switch,
 a connection from the PIC16F877 UART to the serial port of a PC,
 a connection from the PIC16F877 PSP to the parallel port of a PC.
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Pre-Lab ID#
You should complete the pre-lab before coming to the lab. You may not be allowed in the lab if
your pre-lab is incomplete. You will require 6-12 hours in the lab to complete the exercises. Unless
otherwise speci ed, answers should be written on this lab-sheet. Please do not attach any extraneous
pieces of paper. You may keep the help-sheets if you would like to.
At the end of this unit the student will be able to:
 perform I/O programming on the Personal Computer (PC)
 produce programs for communication between a microprocessor and a PC using
the parallel and/or serial port, given UART and/or PPC datasheets/details, and
respective language/instruction sets.
 interface external peripherals/devices to the PIC16F877 microcontroller using a
parallel bus, and interface the PIC16F877 microcontroller as an external device
on a parallel bus (memory mapped).
1. Let’s start with some general questions about parallel and serial communication.
(a) What is the di erence between parallel and serial communications. 3 marks
(b) What is a UART? In your own words, provide a brief description of it’s function. 2 marks
(c) Each UART has 3 pins: transmit, receive, and ground. Draw a diagram to show con-
nections between two UART’s so that they can exchange information. 3 marks
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ECNG2006 (EE25M) March 11, 2008 VIII 53
(d) When establishing a connection across an RS232 link, we must set the baud rate,  ow
control setting, parity, # data bits and # stop bits for the UART. In your own
words (and using diagrams if needed), explain each of these terms. 5 marks
(e) What would you expect to happen if two UARTS were con gured di erently, and they
tried to communicate? Explain your answer. 2 marks
(f) The MAX232 IC and its clones are often used in serial communication circuits. In your
own words, brie y describe what it does. 2 marks
(g) \Serial peripherals can be polled by the CPU or they can interrupt the CPU when
they need service". Explain the highlighted terms in the context of the sentence, and
list the relative advantages/disadvantages of the two methods. 4 marks
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ECNG2006 (EE25M) March 11, 2008 VIII 54
2. Next, some questions speci c to the PIC16F877. The USART allows the PIC to make and an-
swer requests from another microprocessor. The Parallel Slave Port on the PIC16F877 allows
an external \master" to retrieve information from, or send information to the PIC16F877.
(a) Read the PIC16F877 datasheet (10.2.1, 10.2.2) and identify the data, con guration,
state and control bits/registers for the USART operating in asynchronous mode. 5 marks
Processor Peripheral Con guration Control State Data
PIC16F877 USART
(b) The PIC16F877 has several banks in the data memory map. The bottom bytes of each
bank are all mapped onto the same memory locations. Explain, in your own words,
why it is useful to have memory mapped into the same location of each bank. 2 marks
(c) The parallel slave port timing diagrams for the PIC16F877 appear in the datasheet.
For a PIC16F877 which is clocked at 4MHz, determine an appropriate signal sequence
in order for the PSP value to be correctly read. (The PSP should be turned o at the
end of the sequence). 5 marks
(d) Identify the pins associated with the PSP, and the PC connections (for these pins) which
exist in your circuit. 11 marks
There are switches connected to some of the pins. Choose two of these pins. What
voltage do you expect to be applied to pin - when:
 the switch is on? 1 mark
 the switch is o ? 1 mark
What voltage do you expect to be applied to pin - when:
 the switch is on? 1 mark
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ECNG2006 (EE25M) March 11, 2008 VIII 55
 the switch is o ? 1 mark
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ECNG2006 (EE25M) March 11, 2008 VIII 56
3. Now some questions speci c to the PC.
(a) A PC can support more than one serial port or parallel port peripheral. These are
typically labelled COMx or LPTx. Each port has it’s own base address. In your own
words, what is the base address? 2 marks
(b) Now let us look at the parallel port on the PC. Below is the diagram of the male
connector which will  t into the parallel port on the PC.
i. Draw one circuit with a switch for illustration; such that when the switch is ON
the voltage at the pin is 0 volts and 5 volts when it is o . Use one of the Input
Pins on the parallel port. What is the polarity of the signal connected to your
chosen pin? 3 marks
ii. Draw one circuit with an LED for illustration; such that the LED is ON when the
voltage at the pin is 5 Volts and 0 Volts when it is o . Use one of the Output Data
Lines on the Parallel Port. What is the polarity of the signal connected to your
chosen pin? 3 marks
Pins Description
2 { 9 Output Data Lines
18 { 25 Ground
10 Acknowledge
11 Busy
12 Out of Paper
13 Select
15 Error
1 Strobe
iii. The parallel port designations are shown above. Use this information to complete
the table below for the circuit provided: 4 marks
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ECNG2006 (EE25M) March 11, 2008 VIII 57
Switch Number Pin Number Pin Name
1
2
3
4
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4. The LCD display is a device which must controlled by an external \master" across a parallel
bus. LCD displays typically use a standard controller chip (e.g. HD44780) which can com-
municate with the master. The HD44780 LCD character display controller can be operated
in two interface modes: 4 bit (7 wire) and 8 bit (11 wire); where the number of bits indicates
the number of data lines.
(a) The KS066U is an HD44780 instruction-compatible controllers which has di erent tim-
ing under certain conditions. The initialization sequence for 4 bit mode appears in each
datasheet. Write the sequence of instructions, and recommend initialization timings
which would be valid for both controllers so that either would initialise in 4 bit mode
with the block cursor on. 4 marks
(b) We will attempt to write routines which will drive an HD44780 compatible display. Let
us start with a basic delay routine which we can use to build standard times. We will use
instruction based delays. We can write a general delay, which will call a 1 millisecond
delay routine Dly1ms as many times as speci ed in the working register W.
Dly call Dly1ms
addlw 0xFF
btfss STATUS,Z
goto Dly
return
Using this routine, we can write a 4-bit mode initialization routine for the LCD. Assume
that the 4 data bits of the LCD (D7:D4) are connected to the lower half of a PORT
named LCD_DATA, and that the LCD control lines (RW, RS, E) are connected to the
bits (LCD_RW, LCD_RS, LCD_E) of a port named LCD_CNTL. The data direction registers
associated with the ports are LCD_DATA_TRIS and LCD_CNTL_TRIS. It is assumed that
LCD code is always executed from Bank 0. Compare the following code to the datasheet
timings.
LLCDTrigger MACRO
bsf LCD_CNTL, LCD_E
bcf LCD_CNTL, LCD_E
movlw D’15’
call WAIT_MS
ENDM
SLCDTrigger MACRO
bsf LCD_CNTL, LCD_E
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ECNG2006 (EE25M) March 11, 2008 VIII 59
bcf LCD_CNTL, LCD_E
movlw D’1’
call WAIT_MS
ENDM
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ECNG2006 (EE25M) March 11, 2008 VIII 60
LCDTrigger MACRO
bsf LCD_CNTL, LCD_E
bcf LCD_CNTL, LCD_E
ENDM
SetData MACRO dval
movlw 0xF0
andwf LCD_DATA,W
iorlw dval
movwf LCD_DATA
ENDM
LCDInit ; Init LCD
movlw LCD_CNTL_MASK
iorwf LCD_CNTL,F ; Clear port and wait for settling
movlw D’150’
call WAIT_MS
movlw B’0011’ ; Trigger port for first time
movwf LCD_DATA
LLCDTrigger
movlw B’0011’ ; Trigger port for first time
movwf LCD_DATA
LLCDTrigger
movlw B’0010’ ; Trigger port for first time
movwf LCD_DATA
LLCDTrigger
movlw B’0010’ ; Trigger port for second time
movwf LCD_DATA
movlw B’1000’ ; Follow with format
LCDTrigger
movwf LCD_DATA
SLCDTrigger
movlw B’00001110’ ; display and cursor on
call SEND_LCD_CMD
movlw B’00000001’ ; OK now clear display
call SEND_LCD_CMD
return
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ECNG2006 (EE25M) March 11, 2008 VIII 61
i. Use the space below to draw a  owchart to match the preceding code for 4-bit
initialisation. 6 marks
ii. Compare your  owchart to the one in the datasheet. Are there any di erences?
Which algorithm (as illustrated in the two  owcharts) is preferable? Explain your
answer. 2 marks
iii. We have used macros in the preceding code. The preprocessor replaces the macro
name with the code included between the MACRO and ENDM directives. What
are the names of the two macros? What does the code included in each macro do? 2 marks
iv. In the second macro, what is dval? 1 mark
v. Identify at least two di erences between using a macro and using a subroutine. 2 marks
vi. What possible advantage is there to using macros in an assembly language pro-
gram? 1 mark
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ECNG2006 (EE25M) March 11, 2008 VIII 62
vii. Now let us try simple routines to:
 determine if the controller is busy
 send a command
 send a character
We will use two storage variables for these routines: LCD_CHAR to store the char
or command sent/recieved and LCD_TEMP to store the LCD status byte (including
busy  ag).
Let us start with a routine to determine if the controller is busy. Read the datasheet
for the LCD commands, and  ll in appropriate comments for the code below. 9 marks
Busy_check bsf STATUS, RP0
movlw 0x0F
iorwf LCD_DATA_TRIS,F ; _______________________________
bcf STATUS, RP0
bcf LCD_CNTL, LCD_RS
bsf LCD_CNTL, LCD_RW ; _______________________________
bsf LCD_CNTL, LCD_E
nop
swapf LCD_DATA, W
bcf LCD_CNTL, LCD_E ; _______________________________
andlw 0xF0
movwf LCD_TEMP ; _______________________________
bsf LCD_CNTL, LCD_E
nop
movf LCD_DATA, W
bcf LCD_CNTL, LCD_E ; _______________________________
andlw 0x0F
iorwf LCD_TEMP, F ; _______________________________
btfsc LCD_TEMP, 7
goto Busy_check ; _______________________________
bcf LCD_CNTL, LCD_RW ; _______________________________
bsf STATUS, RP0
movlw 0xF0
andwf LCD_DATA_TRIS,F
bcf STATUS, RP0 ; _______________________________
return
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ECNG2006 (EE25M) March 11, 2008 VIII 63
viii. Our next routine sends a command to the LCD. It will check if the LCD is busy
before attempting to send the command. Read the LCD datasheet to determine
the signal line settings needed to send a command. Insert appropriate comments
in the following code. 2 marks
SEND_LCD_CMD movwf LCD_CHAR
call BUSY_CHECK
swapf LCD_CHAR, W ; ____________________
andlw 0x0F
movwf LCD_DATA
bcf LCD_CNTL, LCD_RW
bcf LCD_CNTL, LCD_RS
movf LCD_CHAR, W ; ____________________
andlw 0x0F
LCDTrigger
movwf LCD_DATA
LCDTrigger
return
ix. Modify the above routine so that it will send a character. Call your modi ed rou-
tine SEND_LCD_CHAR, and write it in the space below. 1 mark
x. Now, use the MPASM help, and explain how the CBLOCK directive works 1 mark
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ECNG2006 (EE25M) March 11, 2008 VIII 64
Finally, some general routines to clear the display, and setup the associated ports.
LCDClr movlw B’00001110’ ; display and cursor on
call SEND_LCD_CMD
movlw B’00000001’ ; OK now clear display
call SEND_LCD_CMD
return
LCDSetup clrf LCD_CNTL
clrf LCD_DATA
bsf STATUS, RP0
movlw LCD_CNTL_MASK
andwf LCD_CNTL_TRIS,F
movlw 0xF0
andwf LCD_DATA_TRIS,F
bcf STATUS, RP0
call LCDInit
movlw ’L’
call SEND_LCD_CHAR
movlw ’C’
call SEND_LCD_CHAR
movlw ’D’
call SEND_LCD_CHAR
movlw ’ ’
call SEND_LCD_CHAR
movlw ’O’
call SEND_LCD_CHAR
movlw ’K’
call SEND_LCD_CHAR
return
Place all of this code for the LCD in an include  le named lcdasm.inc. At the top of the
 le, place the following declarations/comments:
; based on http://www.sonic.net/~schlae/lcd.asm
; assumes that: we are connected to the lower 4 bits of the Data Port
; level triggered LCD; code called from Data Bank 0; any bits of CNTL port used
; all variables are in block 0 and that the following are defined e.g.:
;LCD_DATA EQU PORTB
;LCD_DATA_TRIS EQU TRISB
;LCD_CNTL EQU PORTA
;LCD_CNTL_TRIS EQU TRISA
;LCD_E EQU 3
;LCD_RW EQU 2
;LCD_RS EQU 1
; we need 4 locations starting from LCD_Vars in Block 0; define LCD_Vars accordingly
CBLOCK LCD_Vars
LCD_Count
LCD_Count2
LCD_CHAR
LCD_TEMP
ENDC
LCD_CNTL_MASK EQU ~((1<<LCD_E) | (1<<LCD_RW) | (1<<LCD_RS))
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5. Finally let us write/consider some support routines/algorithms which you will use later in
the lab.
(a) Write a 1 millisecond instruction-based delay routine for the PIC16F877 named Dly1ms
whose last line will return on completion, which does not a ect the value of the work-
ing register. You should assume a 4MHz clock. 6 marks
(b) Write a routine to con gure and start Timer0 on the PIC16F877 so that Timer0 will
count the number of falling edges it sees from an external input. 4 marks
(c) Shift and rotate operations can be used to perform fast division by powers of 2. Using 2 marks
a diagram, explain how this works.
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ECNG2006 (EE25M) March 11, 2008 VIII 66
(d) Masking operations can be used to perform modulo (reminder) operations by powers of
2. Using a diagram, explain how this works. 2 marks
(e) Identify and describe an algorithm for: (any ONE of the following)
 estimating the  rst order derivative,
  ltering to remove random or periodic noise,
 integrating,
 predicting the next reading,
from regularly sampled data values. 3 marks
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ECNG2006 (EE25M) March 11, 2008 VIII 67
Helpsheet { A/D and PWM calculations
How to calculate the voltage represented by ADRESH (w/left-justi ed result):
The A/D answer is 10 bits long.
The user reads the 8 most signi cant bits (i.e. the 8 most left most bits in ADRESH).
Let 255 represent VREF+ volts and 0 represent VREF volts.
ADRESH will represent VREF+ VREF 256  ADRESH volts.
How to calculate the PWM frequency and pulse width
\The PWM frequency is de ned as 1/[PWM period]."PIC (2001)
To set the PWM period, we write values to PR2, and set the TMR2 prescaler (bit pattern set in
the two LSB of T2CON).
PWM period = [(PR2)+1] 4 TOSC (prescale value)
To set the duty cycle we write values to CCP1RL and two lower bits. If we ignore the lower 2 bits
of the duty cycle then
PWM duty cycle = CCPR1L 4 TOSC (prescale value)
A/D Timing
A/D sampling time = min’m acquisition time ( 20 s) + # bits (10) * conversion time/bit
(TAD > 1:6 s) + min’m wait time (2TAD)
Acquisition time is the time that the A/D modules holding capacitor is connected to the external
voltage level. Conversion time is started when the GO bit is set, and includes conversion time for
each bit, and a minimum wait time. The signal used to determine TAD is software selectable as
either an internal RC oscillator with a period of between 2 and 6 s, or can be derived from the
chip clock frequency TOSC with scale factor (1/2, 1/8, 1/32). Relative to the instruction cycle, this
sampling time is slow, and the A/D should not be used to measure rapidly changing signals.
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ECNG2006 (EE25M) March 11, 2008 VIII 68
HelpSheet: Hyperterminal and the PC serial port
Hyperterminal is the terminal emulation program supplied with the Windows Operating system.
It may be used to communicate across any serial channel, including the RS232 communications
ports (COMn:).
To start Hyperterminal:
 Click on the Start menu, and then select:
Programs>Accesories>Communications>Hyperterminal.
 Click cancel on the box that is presented. You should now have a window entitled "New
Connection".
 Choose File, Properties, this will bring up a communications box.
 Choose the relevant COM port from the connect using box.
 Click the Con gure button { this will bring up a dialogue box which will allow you to choose
the baud rate, data bits, parity and stop bits, and the  ow control. Once you have chosen
your settings, click OK twice. The settings should now appear in the status bar at the bottom
of the window.
 Click Call> Call to start communicating. (You’ll need to enter a name)
 Click Call> Disconnect to stop communicating.
 If you change the settings while connected, you must disconnect and reconnect for them to
take e ect.
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HelpSheet: Connecting circuits to the Serial port
Level conversion Strictly speaking, driver circuitry must be designed/used in order to convert signals
from RS232 signal levels to/from TTL level UART signals. \ The types of driver ICs used in serial ports can
be divided into three general categories:
 Drivers which require plus (+) and minus (-) voltage power supplies such as the 1488 ...
 Low power drivers which require one +5 volt power supply. This type of driver has an internal charge pump
for voltage conversion ....
 Low voltage (3.3V) and low power drivers which meet the EIA-562 standard ...
... To determine the type of driver used on a serial port without looking at the ICs, put a 3kohm load on the
driver output line to signal ground and measure the voltage. If possible, measure the voltage under both (+) and
(-) voltage conditions. These voltage measurements should give values as shown below.
1488 Driver  9 volts <Vout < 11 volts
Charge Pump driver  7:5 volts <Vout < 8 volts
EIA562 driver  3:7 volts <Vout < 5 volts
" { from Tips for using Port Powered Converters, B&B electronics, Technical Article #2, May 1995,
http://www.bb-europe.com/bb-euro/literature/tech/b&b2.pdf
Some alternative strategies to using driver IC’s:
(see http://www.efplus.com/techref/io/serial/ttl-rs232.htm)
TTL to RS232 { use two TTL outputs (a single output and it’s inverted value) { connect one output to
the RS232 GND and the other to the RS232 RX. Outputs will then appear as 5V (Be careful ... this
won’t work if signal and chassis grounds are connected!).
TTL to RS232 { use the TTL level signal to switch a transistor or opto-coupler, and steal  12V from
other RS232 lines.
RS232 to TTL { use the RS232 level signal to switch a transistor, and provide a 5V supply line.
RS232 to TTL { use a series resistor to reduce the voltage, clamp with a diode, and use an inverting TTL
input/bu er.
Bleeding power \There is one way to get some power out of serial port: steal it from signal lines. When
you are developing you own circuit which connects only the PC, then the only line which can be used are the
output signal lines from PC serial port: DTS, RTS and TD.
In normal operation situation DTR and RTS are raised so they give positive voltage output (about +12V when
not loaded). TD pin is in logic 1 when no data is sent which means that it is most time at negative voltage
(-12V when not loaded) most of the time. The voltage at these outputs drop quite fast when load current
is increase, because they are designed to drive normally only RS-232 input circuits (3-7 kohm resistance). "
http://www.hut.fi/Misc/Electronics/circuits/rspower.html
Surge protection/Opto-isolation \RS-232C serial port lines are quite prone to be damaged by [high
voltages]... Protecting RS-232 serial port against overvoltage is usually quite easy. The standard says that the
signal can be voltage between +25 and -25 volts, so anything outside this range is not allowed. Typical RS-
232C receiver chips can handle well voltages up to +/-30V. So an overvoltage protection [device] which cuts out
everything above 25V and acts fast is the solution. Suitable components for this are varistors (VDR), zener diodes
and other fast semiconductor based overvoltage protection devices.
Another question is what needs to be protected. The answer is that every communication line wire which is
connected between computers must be protected. In simplest case there are only transmit data, receive data and
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ground lines used. In this case only protection components are needed: one between transmit data and ground
and another between receive data and ground. If more lines are used, then all of then must be protected with a
protection component between signal wire and signal ground.
In [potentially] bad overvoltage situations is is recommended to use galvanic isolators in serial port lines or use
serial data converters which provide isolation and protection (short haul modems, current loop adapters etc.)."
http://www.epanorama.net/circuits/rs232_protector.html
8250/16550 UART registers The UART base address is either  xed, set using jumpers, or by using
the BIOS. There are 4 \standard" addresses assigned to the UART in a PC 03f8h, 02f8h, 03e8h, 02e8h
however other addresses may also be used. The COM port typically generates an interrupt at IRQ # 3
(COM2/COM4) or 4(COM1/COM3), however to avoid con icts, some UART’s can also be con gured to
use the "free" interrupts: 10 or 11. The UART registers which will be used in this lab exercise are:
Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 O set
Transmit Register TX Write-only, DLAB=0 { data byte for TX +0x00
Receive Register RX Read-only, DLAB=0 { data byte RX’d +0x00
Line Status Register LSR TX
Bu er
empty
RX
Data
Ready
+0x05
Line Control Register LCR Divisor
Latch
Ac-
cess
Bit
DLAB
0-
even
1-
odd
parity?
0-no
1-yes
stop
bits?
0-1
1-1.5
00 { 5 bits
01 { 6 bits
10 { 7 bits
11 { 8 bits
+0x03
Divisor Latch Low DLL Read/Write, DLAB=1 { Low Byte of 16 bit divisor +0x00
Divisor Latch High DLH Read/Write, DLAB=1 { High Byte of 16 bit divisor +0x01
The 8250/16550 UART transmit and receive circuits clock/sample 16 times per bit. Transmissio/Reception
of the data-word is LSB{ rst. The crystal frequency typically used with these UART’s is is 1.8432 MHz.
Common divisors
Baudrate = clockfrequency16 DLH:DLL
Baud Rate DLH DLL
9600 00 0C
4800 00 18
1200 00 60
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 71
Helpsheet: Interfacing w/Parallel Port
Overview The parallel port is a byte-serial bus, originally designed to provide an interface be-
tween the computer and printer. The standard interface uses a 25 pin D-type connector on the
PC side, and a Centronics-type connector on the printer side. In the standard parallel port, data
is sent in one direction only, from the computer to the printer. Handshaking involves signalling in
both directions. At least three registers are associated with any parallel port: the data, status and
control registers each contain bits which correspond to lines on the port connector, as shown in the
table below:
Signal Name Register bit Signal Pin Ground Return
Data bit 0 D0 2 19
Data bit 1 D1 3 19
Data bit 2 D2 4 20
Data bit 3 D3 5 20
Data bit 4 D4 6 21
Data bit 5 D5 7 21
Data bit 6 D6 8 22
Data bit 7 D7 9 22
ERROR (FAULT) S3 15 23
SELECT S4 13 24
PAPEREND S5 12 24
ACK S6 10 24
BUSY S7 11 23
STROBE C0 1 18
AUTOLF C1 14 25
INIT C2 16 25
SELECTIN C3 17 25
extracted from http://www.lvr.com/ les/pppinout.pdf
When communicating with the printer, the parallel port peripheral \ ... sends data to the data
lines, and then sets the STROBE LOW and then HIGH. These then latch the data to the printer
... [i.e.]
1. Data is written to the data register.
2. The program reads from the status register to test whether the BUSY signal is LOW (that
is, the printer is not busy)
3. If the printer is not busy, then the program sets the STROBE line active LOW.
4. Program then makes the STROBE line HIGH by de-asserting it.
" Buchanan and Wilson (2001)
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 72
A note about locating signals on D-type connectors. Male and female connectors are
designed to mate with each other, so the pin and it’s mating socket are assigned the same numbers.
This means that if the connectors are facing you, pin 1 is located on the right of the female
connector, and on the left of the male connector i.e. the layouts are mirrored. When making up a
cable, always verify by checking the numbers marked on the connector.
Power at the parallel port .
See http://www.hut.fi/Misc/Electronics/circuits/lptpower.html
and http://www.hut.fi/Misc/Electronics/circuits/parallel_output.html.
The parallel port utilises TTL-level signals, and is generally capable of:
 sourcing 0.5mA reliably on the data lines (out),
 sinking 5mA reliably on the data lines (in),
 sourcing 1 mA on the control/signal lines (out),
 sinking 7mA on the control/signal lines (in).
These are only guidelines, actual values will depend on the internal PC circuitry used for the parallel
port. Conservative guidelines are used, because, if the port speci cations are exceeded the voltage
supplied will drop (source), or the port will blow (sink).
If your external circuitry can tolerate the voltage drop associated with higher current draw, then
you may exceed the sourcing guidelines (up to 2.6mA @ 2.4V). Note: If you connect multiple output
pins together in an attempt to source additional current, Use protective diodes on each output to
prevent accidental shorts (blow port).
Apart from circuitry sinking too much current on data/signal lines, unexpected spikes may be fed
back into the computer along the ground lines. The only way to prevent this is to use current
limiting resistors to connect grounds or, opto-isolate the port from the external circuitry.
As a general rule:
try NOT to use a parallel port which is built into the motherboard.
These cannot be replaced if something goes wrong.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 73
In the Lab ID#
Check that the circuit matches the circuit diagram on page 51; if there are any di erences, make
appropriate corrections to the diagram and/or circuit.
Please let the lab technician check your circuit before you proceed.
1. This question will allow the student to investigate and develop code that converts Analog
Data into Digital Data using the A/D peripheral. Con gure ADCON0, ADCON1 so that the A/D
meets the following speci cations:
A/D Conversion Clock Fosc8
Analog Channel Channel 0 selected
State A/D Conversion not in progress, A/D On
Result rightt justi ed
Reference voltages Vdd, Vss
Port Con guration AN0 as the ONLY analog input
Bit <7> <6> <5> <4> <3> <2> <1> <0>
ADCON0
ADCON1
2 marks
The result produced by the A/D module is 10 bits long and not 8 bits nor 16 bits. This
means that the answer is put into two registers named ADRESH and ADRESL. When the answer
is left justi ed, valid bits are in ADRESH<7:0>, ADRESL<7:6>. When the answer is right
justi ed, valid bits are in ADRESH<1:0>, ADRESL<7:0>. We will only be using ADRESH. The
code appears on the following page. Fill in the values from the table above.
Check that the code is consistent with the comments before moving on. If not, what changes
did you make?
In the code, Timer0 is used to set the sampling rate. If Timer0 has just over owed, how long
will it take to over ow again? Show your working. 2 marks
Compile the code and ensure that it builds successfully.
Did your code compile? Yes or No (Circle your answer)
If not, what changes did you make?
If you have problems ask the TA for assistance.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 74
LIST p=16f877
INCLUDE "p16f877.inc"
ORG 0x00
nop
goto Start ; Start at the reset vector + 1 for ICD
ORG 0x20
Start
BANKSEL PORTD
clrf PORTD ;Clear PORTD
movlw ___________ ;Fosc/8, A/D enabled, Sample Channel 0
movwf ADCON0
BANKSEL OPTION_REG
movlw B’10000111’ ;TMR0 prescaler, 1:256
movwf OPTION_REG
clrf TRISD ;PORTD all outputs
movlw ___________ ;Right justify, 1 analog channel
movwf ADCON1 ;VDD and VSS references
Main bcf INTCON,T0IF
Loop btfss INTCON,T0IF ;Wait for Timer0 to timeout
goto Loop
bsf ADCON0,GO ;Start A/D conversion for channel 0
Wait btfss PIR1,ADIF ;Wait for conversion to complete
goto Wait
movf ADRESH,W ;Write A/D result to PORTD
movwf PORTD ;LEDs
bcf PIR1,ADIF ;Clear the completion flag
goto Main ;Do it again
sleep
END ;this is the last line in the file
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 75
Does the program work as you expected? Yes or No (Circle your answer)
If it didn’t what was the problem? How did you  x it?
Program the 16F877, run the program and use a voltmeter to read the actual input voltage
to the PIC from the potentiometer. Note the voltage and the binary value displayed on
the LED’s in the chart below. Take 5 readings between 0 and 5 Volts, then work out the
equivalent voltages for the readings you observed on the LED’s.
Input Voltage PIC LED’s Equivalent Voltage
Comment on the accuracy and resolution of your results. 2 marks
Would it improve if we used more bits of the A/D result? How could we modify the code to
achieve this? Justify your answers. 2 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 76
2. We shall now investigate Pulse Width Modulation (PWM). Compile the code below and
PROGRAM the PIC16F877. Then vary the POT.
If the circuit is working properly the Motor speed should change as you turn the POT.
If you have problems ask the TA for assistance.
Answer the following questions based on the code on the following page, and your observations.
(a) Fill in appropriate comments at the places indicated. 4 marks
(b) What is the PWM frequency? Show your working. 4 marks
(c) Explain, in your own words, why the motor speed changes as you vary the POT. 2 marks
(d) What happens if you change the PWM frequency speci ed by the Timer2 pre-scaler
and re-compile and re-run the program? Your answer should state how you changed
the Timer2 pre-scaler for this investigation. 4 marks
(e) Based on your observations, is PWM an appropriate technique for D/A conversion?
Explain your answer. 2 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 77
LIST p=16F877
INCLUDE <p16F877.inc>
ORG 0x00
goto main
ORG 0x20
main
BANKSEL PR2 ; __________________________________
movlw 0xFF ; __________________________________
movwf PR2 ; __________________________________
BANKSEL T2CON ; __________________________________
movlw B’00000110’ ; __________________________________
movwf T2CON ; __________________________________
movlw B’00001100’ ; __________________________________
movwf CCP1CON ; __________________________________
BANKSEL TRISD ;
clrf TRISD ;Configure PORTD -- all output pins.
bcf TRISC,2 ;Configure PORTC<2> as an output pin.
BANKSEL ADCON0
movlw B’01000001’ ;Fosc/8, A/D enabled, Sample Channel 0
movwf ADCON0
BANKSEL OPTION_REG
movlw B’10000111’ ;TMR0 prescaler, 1:256
movwf OPTION_REG
movlw B’00001110’ ;Left justify, 1 analog channel
movwf ADCON1 ;VDD and VSS references
BANKSEL PORTD
movlw 0xFF ;put an indicator value on the LED’s
movwf PORTD
Loop bcf INTCON,T0IF
Here btfss INTCON,T0IF ;Wait for Timer0 to timeout
goto Here
bcf PIR1,ADIF ;Clear the completion flag
bsf ADCON0,GO ;Start A/D conversion for channel 0
Wait btfss PIR1,ADIF ;Wait for conversion to complete
goto Wait
movf ADRESH,W ;_______________________________________
movwf CCPR1L
movwf PORTD
goto Loop ;Do it again
sleep
END
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 78
3. We will move on to investigate the parallel slave port (PSP).
Examine the following program and answer the questions.
INCLUDE <P16f877.inc>
LIST P=16F877
ORG 0x00
nop
goto main
ORG 0x20
main bsf STATUS,RP0 ; Line A
movlw B’00000111’
movwf ADCON1
bsf TRISE,PSPMODE ; Line B
bcf STATUS,RP0 ; Line C
movlw 0x56 ; Line D
movwf PORTD ; Line E
loop goto loop
sleep
END
 What does Line B do? 1 mark
 What does Line E do? 1 mark
 What is the value in TRISD when this program runs? 1 mark
 Can you tell (just from the code) whether the value in the PORTD output latch will
appear on the output pins? Explain your answer. 2 marks
Type in the above program. Compile it and download it to the PIC16F877. Run the program,
and record what happens when the switches are ALL on. 1 mark
If you have problems ask the TA for assistance.
In your own words, explain how the PSP can be used by an external microprocessor, to access
data within the PIC16F877. 2 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 79
Turn o all switches before proceeding.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 80
4. This question will familiarize us with the parallel port on the PC47.
(a) What kind of PC are you running this lab on? 1 mark
(b) What operating system is loaded on the machine? 1 mark
(c) How do you tell the di erence between a parallel port and a serial port? 1 mark
(d) How many parallel ports are there on the machine? 1 mark
(e) To be safe we should use a parallel port that is NOT on the motherboard; this way we
would avoid accidentally blowing the computer. If this is not possible then our circuit
should have appropriate protection circuitry.
 Choose a parallel port on your computer, and write down the designator (LPTx),
the associated interrupt number, the port address(es) and describe where the port
connector is located. 3 marks
 For your circuit, identify the protection circuitry which will prevent excess current
being fed into the parallel port. 3 marks
47To get information about the PC, Right click on the \My Computer" icon, and select Properties from the menu.
Some information can be found on the General Tab. Other information can be found on the Hardware Tab (you will
need to press the device manager button, and  nd the parallel port under the \Ports" tree-item )
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 81
Connect the Parallel Port Cable before proceeding.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 82
5. We are going to use the Data Acquisition Toolbox in MATLAB to control the pins of the
parallel port.
The registers are contained at the addresses you wrote down for your chosen parallel port.
The lowest number (e.g. hex 0378) is referred to as the BASE ADDRESS of the parallel
port. All parallel registers have an address which is o set from the BASE ADDRESS.
The data register is located at BASE ADDRESS+0x00, and contains bits which correspond
to the values output on pins 2 thru 9.
The status register is located at BASE ADDRESS+0x01, and contains bits which correspond
to the values input from the Error,Select, Out of Paper, Acknowledge and Busy lines (see
helpsheet to determine which is which).
The control register is located at BASE ADDRESS+0x02. It contains the bits corresponding
to the values output on the Strobe, LineFeed, Initialise, and Select lines (see helpsheet to
determine which is which).
(a) Start MATLAB on your computer. Use the daqhwinfo command to access information
about ports on your machine, which MATLAB can recognise.
parportinfo=daqhwinfo(’parallel’);
parportinfo.InstalledBoardIds
Ensure that your chosen port is available to MATLAB.
(b) Create a MATLAB digital Input/output object for your chosen port (be sure to replace
x with the appropriate number):
parport = digitalio(’parallel’,’LPTx’);
(c) Examine the hardware information for the object you have just created.
hwinfo = daqhwinfo(parport);
hwinfo.Port(1)
hwinfo.Port(2)
hwinfo.Port(3)
Relate the information to that given in the helpsheet. Use the space below to make
notes.
If you have problems ask the TA for assistance.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 83
(d) Add the following lines to your digital input/output object (corresponding to the parallel
port lines you use in your circuit):
addline(parport,0:7,0,’out’);
addline(parport,1:3,2,’out’);
parport
We can set and clear individual bits using the command putvalue. For example
putvalue(parport.pin2,1);
Use the LED’s or a meter to determine whether each of the 11 lines is active high, or
active low. If you have problems ask the TA for assistance.
We can also set multiple lines simultaneously for example:
putvalue(parport,75);
What value(s) would you have to use, and in what sequence, in order to get the value
2310 into the Parallel Slave Port of the PIC16F877? 3 marks
Run the PIC program, use the appropriate commands in MATLAB and then check
(use watch window) to see if you were successful. If you have problems ask the TA for
assistance.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 84
(e) Write an assembly language program which varies motor speed according to value re-
ceived from PSP.
Compile and test your code using MATLAB to change the parallel port output from
the PC. Does it work as you expected? Make changes until it does.
If your program didn’t work on the  rst try what were the problems? How did you  x it?
Write down the  nal version of your main loop here.
Did this code work? Yes or No (Circle your answer)
If so, demonstrate the working code to your TA/lecturer who will sign your script. 10 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 85
6. The PIC16F877 can also control a parallel bus connection. We are going to use the LCD
functions we wrote in the pre-lab to write a simple program which will display a string, and
a value.
(a) Let us start with a basic program which will use the include  le, and simply initialise
the LCD. Make a new project, a new  le, and enter the following code (N.B. the LCD
include  le must be in the same directory).
INCLUDE <P16f877.inc>
LIST P=16F877
ORG 0x00
nop
goto main
ORG 0x10
main bsf STATUS,RP0
movlw B’00000110’ ; we need this for PORTA
movwf ADCON1 ; to operate properly as digital pins
bcf STATUS,RP0
call LCDSetup
loop goto loop
sleep
ORG 0x200 ; place this somewhere where it won’t clash with the program
LCD_DATA EQU PORTB
LCD_DATA_TRIS EQU TRISB
LCD_CNTL EQU PORTA
LCD_CNTL_TRIS EQU TRISA
LCD_E EQU 3
LCD_RW EQU 2
LCD_RS EQU 1
LCD_Vars EQU 0x20 ; we need 4 locations starting from LCD_Vars in Block 0
INCLUDE <LCDASM.INC>
END
Compile and run all code, and make sure it works in the simulator.
Program and run the code on the cirucit using the ICD. If the program is working your
LCD will initialise and display Ok. If it doesn’t work, apply your troubleshooting skills to
locating the problem. Use the space below to record your troubleshooting investigations.
Demonstrate working code to your TA/lecturer who will sign your script. 4 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 86
(b) Choose an appropriate place in your code to place the following lookup tables.
lookup_nibble addwf PCL,F
DT "0123456789ABCDEF\0"
lookup_string addwf PCL,F
DT "The value is: \0"
Where did you put them? Explain why you chose that position. 2 marks
Now, alter the main loop so that it uses a register named indx which starts from 0
and increments until we get to the end of the string. On each iteration, fetch the
character from lookup_string associated with indx, and display it on the LCD using
the LCD_SEND_CHAR routine.
Compile and test your code. Does it work as you expected? Make changes until it does.
If your program didn’t work on the  rst try what were the problems? How did you  x it?
Write down the  nal version of your main loop here.
Did this code work? Yes or No (Circle your answer)
If so, demonstrate the working code to your TA/lecturer who will sign your script. 5 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 87
(c) Strings can also be stored in RAM. Write a routine that will take the following 16-byte
string:
MyStrInit addwf PCL,F
DT "The value is: ",0
and copy it using indirect addressing to locations starting from MyStr. You should
de ne the 16 spaces for MyStr using the CBLOCK directive.
Alter the main loop so that it will convert the number stored in a register num into the
ASCII equivalent of the hexadecimal number, and store it in MyStr at positions 13 and
14, before displaying MyStr on the LCD display.
(Hint: Use the lookup_nibble table to translate each nibble of num separately.)
Initialise num with a value.
Compile and test your code. Does it work as you expected? Make changes until it does.
If your program didn’t work on the  rst try what were the problems? How did you  x it?
Write down the  nal version of your main loop here.
Did this code work? Yes or No (Circle your answer)
If so, demonstrate the working code to your TA/lecturer who will sign your script. 10 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 88
(d) Now alter the main loop so that:
 the motor runs at a speed determined by the value written to the PSP from the
PC,
 Timer0 is con gured as a counter, whose value is continuously copied to num for
display.
As the motor turns we should see the LCD displaying the number of encoder pulses
counted by Timer0.
Compile and test your code. Does it work as you expected? Make changes until it does.
If your program didn’t work on the  rst try what were the problems? How did you  x it?
Write down the  nal version of your main loop here.
Did this code work? Yes or No (Circle your answer)
If so, demonstrate the working code to your TA/lecturer who will sign your script. 16 marks
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 89
7. We would like to write a program for the PIC16F877, which communicates with the PC over
an RS232 link, instead of across the PSP.
Disconnect the parallel cable and connect the serial cable.
Firstly, let us locate all the serial ports on the computer that you are using. Look at the back
of the computer. How many serial ports do you see?
(N.B. remember that the computer is DTE, so you are looking for male D-type connectors,
either 9 pin or 25 pin.)
The computer \labels" it’s serial ports COM1, COM2, COM3, COM4 etc Which serial port
are you connected to?
The circuit you have been provided with has a PIC in a header, and an RS232 driver IC.
What is the IC part #?
You have already located the registers of the USART peripheral in the PIC (as part of
your pre-lab exercises). We will start with a simple program which polls for a character,
and then echoes the character received to the transmit register. We will use the following
communication settings:
 2400 baud
 8 data bits
 no parity bits
 no  ow control
 1 stop bit
The structure of the program will be:
con gure and control
loop:
wait for state
read data
write data
wait for state
goto loop
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 90
Fill in appropriate settings, and register/ ag names in the code below. 7 marks
LIST P=16F877
INCLUDE <P16F877.inc>
ORG 0x00
nop
goto main
ORG 0x10
main call UART_Cnfg
goto UART_poll
UART_Cnfg bsf STATUS,RP0 ; Go to Bank1
movlw __________ ; Set Baud rate to 2400
movwf SPBRG
movlw __________ ; 8-bit transmit, transmitter enabled,
movwf TXSTA ; asynchronous mode, low speed mode
bcf STATUS,RP0 ; Go to Bank 0
movlw __________ ; 8-bit receive, receiver enabled,
movwf RCSTA ; serial port enabled
return
UART_poll clrw
loop btfss ______,RCIF; wait on char
goto loop
nchar movf _________,W; transmit what you rx (NB reading clears the flag)
movwf ___________
wait btfss ______,TXIF; flag clears when character is gone from buffer
goto wait
goto loop
sleep
END
Start up MPLAB, and create a new project and  le for the code. Compile your code, and
make sure there are no errors. Power up your circuit, and download the code.
Start Hyperterminal (as per helpsheet) and choose the required communication settings.
Connect to the communication channel, and then run the program on the PIC16F877.
What happens when you type in the Hyperterminal window? Is it what you expected? If
not, why not, and/or what did you do to  x it?
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 91
What happens if you disconnect, change the communication settings, and then reconnect and
start typing? Is that also what you expected? If not, why not, and/or what did you do to  x
it?
Explain your observations. 4 marks
Change the settings in HyperTerminal back to their original values before pro-
ceeding.
8. We can modify the previous example, so that instead of just repeating what it received, the
microcontroller changes the content somehow.
Add the routine lookup_string (that you used earlier) between return and UART_poll.
Change the original program by inserting a call to your routine, on a new line just after the
nchar labelled line.
Compile your code, and make sure there are no errors. Download the code and test your
program by typing di erent characters in HyperTerminal. Does it work as you expected?
Make changes until it does.
If your program didn’t work on the  rst try what were the problems? How did you  x it?
Write down the  nal version of your routine here. 5 marks
Did this code work? Yes or No (Circle your answer)
If so, demonstrate the working code to your TA/lecturer who will sign your script.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 92
9. So far we have been polling. Let us convert the polling program to an interrupt driven one.
We can rewrite the main part of the code as:
con gure and control
loop:
{ do something else
goto loop
and then write an interrupt service routine, and handlers for the individual interrupts.
Let us start with the interrupt subroutine. Choose appropriate locations to store _W, _STATUS
and _PCLATH, so that the code will work in all banks. Justify your choices. 3 marks
_W EQU _______
_STATUS EQU _______
_PCLATH EQU _______
isr ; save all registers
movwf _W ; Copy W to TEMP register
swapf STATUS,W ; Swap status to be saved into W
clrf STATUS ; bank 0, clears IRP,RP1,RP0
movwf _STATUS ; Save status to bank zero STATUS_TEMP register
movf PCLATH, W ; Only required if using pages 1, 2 and/or 3
movwf _PCLATH ; Save PCLATH into W
clrf PCLATH ; Page zero, regardless of current page
; no need to clear flags as they change automatically for this device
btfsc PIR1, RCIF
goto rx_handler
rtn_isr movf _PCLATH, W ; Restore PCLATH
movwf PCLATH ; Move W into PCLATH
swapf _STATUS,W ; Swap STATUS_TEMP register into W
; (sets bank to original state)
movwf STATUS ; Move W into STATUS register
swapf _W,F ; Swap W_TEMP
swapf _W,W ; Swap W_TEMP into W
; all registers restored
retfie
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 93
Next we have the individual handlers. We would like to transmit as soon as we receive, so
the transmit handler can be empty for now.
rx_handler ; on rx, tx the next character
movf ________,W ; read the byte to clear the flag
movwf ___________
goto rtn_isr
Now for the associated main loop,  ll in the blanks in the following code . 2 marks
LIST P=16F877
INCLUDE <P16F877.inc>
ORG 0x00
nop
goto main
ORG 0x04
goto isr
ORG 0x10
main call UART_Cnfg
goto otherloop
UART_Cnfg bsf STATUS,RP0 ; Go to Bank1
movlw __________ ; Set Baud rate to 2400
movwf SPBRG
movlw __________ ; 8-bit transmit, transmitter enabled,
movwf TXSTA ; asynchronous mode, low speed mode
bsf PIE1, RCIE ; enable receive interrupts
bcf STATUS,RP0 ; Go to Bank 0
movlw __________ ; 8-bit receive, receiver enabled,
movwf RCSTA ; serial port enabled
bsf INTCON, ___; enable peripheral & general interrupts
bsf ______, PEIE
bsf RCSTA , CREN
return
otherloop goto otherloop
sleep
END
Type in all your code, compile and run it. Test your program by using HyperTerminal.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) March 11, 2008 VIII 94
Does it work as you expected? If not, why not, and/or what did you do to  x it?
Demonstrate the working code to your TA/lecturer who will sign your script. 4 marks
10. Previously, we adjusted motor speed from the PC using the PSP. Write a program which will
change motor speed based on a single digit typed in HyperTerminal, where ’0’means stop,
9 means full speed, and digits 1 through 8 adjust speed proportionally between no and full
speed.
Compile and test your code. Does it work as you expected? Make changes until it does.
If your program didn’t work on the  rst try what were the problems? How did you  x it?
Write down the  nal version of your main loop here.
Did this code work? Yes or No (Circle your answer)
If so, demonstrate the working code to your TA/lecturer who will sign your script. 10 marks
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11. Modify your program so that it in response to a single character, it sends the 3-character
string ‘OK\n\0’. You should use TX interrupts, and indirect addressing in a way which will
allow you to send any null-terminated string. 15 marks
Compile and test your code. Does it work as you expected? Make changes until it does.
If your program didn’t work on the  rst try what were the problems? How did you  x it?
Write down the  nal version of your main loop here.
Did this code work? Yes or No (Circle your answer)
If so, demonstrate the working code to your TA/lecturer who will sign your script.
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ECNG2006 (EE25M) March 11, 2008 VIII 96
12. Modfy your program to report the measured motor speed(Timer0) across the serial link when
the character S is typed in HyperTerminal. 20 marks
Compile and test your code. Does it work as you expected? Make changes until it does.
If your program didn’t work on the  rst try what were the problems? How did you  x it?
Write down the  nal version of your main loop here.
Did this code work? Yes or No (Circle your answer)
If so, demonstrate the working code to your TA/lecturer who will sign your script.
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13. Challenge: The pre-lab asked you to explain an algorithm for manipulating regularly sampled
data.
(a) Use either instruction-based delays or Timer1 (with/without interrupts) to regularly
sample the distance turned by the motor (Timer0 counts). 20 marks
(b) All results should be reported serially using RS232. 10 marks
(c) Bonus(30 marks): For the sampled data, implement the algorithm you explained in the
pre-lab.
Compile and test your code. Does it work as you expected? Make changes until it does.
If your program didn’t work on the  rst try what were the problems? How did you  x it?
Attach a printout of your  nal code.
Did this code work? Yes or No (Circle your answer)
If so, demonstrate the working code to your TA/lecturer who will sign/stamp your script/printout.
Total marks 300.
PLAIGIARISM DECLARATION:
For the purposes of this exercise, unauthorised collaboration is any form of collaboration which does NOT fall into one of the following categories:
 verbal or written discussion/clari cation of question and/or related concepts
 assistance in troubleshooting circuitry and/or using/operating the IDE/debugger
Department of Electrical and Computer Engineering
PLAGIARISM Plagiarism is the presentation by a student of an assignment which has in fact been copied in whole or in part from another student’s
work, or from any other source (e.g. published books or periodicals), without due acknowledgement in the text.
COLLUSION Collusion is the presentation by a student of an assignment as his or her own which is in fact the result in whole or part of unauthorised
collaboration with another person or persons.
DECLARATION I declare that this assignment is my own work and does not involve plagiarism or collusion. I have read and understood University
Examination Regulations 73,75,76 and 79 regarding cheating.
Signed: Date:
(Department of Electrical and Computer Engineering)
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Re ection & Feedback
 Indicate the objectives that you feel you have achieved in this lab exercise.
{ perform I/O programming on the Personal Computer (PC)
{ produce programs for communication between a microprocessor and a PC using the
parallel and/or serial port, given UART and/or PPC datasheets/details, and respective
language/instruction sets.
{ interface external peripherals/devices to the PIC16F877 microcontroller using a parallel
bus, and interface the PIC16F877 microcontroller as an external device on a parallel
bus (memory mapped).
 Which aspect of this lab exercise did you have the most di culty understanding?
 Which aspect of this lab exercise did you like best? Why did you like it?
 Identify one thing (if any) that you learned while doing this lab exercise.
 Identify one way in which this lab exercise could be improved.
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Communication
In this section we have looked at commercially available methods of interfacing peripherals with
microprocessors. In doing so, we have completed the third learning objective \design and implement
an appropriate interface between a microprocessor-based system and another microprocessor-based
system, given relevant data-sheets and suitable parts".
The course is now complete.
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ECNG2006 (EE25M) March 11, 2008 VIII 100
Final Feedback
This form should be  lled in together with the standard course evaluation form.
Please re-read all the course/unit objectives, and respond to the following questions.
At the end of this unit/course the student will be able to:
p Objective Description
A in general, or (given relevant diagrams and speci cations) for a particular
system, identify, and describe the role of, the components of a micropro-
cessor, a microprocessor-based system, a development suite (hardware, soft-
ware) for a microprocessor-based system.
B illustrate how data can be represented in (and accessed from) memory, im-
plement arithmetic and data manipulation operations in assembly language,
and assess how well code will perform in a given context, given the architec-
ture and instruction set of a microprocessor
C design and implement an appropriate interface between a microprocessor-
based system and a peripheral device (or another microprocessor-based sys-
tem), given relevant datasheets and suitable parts
D select (and critique the selection of) a microprocessor-based system for an
application, given relevant datasheets and application requirements
1 identify the components of a typical microprocessor (ALU, registers, stack),
describe the operation of the components of a typical microprocessor, de-
scribe the representation of instructions in the operation of the instruction
cycle, and illustrate how these may be implemented in digital logic.
2 identify the components of a microprocessor-based system (CPU, bus,
memory/layout, peripherals), describe the operation of typical components
of a microprocessor-based system, and discuss issues involved in the de-
sign/choice of the various components of the microprocessor(-based system)
3 describe (and/or illustrate) how bus addressing, con ict resolution and mem-
ory systems may be implemented using digital logic, and explain the function
of basic support circuitry for microprocessors such as clocks, reset circuitry,
watch dog timer, capacitors to ground etc. .
4 di erentiate between organization and architecture, and relate instruction
set design to the programmers model of the microprocessor,.
5 explain the operation of typical machine instructions, addressing modes,
status bits, and infer the consequences of sequences of generic instructions.
6 explain the roles of the compiler, linker, assembler, simulator, emulator, de-
bugger in the software development tool chain, and the role of scripting lan-
guages, make/con gure-like utilities and IDEs in the software development
process.
7 explain the role of  rmware, operating systems, oscilloscopes, logic
probes/analyzers, terminals, disk drives, external memory, debug protocols
e.g. JTAG and host computers in the development and support of micro-
processor based systems.
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ECNG2006 (EE25M) March 11, 2008 VIII 101
p Objective Description
8 recognize and di erentiate between the di erent commercially avail-
able families/forms of microprocessor based systems (Motorola, AVR,
PIC chip/component packages { SOC, microcontroller, PC, back-
plane bus, PC104, standalone, embedded) based on their pic-
tures/properties/descriptions and supported development tools.
9 explain the memory layout, operation of the instruction cycle, and the basic
instructions (move, add, subtract, shifts) for the PIC16F877 microcontroller.
10 utilize the MPLAB IDE and CCS compiler, to develop software, for the
MicroChip PIC16Cxxx series of microcontroller, in C, C++ and assembly
language.
11 represent real numbers using  xed and  oating point binary representations.
12 perform integer arithmetic operations using binary representations of the
operands.
13 perform  xed/ oating point arithmetic operations using binary representa-
tions of the operands.
14 describe how data (and data structures) can be represented and manipulated,
with reference to alignment and byte/bit ordering issues.
15 implement basic programming operations (loops, swaps, lookups), integer
arithmetic, and other computation/numerical methods, in assembly lan-
guage on the PIC16Cxxx series of microcontrollers.
16 contrast alternate programming techniques in terms of size/speed of the code
produced for the PIC16Cxxx series of microcontrollers.
17 give examples of the limitations placed on compiler-generated code, for the
PIC16Cxxx series of microcontrollers, by the need for generality.
18 identify alternate ways in which components may be interfaced with a mi-
croprocessor (i.e. common idioms for interacting with hardware): single bit
(with debounce), matrix, map into memory space, etc. .
19 explain the operation of interrupts, the di erent types of interrupts (h/w,
s/w), how interrupts may be prioritized (peripheral interrupt controller),
and identify common applications of interrupts.
20 explain the operation of commonly used peripherals: PPI/PIA, A/D,
Timers/Counters, PWM, D/A.
21 select and apply techniques for troubleshooting digital circuitry using stan-
dard laboratory equipment.
22 utilize built-in peripherals of the PIC16F877 microcontroller in simple ap-
plications.
23 understand the implications of (nested/multiple) interrupts for execution
times, and the need for context saving.
24 interpret and recognize the implications of, microprocessor timing and in-
terfacing requirements.
25 determine the noise, voltage, current and power considerations due to mi-
croprocessors and peripheral fan-in/out and how di erent logic families (as
well as inverted voltage logic) may be used in conjunction in a design.
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ECNG2006 (EE25M) March 11, 2008 VIII 102
p Objective Description
26 compare alternate microprocessors in terms of the instruction set, and fea-
tures(such as cache pipelining etc.) and the best performance, which may
be facilitated by the instruction set and features.
27 recognize the issues and tradeo s involved in the design of a microprocessor
based (embedded) system, and justify the role of a microprocessor(-based
system) in an applied context.
28 remember and follow a checklist of design guidelines, in order to perform a
simple design for a microprocessor based system.
29 understand commonly used parallel and serial communication protocols
(I2C, CAN, RS232, USB, Firewire) and interpret descriptions of proprietary
protocols (Dallas 1-wire).
30 di erentiate between bit-banging and dedicated hardware interfaces for se-
rial/parallel communication (PPI/PIA vs ACIA/UART, PPC)
31 perform I/O programming on the PC in C/Assembly using the Visual C++
compiler
32 produce assembly language programs for communication between a micro-
processor and a PC using the parallel and/or serial port, given UART and
PPC datasheets/details, and respective instruction sets.
33 interface external peripherals/devices to the PIC16F877 microcontroller us-
ing a parallel bus, and interface the PIC16F877 microcontroller as an exter-
nal device on a parallel bus (memory mapped).
 Do you think that you have achieved all four of the course objectives? (Yes/No)
 Tick the objectives you feel that you have achieved, and identify the objectives you have not
achieved, by writing NA next to them.
 Identify the three learning objectives which you had the most di culty in achieving, by
writing D next to them.
 Identify the three learning objectives which you liked best, by writing B next to them.
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 Do you have any comments/suggestions regarding the order in which material was presented
in this course?
 Do you have any comments/suggestions regarding the depth with which material was covered
in this course?
 Do you have any comments/suggestions regarding the content covered in this course?
 Do you feel that you were adequately prepared to take this course, and if not, why not?
 Do you feel that this course has adequately prepared you to take  nal year courses? Please
give a brief explanation of your answer.
c DECE, UWI, St. Augustine, Trinidad
ECNG2006 (EE25M) Introduction to microprocessors
c DECE, UWI, St. Augustine, Trinidad
original author: Feisal Mohammed; updated: March 11, 2008{ CLR
Part IX
Appendices
A Data Sheets
Selected pages from documents listed below are included in the data-sheets used in this course. **
documents do not appear in the electronic copy
 A Comparison of Low End 8 Bit Microcontrollers, M. Palmer, Microchip Application Note
520, 1997. pp 1-9.
 PIC16F87x 20/40 Pin 8-bit CMOS FLASH Microcontrollers, Microchip Data Sheet DS30292C,
2001. pp 1, 6, 8, 9, 11-13, 15-17, 26-27, 29-39, 41-49, 51-63, 95-102, 111-117, 129-133, 135-142,
149, 159-163, 165-167, 173, 175.
 Chapter 6 from Programming and Customizing PICmicroR Microcontrollers, M. Predko, 2001.
 HD44780U (LCD-II) (Dot Matrix Liquid Crystal Display Controller/Driver), Hitachi datasheet,
pp. 24,25,33,46,52,58.
 KS0066U 16COM / 40SEG DRIVER & CONTROLLER FOR DOT MATRIX LCD, Sam-
sung Electronics datasheet,pp. 1-2,18-19,26-27, 31-32
 8XC51FX CHMOS SINGLE-CHIP 8-BIT MICROCONTROLLERS, Intel Preliminary DataSheet,
April 1996. pp 1, extract from 2-3, extract from 8-9, 4-5
 **80C51 External Memory Interfacing, Phillips Semiconductors Application Note AN457,
May 1996. p 2
 8051 Compact Assembly Reference from http://www.stack.nl/~wvengen/uni/elec/8051ref.pdf,
page 1
 **Converting from 8051 to Microchip Assembler: A Quick Reference, G. Kavaiya, Microchip
Application Note 880, 2003. pp 1-8.
 ATMEL AT91 Electrical Characteristics M55800A, pp 1,2,3,11
 ATMEL AT91 Summary M55800A, p. 9
 ARM 7 Datasheet Document Number: ARM DDI 0020C Issued: Dec 1994 , pp 13-15
 ARM7TDMI Datasheet Document Number: ARM DDI 0084D Issued: Chapter 4 Arm In-
struction Sept. pp. 2-4
 Z8 CPU User Manual UM001602-0904, ZiLOG Sept. 2004, pp.1-3, 7-9, 201-204
ECNG2006 (EE25M) March 11, 2008 IX 2
 **MC68HC705C8A Rev. 2.1 Technical Data, 2001. pp 22,25,35,36,38,44-48,57-58,154,164-
170
 Micro-reader: RI-STU-MRD1 Reference Manual, Texas Instruments Series 2000 Reader Sys-
tem, May 2000. pp 6,8,15-19.
 MicroLAN Design Guide,Tech Brief 1,Dallas Semiconductor. pp1-2
 Intel Hexadecimal Object File Format Speci cation,Intel, Revision A, 1988. pp 1-10.
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B Glossary
Where de nitions have been taken from other documents, the references are cited.
access time The average time interval between a storage peripheral (usually a disk drive or semi-
conductor memory) receiving a request to read or write a certain location and returning the
value read or completing the write. (FOL 1993)
accumulator The name of the CPU register in a single-address instruction format. The accumu-
lator, or AC, is implicitly one of the two operands for the instruction.(Stallings 2000) Also
known as the working register.
addressing modes One of a set of methods for specifying the operand(s) for a machine code in-
struction. Di erent processors vary greatly in the number of addressing modes they provide.
The more complex modes described below can usually be replaced with a short sequence of
instructions using only simpler modes.
The most common modes are "register" - the operand is stored in a speci ed register; "ab-
solute"(direct) - the operand is stored at a speci ed memory address; and "immediate" - the
operand is contained within the instruction.
Most processors also have indirect addressing modes, e.g. "register indirect", "memory in-
direct" where the speci ed register or memory location does not contain the operand but
contains its address, known as the "e ective address". For an absolute (direct) addressing
mode, the e ective address is contained within the instruction. (FOL 1993)
ALU Arithmetic and Logic Unit. A part of a CPU that performs arithmetic operations, logic
operations and associated operations.(Stallings 2000)
analog-to-digital converter
anti-aliasing
arbiter
architecture
arithmetic shift
ASCII American Standard for Information Interchange. ASCII is a 7-bit code used to repre-
sent numeric, alphabetic, and special printable characters. It also includes codes for control
characters which are not printed or displayed but specify some control function.(Stallings
2000)
assembler A language tool that translates a users assembly source code (.asm) into machine
code.(MPL 2000)
assembly language A programming language that is once removed from machine language. Ma-
chine languages consist entirely of numbers and are almost impossible for humans to read and
write. Assembly languages have the same structure and set of commands as machine lan-
guages, but they enable a programmer to use names (mnemonics) instead of numbers.(MPL
2000)
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ECNG2006 (EE25M) March 11, 2008 IX 4
assert
asynchronous bus timing A technique in which the occurrence of one event on a bus follows
and depends upon the occurrence of a previous event.(Stallings 2000)
Automation
background debug
balanced
banking/paging
base/radix In a positional representation of numbers, that integer by which the signi cance of
one digit place must be multiplied to give the signi cance of the next higher digit place.
Conventional decimal numbers are radix ten, binary numbers are radix two.(FOL 1993)
BCD Binary Coded Decimal.
benchmark A standard program or set of programs which can be run on di erent computers to
give an inaccurate measure of their performance.
"In the computer industry, there are three kinds of lies: lies, damn lies, and benchmarks."
A benchmark may attempt to indicate the overall power of a system by including a "typical"
mixture of programs or it may attempt to measure more speci c aspects of performance,
like graphics, I/O or computation (integer or  oating-point). Others measure speci c tasks
like rendering polygons, reading and writing  les or performing operations on matrices. The
most useful kind of benchmark is one which is tailored to a user’s own typical tasks. While
no one benchmark can fully characterise overall system performance, the results of a variety
of realistic benchmarks can give valuable insight into expected real performance.
Benchmarks should be carefully interpreted, you should know exactly which benchmark was
run (name, version); exactly what con guration was it run on (CPU, memory, compiler
options, single user/multi-user, peripherals, network); how does the benchmark relate to your
workload? (FOL 1993)
biased
binary
bit mask A pattern of binary values which is combined with some value using bitwise AND with
the result that bits in the value in positions where the mask is zero are also set to zero.(FOL
1993)
bit In the pure binary numeration system, either 0 or 1.
bit-parallel
bit-serial
bit-width
Booth’s algorithm
boot To load and initialise the operating system on a computer. (FOL 1993)
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boot-loader (from "bootstrap" or "to pull oneself up by one’s bootstraps") A short program that
was read in from cards or paper tape, or toggled in from the front panel switches, which read
in a more complex program to which it gave control.
On early computers the bootstrap loader was always very short (great e orts were expended
on making it short in order to minimise the labour and chance of error involved in toggling
it in), but was just smart enough to read in a slightly more complex program (usually from a
card or paper tape reader), to which it handed control; this program in turn was smart enough
to read the application or operating system from a magnetic tape drive or disk drive. Thus, in
successive steps, the computer "pulled itself up by its bootstraps" to a useful operating state.
Nowadays the bootstrap is usually found in ROM or EPROM, and reads the  rst stage in
from a  xed location on the disk, called the "boot block". When this program gains control,
it is powerful enough to load the actual OS and hand control over to it.(FOL 1993)
branch prediction A mechanism used by the processor to predict the outcome of a program
branch prior to it’s execution.(Stallings 2000)
breakpoint In hardware, an event whose execution will cause a halt. In software, an address
where execution of the  rmware will halt.(MPL 2000)
bu ered
bug An unwanted and unintended property of a program or piece of hardware, especially one
that causes it to malfunction. The identi cation and removal of bugs in a program is called
"debugging".(FOL 1993)
built-in hardware support for debugging
bus arbitration The process of determining which competing bus master will be permitted access
to the bus.(Stallings 2000)
bus cycle
bus A shared communications path consisting of one or a collection of lines. In some computer
systems, CPU, memory, and I/O components are connected by a common bus. Since the
lines are shared by all the components, only one component at a time can successfully trans-
mit.(Stallings 2000)
byte (a collective term for) 8 bits.
CCS
cell
CISC Complex Instruction Set Computer. Inspired by a desire to improve performance by re-
ducing the number of instruction fetches, CISC processers feature rich instruction sets, and
smaller program sizes.
clock (cycle) (period) The relative execution times of instructions on a computer are usually
measured by number of clock cycles rather than seconds. One good reason for this is that
clock rates for various models of the computer may increase as technology improves, and it is
usually the relative times one is interested in when discussing the instruction set.(FOL 1993)
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clock speed The fundamental rate in cycles per second at which a computer performs its most
basic operations such as adding two numbers or transfering a value from one register to
another.(FOL 1993)
CMOS
Communication protocols A set of formal rules describing how to transmit data, especially
across a network. Low level protocols de ne the electrical and physical standards to be
observed, bit- and byte-ordering and the transmission and error detection and correction of
the bit stream. High level protocols deal with the data formatting, including the syntax of
messages, the terminal to computer dialogue, character sets, sequencing of messages etc.(FOL
1993)
compiler A language tool that translates a users C source code into machine code.(MPL 2000)
contiguous
control characters
control signals
counter mode
Counter/Timer
CPU bound
CPU Central Processing Unit. The portion of a computer that fetches and executes instructions.
It consists of an arithmetic and logic unit (ALU), a control unit, and registers. Often simply
referred to as a processor.(Stallings 2000)
CU Control Unit. That part of the CPU that controls CPU operations, including ALU operations,
the movement of data within the CPU, and the exchange of data and control signals across
external interfaces (e.g. the system bus).(Stallings 2000)
cycle stealing
data
data-word
debug kernel
debugger
decouple
decrement decrease by 1 unit.
density
Development boards
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ECNG2006 (EE25M) March 11, 2008 IX 7
di erential line (signal) A kind of electrical connection using two wires, one of which carries
the normal signal (V) and the other carries an inverted version the signal (-V). A di erential
ampli er at the receiver subtracts the inverted signal from the normal signal to yield a signal
proportional to V. This subtraction is intended to cancel out any noise induced in the wires,
on the assmption that the same level of noise will have been induced in both wires. Twisted
pair wiring is often used to try to ensure that this is the case.(FOL 1993)
digit In the decimal system, 0,1,2,3,4,5,6,7,8,9. Often used to refer to the numbers of other numeric
bases e.g. hexadecimal digits, binary digits.
digital-to-analog converter
directive
disassembling
discrete event simulator
DMA Direct Memory Access. A form of I/O in which a special module, called a DMA module,
controls the exchange of data between main memory and an I/O module. The CPU sends a
request for the transfer of a block of data to the DMA module, and is interrupted only after
the entire block has been transferred.(Stallings 2000)
EEPROM Electrically Erasable Programmable Read Only Memory. A special type of PROM
that can be erased electrically. Data is written or erased one byte at a time. EEPROM
retains its contents even when power is turned o .
emulator Hardware that performs the process of executing software (loaded into emulation mem-
ory) as if the  rmware resided on the microcontroller device under development.(MPL 2000)
enabled
EPROM
executable
exponent
fan- in/out
fetch
 le register Register within the register  le.
 le storage system
 rmware
 xed point representation system. A radix numeration system in which the radix point is im-
plicitly  xed in the series of digit places by some convention upon which agreement has been
reached.(Stallings 2000)
 ags (Z,C,DC, and others??)
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ECNG2006 (EE25M) March 11, 2008 IX 8
Flash A type of EEPROM where data is written or erased in blocks instead of bytes.
 oating point representation system. A numeration system in which a real number is represented
by a pair of distinct numerals, the real number being the product of the  xed point part,
one of the numerals, and a value obtained by raising the implicit  oating point base to a
power denoted by the exponent in the  oating point representation, indicated by the second
numeral.(Stallings 2000)
full-duplex A type of duplex communications channel which carries data in both directions at
once. (FOL 1993)
general purpose register
Ground bounce
half-duplex A type of communication channel using a single circuit which can carry data in either
direction but not both directions at once.(FOL 1993)
Harvard architecture A computer architecture in which program instructions are stored in dif-
ferent memory from data. Each type of memory is accessed via a separate bus, allowing
instructions and data to be fetched in parallel.(FOL 1993)
hexadecimal
hold time
host machine The machine on which the IDE, compiler, assembler and other development tools
are run to produce the hex  le for the target machine.
I/O bound
I/O module One of the major components of a computer. It is responsible for the control of one
or more external devices (peripherals) and for the exchange of data between those devices
and main memory and/or CPU registers.(Stallings 2000)
IDE An application that has multiple functions for  rmware development. The MPLAB IDE
integrates a compiler, an assembler, a project manager, an editor, a debugger, a simulator,
and an MPLAB IDE Users Guide assortment of other tools within one Windows application.
A user developing an application can write code, compile, debug, and test an application
without leaving the MPLAB IDE desktop.(MPL 2000)
in-circuit debug
in-circuit programming PIC16F87X microcontrollers can be serially programmed while in the
end application circuit. This is simply done with two lines for clock and data and three other
lines for power, ground, and the programming voltage. This allows customers to manufac-
ture boards with unprogrammed devices, and then program the microcontroller just before
shipping the product. This also allows the most recent  rmware, or a custom  rmware to be
programmed. (PIC 2001)
in-circuit
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ECNG2006 (EE25M) March 11, 2008 IX 9
increment increase by 1 unit.
indirect (decode)
instruction cycle { 5 stages
instruction format  xed/var
instruction width/length  xed/var
instruction register A register that is used to hold an instruction for interpretation.
interrupt A suspension of a process, such as the execution of a computer program, caused by
an event external to that process, and performed in such a way that the process can be
resumed.(ATI 2001)
interrupt-driven
IR see instruction register.
isolated I/O A method of addressing I/O modules and external devices. The I/O address space
is treated separately from main memory address space. Speci c I/O machine instructions
must be used. Compare memory-mapped I/O.(Stallings 2000)
ISR
JTAG
jump/goto/branch
kernel
level translators/shifters
library
line coding
linker scripts
linker A language tool that combines object  les and libraries to create executable code.(MPL
2000)
load-and-store
logic analyser
logic high
logic low
logical shift
bitwise-and
bitwise-or
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ECNG2006 (EE25M) March 11, 2008 IX 10
bitwise-xor
machine cycle The (minimum) time it takes to fetch two operands from registers, perform an
ALU operation, and store the result in a register.(Stallings 2000)
machine instruction
make le
mantissa For a real number x, the mantissa is de ned as the positive fractional part.(WOL) The
part of a  oating point number which, when multiplied by its radix raised to the power of
its exponent, gives its value. The mantissa may include the number’s sign or this may be
considered to be a separate part.(REF)
map
masked/unmasked; set/reset; set/clear
mechanical connection
memory mapped I/O A method of addressing I/O modules and external devices. A single
address space is used for both main memory and I/O addresses, and the same machine
instructions are used both for memory read/write and for I/O.(Stallings 2000)
micro-code
microcontroller A highly integrated chip that contains all the components comprising a con-
troller. Typically this includes a CPU, RAM, some form of ROM, I/O ports, and timers.
Unlike a general-purpose computer, which also includes all of these components, a microcon-
troller is designed for a very speci c task - to control a particular system. As a result, the
parts can be simpli ed and reduced, which cuts down on production costs.(MPL 2000)
micro-operation an elementary CPU operation, performed during 1 clock pulse.(Stallings 2000)
microprocessor A processor whose elements have been miniaturized into one or a few integrated
circuits.(Stallings 2000)
mnemonics Instructions that are translated directly into machine code. Mnemonics are used to
perform arithmetic and logical operations on data residing in program or data memory of
a microcontroller. They can also move data in and out of registers and memory as well as
change the  ow of program execution. Also referred to as Opcodes.(MPL 2000)
MPLAB
multiplex (time)
network
nibble (a collective term for) 4 bits.
object  les
opcode Operation Code. A code used to represent the operations of a computer.(Stallings 2000)
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ECNG2006 (EE25M) March 11, 2008 IX 11
open drain output
open-collector
open-drain (output)
operand An entity on which an operation is performed.(Stallings 2000)
operating system
organization
oscilloscope
packed BCD
pad
parity
peripheral In a computer system, with respect to a particular processing unit, any equipment
that provides the processing unit with outside communication.(Stallings 2000)
PIC Peripheral Interrupt Controller
PIC16F877
pipeline branch prediction
pipeline a processor organization in which the processor consists of a number of stages, allowing
multiple instructions to be executed concurrently.(Stallings 2000)
point-to-point
polling
Power conditioning
Power supply supervision
preprocessor A program invoked by various compilers to process code before compilation. For
example, the C preprocessor, cpp, handles textual macro substitution, conditional compilation
and inclusion of other  les. (FOL 1993)
priority
processor
program branch
program counter PC. A special purpose register used to hold the address of the next instruction
to be executed.(Stallings 2000) Also known as the instruction address register (IAR).
program The noun "program" describes a single, complete and more-or-less self-contained list of
instructions, often stored in a single  le.(FOL 1993)
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ECNG2006 (EE25M) March 11, 2008 IX 12
project  le
project nodes
PROM
push-pull (output)
PWM
RAM Random Access Memory. Memory in which each addressable location has a unique address-
ing mechanism. The time to access a given location is independent of the sequence of prior
access.(Stallings 2000) Variants: static/dynamic.
register  le Collection of registers within the CPU. Also used to refer to RAM within a micro-
controller which can be accessed in a register-like fashion.
register(s) High speed memory internal to the CPU. Some registers are user visible; that is
available to the programmer via the machine instruction set. Other registers are used only
by the CPU, for control purposes.(Stallings 2000)
relocatable code A section of code whose address is not  xed (absolute). The linker assigns
addresses to relocatable sections through a process called relocation. Relocation is a process
in which absolute addresses are assigned to relocatable sections and all identi er symbol
de nitions within the relocatable sections are updated to their new addresses.(MPL 2000)
Requirements List
Reuse
RISC Reduced Instruction Set Computer(ing). Generally characterised by a large number of
general purpose registers, a limited, simple instruction set and an emphasis on optimizing the
instruction pipeline.(Stallings 2000)
ROM Read-Only Memory. Semiconductor memory whose contents cannot be altered, except
by destroying the storage unit. Non-erasable memory.(Stallings 2000) Variants: EEPROM,
Flash.
rotate
Schmitt trigger
setup time
shield
sign extension/reduction In order to represent a two’s complement number using more/less
bits, it is necessary to consider the sign bit. For example the number negative 8 may be
expressed as 1000 in 4 bits, 1111 1000 in 8 bits or 1111 1111 1111 1000 in 16 bits. The
uppermost bit is extended to  ll all empty bits. Similarly to reduce the number of bits, all
bits may be removed up until there is a change. For example 0000 0111 may be reduced to
0111, and no further.
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ECNG2006 (EE25M) March 11, 2008 IX 13
signal ground
signi cand
simplex
simulator A software program that models the operation of a microprocessor.(MPL 2000)
single-ended
single-phrase function statement
special function register
stack { di erent uses
static branch prediction
static RAM
step The Single Step command steps though code, one instruction at a time. After each instruc-
tion, MPLAB IDE updates register windows, watch variables, and status displays so you can
analyze and debug instruction execution.(MPL 2000)
stimulus Data generated to exercise the response of simulation to external signals. Often the
data is put into the form of a list of actions in a text  le. Stimulus may be asynchronous,
synchronous (pin), clocked and register. (MPL 2000)
subroutine/call
synchronous bus timing A technique in which the occurrence of events on a bus is determined
by a clock. The clock de nes equal-width time slots, and events begin only at the beginning
of a time slot.(Stallings 2000)
synchronous
target (machine) The machine on which the hex  le (produced by a compiler, assembler or other
development tools) is intended to run.
ten’s complement
terminal
time multiplexed
timer mode
timing diagrams
tool chain
trace
transducer A device for converting sound, temperature, pressure, light or other signals to or from
an electronic signal.(FOL 1993)
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ECNG2006 (EE25M) March 11, 2008 IX 14
transition
tri-state (output)
two’s complement Used to represent binary integers. A positive integer is represented as the
unsigned binary number. A negative number is represented by the two’s complement of
the positive number. The two’s complement may be found by adding one to the comple-
ment(inverted bits) of the positive number.(Stallings 2000)
unbalanced
unbu ered
uni/bi-directional
universal or chip programmer
vectored
Veri cation
visible to the programmer
volatile
von Neumann architecture A computer architecture conceived by mathematician John von
Neumann, which forms the core of nearly every computer system in use today (regardless of
size). In contrast to a Turing machine, a von Neumann machine has a random-access memory
(RAM) which means that each successive operation can read or write any memory location,
independent of the location accessed by the previous operation.
A von Neumann machine also has a central processing unit (CPU) with one or more registers
that hold data that are being operated on. The CPU has a set of built-in operations (its
instruction set) that is far richer than with the Turing machine, e.g. adding two binary
integers, or branching to another part of a program if the binary integer in some register is
equal to zero (conditional branch).
The CPU can interpret the contents of memory either as instructions or as data according to
the fetch-execute cycle.(FOL 1993)
Watch Watch Variables are variables that you may monitor during a debugging session in a watch
window. Watch windows contain a list of watch variables that are updated at each break
point.(MPL 2000)
Watchdog timers
working register see accumulator.
write
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C Study tips
The solutions should be consulted only AFTER you have attempted the review exercises.
If you read the solutions without attempting the questions, you will have LOST the opportunity to
assess/re ect on your performance, and make an impact on your memory (i.e. you’ll forget faster).
One technique for making best use of solutions is to perform an error analysis. For each question
you get wrong, ask yourself the following questions: (Hartman 2001)
1. What answer did I have? AND What was the answer really?
OR
What did I do wrong? AND What should I have done?
2. Why did I choose the wrong answer?
OR
Why did I do it wrong?
3. How will I remember what I now know is the correct answer?
OR
How will I make sure I don’t make the same mistake again?
Try to focus on the speci c content involved in the errors, rather than on general causes like not
studying enough, or feting the night before.
Another study technique from (Hartman 2001):
When reviewing your notes, or reading any reference books, ask yourself:
1.  What do I already know about this topic?
 Is there anything I don’t understand?
 Did I understand and remember everything?
2.  What am I expected to learn from this reading?
 Can I  gure it out on my own?
 Which ideas are most important?
3.  How much time should it take me to read this?
 How can I remember what I have read so far?
 How can I read with better understanding next time?
If you are working/studying in pairs or groups, you may wish to try the Think Aloud technique
while attempting the questions/projects. This technique involves one person (thinker) saying all
their thoughts, feelings etc. out loud, as they solve the problem (no swear words please!). The
listener(s) then pay(s) attention to what the thinker says, examines the accuracy, points out errors,
and keep(s) the thinker talking aloud.
Any people/groups who are interested can pass by my o ce or access
http://www.ccny.cuny.edu/ctl/handbook/hartman.html
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ECNG2006 (EE25M) March 11, 2008 IX 16
to get a copy of the guidelines for using the Think Aloud technique. This technique may help
you to discover, \errors, misconceptions, disorganization, and other impediments to intellectual
performance"(Hartman 2001).
The Think Aloud technique can also be useful if you are studying alone, but don’t be surprised if
passers-by give you strange looks!
Happy studying!
c DECE, UWI, St. Augustine, Trinidad