1. 16.317 Microprocessor Systems Design I
Instructor: Dr. Michael Geiger
Summer 2012
Lecture 10
PIC16F684 programming examples
Exam 3 Preview

2. Microprocessors I: Lecture 10
Lecture outline
Announcements/reminders
Lab 4 due today
HW 3 due Friday, 8/10
Do not do problem 2j (JL instruction)
Turn in to my office by 1:45, or
Exam 3: Monday, 8/13
Exam 2 regrade requests also due that day
Lab 5 due Tuesday, 8/14
Same turn-in procedure as HW 3
Today’s lecture
PIC programming examples
Exam 3 Preview
10/27/2017
Microprocessors I: Lecture 10

3. Review: PIC instructions
Logical operations
andlw/andwf
iorlw/iorwf
xorlw/xorwf
Rotates
rrf
rlf
Jumps/calls/return
goto
call
return/retlw/retfie
Miscellaneous
nop
sleep/clrwdt
Conditional execution
Test bit and skip next instruction if clear/set: btfsc/btfss
Increment/decrement register and skip next instruction if zero: incfsz/decfsz
Example use: combined with goto to create conditional jump
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Microprocessors I: Lecture 10

4. Microprocessors I: Lecture 10
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Working with 16-bit data
Assume a 16-bit counter, the upper byte of the counter is called COUNTH and the lower byte is called COUNTL. Decrement a 16-bit counter movf COUNTL, F ; Set Z if lower byte == 0 btfsc STATUS, Z decf COUNTH, F ; if so, decrement COUNTH decf COUNTL, F ; in either case decrement COUNTL Test a 16-bit variable for zero btfsc STATUS, Z ; If not, then done testing movf COUNTH, F ; Set Z if upper byte == 0 btfsc STATUS, Z ; if not, then done goto BothZero ; branch if 16-bit variable == 0 CarryOn
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Microprocessors I: Lecture 10
Chapter 9

5. Microprocessors I: Lecture 10
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A Delay Subroutine
; ***********************************************************************************
; TenMs subroutine and its call inserts a delay of exactly ten milliseconds
; into the execution of code.
; It assumes a 4 MHz crystal clock. One instruction cycle = 4 * Tosc.
; TenMsH equ 13 ; Initial value of TenMs Subroutine's counter
; TenMsL equ 250
; COUNTH and COUNTL are two variables
TenMs
nop ; one cycle
movlw TenMsH ; Initialize COUNT
movwf COUNTH
movlw TenMsL
movwf COUNTL
Ten_1
decfsz COUNTL,F ; Inner loop
goto Ten_1
decfsz COUNTH,F ; Outer loop
goto Ten_1
return
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Microprocessors I: Lecture 10
Chapter 9

6. Microprocessors I: Lecture 10
Blinking LED example
Assume three LEDs (Green, Yellow, Red) are attached to Port D bit 0, 1 and 2. Write a program for the PIC16F874 that toggles the three LEDs every half second in sequence: green, yellow, red, green, …. For this example, assume that the system clock is 4MHz.
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Microprocessors I: Lecture 10

7. Microprocessors I: Lecture 10
Top Level Flowchart
Initialize: Initialize port D, initialize the counter for 500ms.
Blink: Toggle the LED in sequence, green, yellow, red, green, …. Which LED to be toggled is determined by the previous state.
Wait for 500ms: Keep the LED on for 500ms and then toggle the next one.
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Microprocessors I: Lecture 10

8. Microprocessors I: Lecture 10
Strategy to “Blink”
The LEDs are toggled in sequence - green, yellow, red, green, yellow, red…
Let’s look at the lower three bits of PORTD
001=green, 010=yellow, 100=red
The next LED to be toggled is determined by the current LED.
001->010->100->001->…
10/27/2017
Microprocessors I: Lecture 10

9. Microprocessors I: Lecture 10
“Blink” Subroutine
Blink
btfsc PORTD, 0 ; is it Green?
goto toggle1 ; yes, goto toggle1
btfsc PORTD, 1 ; else is it Yellow?
goto toggle2 ; yes, goto toggle2
;toggle0
bcf PORTD, 2 ; otherwise, must be red, change to green
bsf PORTD, 0 ; 100->001
return
toggle1
bcf PORTD, 0 ; change from green to yellow
bsf PORTD, 1 ; 001->010
toggle2
bcf PORTD, 1 ; change from yellow to red
bsf PORTD, 2 ; 010->100
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Microprocessors I: Lecture 10

10. Another way to code “Blink” ---- Table Use
movf PORTD, W ; Copy present state of LEDs into W
andlw B' ' ; and keep only LED bits
addwf PCL,F ; Change PC with PCLATH and offset in W
retlw B' ' ; (000 -> 001) reinitialize to green
retlw B' ' ; (001 -> 010) green to yellow
retlw B' ' ; (010 -> 100) yellow to red
retlw B' ' ; (011 -> 001) reinitialize to green
retlw B' ' ; (100 -> 001) red to green
retlw B' ' ; (101 -> 001) reinitialize to green
retlw B' ' ; (110 -> 001) reinitialize to green
retlw B' ' ; (111 -> 001) reinitialize to green
In calling program
call BlinkTable ; get bits to change into W
xorwf PORTD, F ; toggle them into PORTD
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Microprocessors I: Lecture 10

11. Lab 5 Intro: Stepper motors
Magnet attached to shaft
Current through coil  magnetic field
Reverse current  reverse field
Pair of coils used to attract magnet to one of 4 different directions
Unipolar stepper motor: center taps on coil make current reversal easy
Microcontroller can activate drive transistors
Major differences between uni-polar and bi-polar.
Uni-polar
has logically two windings per phase, so a magnetic pole can be reversed without switching the direction of current,
A microcontroller or stepper motor controller can be used to activate the drive transistors in the right order,
Less torque
bi-polar
has logically a single winding per phase, so current in a winding needs to be reversed in order to reverse a magnetic pole,
driving circuit must be more complicated, typically with an H-bridge arrangement
More torque
Magnet rotor
coil
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Microprocessors I: Lecture 10

12. How Bi-polar Stepper Motor Works
More torque than unipolar motor
Similar principle, but no center taps
Need glue circuitry (use H-bridge)
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Microprocessors I: Lecture 10

13. Sequences (1 = phase activated)
Table of Stepping Sequences
Sequences (1 = phase activated)
Sequence
Polarity
Name
Description
Wave Drive, One-Phase
Consumes the least power. Only one phase is energized at a time. Assures positional accuracy regardless of any winding imbalance in the motor.
Hi-Torque, Two-Phase
Hi Torque - This sequence energizes two adjacent phases, which offers an improved torque-speed product and greater holding torque.
Half-Step
Half Step - Effectively doubles the stepping resolution of the motor, but the torque is not uniform for each step.  (Since we are effectively switching between Wave Drive and Hi-Torque with each step, torque alternates each step.)  Note that this sequence is 8 steps.
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Microprocessors I: Lecture 10

14. Microprocessors I: Lecture 10
The Schematic
Because of power requirements, induction of the windings, and temperature management, motors cannot be directly powered by most digital controllers. Some "glue circuitry," such as a motor controller (H-bridge) is necessary between digital controller and motor. The above image shows the basic circuit of a motor controller which can also sense motor current. (One wire of the motor is shown; a stepper motor would require such a circuit for four wires, and a normal DC motor for two. This circuitry is typically all included in an integrated H-bridge chip.
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Microprocessors I: Lecture 10

15. Our energization pattern
Step
Up-down Coil
East-West Coil 1
South
Off 2 3 4
North 5 6 7 8
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Microprocessors I: Lecture 10

16. Microprocessors I: Lecture 10
Our control sequence
RC5
RC4
RC3
RC2 1
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17. Microprocessors I: Lecture 10
Sequence 0111
OFF 1 1 1
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Microprocessors I: Lecture 10

18. Microprocessors I: Lecture 10
Sequence 0101 1 1
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Microprocessors I: Lecture 10

19. The code (comments, directives)
title "asmStepper - PIC16F684 Bipolar Stepper Motor Control" ;
; This Program Outputs a new Bipolar Stepper Motor Sequence
; once every 250 ms.
; Hardware Notes:
; PIC16F684 running at 4 MHz Using the Internal Clock
; Internal Reset is Used
; RC5:RC2 - L293D Stepper Motor Control ;;
; Myke Predko ;
LIST R=DEC ;yluo note: list directive to specify assembler options
INCLUDE "p16f684.inc"
List directive is used to specify different assembly and listing commands to the assembler program.
R=DEC means the default number base in 10.
_CONFIG directive is used to specify the configuration word bits. Each parameter is ANDed together to specify which bits are reset and set.
10/27/2017
Microprocessors I: Lecture 10

20. The Code (configuration code and data variables)
__CONFIG _FCMEN_OFF & _IESO_OFF & _BOD_OFF & _CPD_OFF & _CP_OFF & _MCLRE_OFF & _PWRTE_ON & _WDT_OFF & _INTOSCIO
; Variables
CBLOCK 0x20
Dlay, i
ENDC
PAGE
; Mainline
org 0
nop ; For ICD Debug
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Microprocessors I: Lecture 10

21. The code (initialization)
movlw 1 << ; Start with Bit 2 Active
movwf PORTC
movlw ; Turn off Comparators
movwf CMCON0
bsf STATUS, RP ; Execute out of Bank 1
clrf ANSEL ^ 0x ; All Bits are Digital
movlw b'000011' ; RC5:RC2 are Outputs
movwf TRISC ^ 0x080
bcf STATUS, RP ; Return Execution to Bank 0
clrf i
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Microprocessors I: Lecture 10

22. Microprocessors I: Lecture 10
The code (main loop)
Loop: ; Return Here for Next Value
movlw HIGH (( / 5) + 256)
movwf Dlay
movlw LOW (( / 5) + 256)
addlw ; 250 ms Delay
btfsc STATUS, Z
decfsz Dlay, f
goto $ - 3
movf i, w
call SwitchRead
movwf PORTC
incf i, f ; i = (i + 1) % 8;
bcf i, 3
goto Loop
SwitchRead:
addwf PCL, f ; Staying in First 256 Instructions
dt b'011100', b'010100', b'000100', b'100100'
dt b'100000', b'101000', b'111000', b'011000'
end
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Microprocessors I: Lecture 10

23. Microprocessors I: Lecture 10
Exam 3 notes
Allowed
One 8.5” x 11” double-sided sheet of notes
Calculator
No other notes or electronic devices (phone, laptop, etc.)
Exam will last three hours
Will be written for ~50 minutes, but you’ll have whole exam period
Covers all lectures after Exam 2
Format similar to previous exams
1 multiple choice question
2 short problems to solve/code sequences to evaluate
10/27/2017
Microprocessors I: Lecture 10

24. Review: Microcontroller/PIC basics
Microcontrollers: CPU integrated with storage, I/O devices
Examples
Timers/event counters
Parallel & serial ports
Clock generator
Analog to digital converter
Benefits: low cost/low power, easy to program
Limitations: storage, computational power
Introduced PIC 16F684 microcontroller
14 pins—12 multiplexed I/O + power/ground
All computations using 2 values use accumulator
Harvard memory architecture
Memory divided into SFR / GPR
Dedicated 8-entry system stack for return addresses (subroutines/interrupts)
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Microprocessors I: Lecture 10

25. Review: PIC Data Memory Organization
Made up of SFRs and GFRs
Banking: 128 byte chunks
Max offset within bank 0x7f
Controller may have 2 or 4 banks
Addressing Modes
Direct addressing:
7 bit address (within bank)
RP1:RP0 selects bank
Indirect addressing:
Access to INDF causes indirect addressing
Actual memory address in IRP+FSR
I/O ports
Control register (e.g. TRISA) controls direction of each pin
Bit = 1  input, Bit = 0  output
Data register (e.g. PORTA) contains actual port state
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Microprocessors I: Lecture 10

26. Review: PIC instructions
Four instruction formats
Upper bits of all hold opcode
Byte-oriented includes 1 bit destination, 7 bit direct address
Bit-oriented includes 3 bit position (0-7), 7 bit direct address
Literal/control includes 8 bit literal
CALL/GOTO includes 11 bit literal
Variable declarations
cblock <start_address>: start of variable declarations
All names between cblock/endc directives assigned to consecutive bytes starting at <start_address>
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Microprocessors I: Lecture 10

27. Microprocessors I: Lecture 10
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Microprocessors I: Lecture 10

28. Review: PIC instructions (cont.)
Clearing register: clrw/clrf
Moving values: movlw/movwf/movf
Swap nibbles: swapf
Single bit manipulation: bsf/bcf
Unary operations: incf/decf/comf
Arithmetic: addlw/addwf/sublw/subwf
10/27/2017
Microprocessors I: Lecture 10

29. Review: PIC instructions (cont.)
Logical operations
andlw/andwf
iorlw/iorwf
xorlw/xorwf
Rotates
rrf
rlf
Jumps/calls/return
goto
call
return/retlw/retfie
Miscellaneous
nop
sleep/clrwdt
Conditional execution
Test bit and skip next instruction if clear/set: btfsc/btfss
Increment/decrement register and skip next instruction if zero: incfsz/decfsz
Example use: combined with goto to create conditional jump
10/27/2017
Microprocessors I: Lecture 10

30. Review: A Delay Subroutine
10/27/2017
Review: A Delay Subroutine
; ***********************************************************************************
; TenMs subroutine and its call inserts a delay of exactly ten milliseconds
; into the execution of code.
; It assumes a 4 MHz crystal clock. One instruction cycle = 4 * Tosc.
; TenMsH equ 13 ; Initial value of TenMs Subroutine's counter
; TenMsL equ 250
; COUNTH and COUNTL are two variables
TenMs
nop ; one cycle
movlw TenMsH ; Initialize COUNT
movwf COUNTH
movlw TenMsL
movwf COUNTL
Ten_1
decfsz COUNTL,F ; Inner loop
goto Ten_1
decfsz COUNTH,F ; Outer loop
goto Ten_1
return
10/27/2017
Microprocessors I: Lecture 10
Chapter 9

31. Review: Strategy to “Blink”
The LEDs are toggled in sequence - green, yellow, red, green, yellow, red…
Let’s look at the lower three bits of PORTD
001=green, 010=yellow, 100=red
The next LED to be toggled is determined by the current LED.
001->010->100->001->…
10/27/2017
Microprocessors I: Lecture 10

32. Microprocessors I: Lecture 10
“Blink” Subroutine
Blink
btfsc PORTD, 0 ; is it Green?
goto toggle1 ; yes, goto toggle1
btfsc PORTD, 1 ; else is it Yellow?
goto toggle2 ; yes, goto toggle2
;toggle0
bcf PORTD, 2 ; otherwise, must be red, change to green
bsf PORTD, 0 ; 100->001
return
toggle1
bcf PORTD, 0 ; change from green to yellow
bsf PORTD, 1 ; 001->010
toggle2
bcf PORTD, 1 ; change from yellow to red
bsf PORTD, 2 ; 010->100
10/27/2017
Microprocessors I: Lecture 10

33. Another way to code “Blink” ---- Table Use
movf PORTD, W ; Copy present state of LEDs into W
andlw B' ' ; and keep only LED bits
addwf PCL,F ; Change PC with PCLATH and offset in W
retlw B' ' ; (000 -> 001) reinitialize to green
retlw B' ' ; (001 -> 010) green to yellow
retlw B' ' ; (010 -> 100) yellow to red
retlw B' ' ; (011 -> 001) reinitialize to green
retlw B' ' ; (100 -> 001) red to green
retlw B' ' ; (101 -> 001) reinitialize to green
retlw B' ' ; (110 -> 001) reinitialize to green
retlw B' ' ; (111 -> 001) reinitialize to green
In calling program
call BlinkTable ; get bits to change into W
xorwf PORTD, F ; toggle them into PORTD
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Microprocessors I: Lecture 10

34. Microprocessors I: Lecture 10
Final notes
Next time
Exam 3, Monday, 8/13
Reminders
Lab 4 due today
HW 3 due Friday, 8/10
Do not do problem 2j (JL instruction)
Turn in to my office by 1:45, or
Exam 2 regrade requests due Monday, 8/13
Lab 5 due Tuesday, 8/14
Same turn-in procedure as HW 3
10/27/2017
Microprocessors I: Lecture 10

35. Microprocessors I: Lecture 10
References
Myke Predko, "Programming and Customizing PICmicro Microcontrollers" 2nd Ed, McGrawHill, 2002, ISBN
R. Laidman, Stepper Motors and Control, Part II - Bipolar Stepper Motor and Control,
D. Jones, Stepping Motor Types,
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Microprocessors I: Lecture 10