This is assembly language program for PIC IC16F84A. This program controls A DC motor’s Speed and Direction using Interrupts.

Interrupts used : Timer0 overflow Interrupt, External Interrupt, PORTB Inerrupt
Timers used      : TIMER0
Ports used        : PORTA as output, PORTB as input

Program:

list p=16F84A
#include “p16f84A.inc”
__CONFIG  _CP_OFF & _WDT_OFF & _PWRTE_ON &  _XT_OSC
cblock 0x0c      ;Declaring variables
count
level
sel_delay
dir
endc

;************************* MACRO **************************************
send1 macro dir         ;Macro for sending high to portA
banksel PORTA
btfsc dir,0             ;If dir=0 rotate CLOCKWISE else ANTI-CLOCKWISE
goto next16
goto next15
next15:bsf PORTA,0   ;Setting or Resetting the PORTA pins
bsf PORTA,1         ;For CLOCKWISE direction
bcf PORTA,2
bcf PORTA,3
goto next19
next16:bcf PORTA,0  ;Setting or Resetting the PORTA pins
bcf PORTA,1         ;For ANTI-CLOCKWISE direction
bsf PORTA,2
bsf PORTA,3
goto next19
next19:
ENDM
send0 macro              ;Macro for sending low to portA
banksel PORTA     ;Resetting the PORTA pins done here
bcf PORTA,0
bcf PORTA,1
bcf PORTA,2
bcf PORTA,3
ENDM
org 0×00            ;Reset vector address
goto MAIN           ;Begining of main programm
org 0×04            ;Interrupt vector address
goto int_service  ;Beginig of interrupt routine

;************************ MAIN PROGRAM ********************************
MAIN
banksel TRISA     ;Select BANK1
movlw 0×00
movwf TRISA       ;Set PORTA as output port
movlw 0xff
movwf TRISB       ;Set PORTB as input1
movlw 0×00         ;Regester for indicating 1 or 0 for PORTA pins
movwf level         ;By default high is sected
banksel PORTA
movlw 0×03
movwf PORTA       ;Initialise PORTA (pin 0,1) as high
movlw 0×37
movwf TMR0         ;Initialise TIMER0
movlw 0×00          ;Regester for indicating SPEED of motor
movwf sel_delay   ;By defult maximum speed is selected
movlw 0×00          ;Regester for indicating DIRECTION of motor
movwf dir              ;By default ANTI-CLOCKWISE is selected
banksel INTCON    ;Enable PORTB(bit3=0)EXTERNAL INTERRUPT(bit4=0)
movlw b’10111000′ ;TIMER0 (bit 5=0) interrupts
movwf INTCON
loop:
banksel OPTION_REG
btfsc sel_delay,0;If el_delay=0prescaler=1:256 else prescaler=1:1
goto next20
goto next21
next20:
banksel OPTION_REG ;Intialise OPTION_REG with 000 for TIMER0
movlw 0×00        ;Prescaler=1:1 ,and bit 7(=0) for activating
movwf OPTION_REG  ;”PULL UP” resistor of the PORTB
goto end_rt
next21:
banksel OPTION_REG
movlw 0×07        ;Intialise OPTION_REG with 111 for
movwf OPTION_REG ;TIMER0 prescaler=1:256
goto end_rt
end_rt:
goto loop             ;Main programm ends here

;***************** INTERRUPT SERVICE ROUTINE **************************
int_service
banksel PORTA    ;Select BANK0
btfsc INTCON,2     ;Check for TIMER0 overflow interupt
goto isr_time         ;if occured goto TIMER0  ISR
btfsc INTCON,0      ;Check for PORTB(Pin 4/5/6/7)interrupt
goto isr_prtrinc      ;if occured goto PORTB ISR
btfsc INTCON,1       ;Check for EXTERNALINTERRUPT (RB0/INT)
goto isr_direct        ;if occured got EXTERNAL ISR

;***************** TIMER OVERFLOW ISR *****************
isr_time:
btfss level,0       ;If level=0 (bit 0) call send0 macro
goto next4
decf level,1        ;Else update level
banksel TMR0     ;Select BANK0
movlw 0xc8
movwf TMR0      ;Initialise TIMER0 with ‘ON’ time
send1 dir           ;send HIGH to PORTA pins (MACRO)
goto end_isr
next4:
banksel TMR0
movlw 0×37       ;Initialise TIMER0 with ‘OFF’ time
movwf TMR0
send0                 ;Send LOW to PORTA pins (MACRO)
banksel PORTA
incf level             ;Update level
goto end_isr

;********************** PORTB ISR **************************
isr_prtrinc:
banksel PORTB    ;If there is an Interrupt on PORTB Pin 4
btfsc PORTB,5      ;Alternativelly select max. and min. speed
goto end_isr
incf sel_delay,1    ;sel_delay(bit0)changes to 0and1 alternatively
goto end_isr

;************************** EXTERNAL ISR **************************
isr_direct:
banksel PORTB     ;If INTERRUPT on RB0/INT pin occured
btfsc PORTB,0       ;Regester dir = 0 for CLOCKWISE ROTATION
goto end_isr          ; dir = 1 for ANTI-CLOCKWISE ROTATION
incf dir,1                ;Incriment Direction regester (bit0=1)
goto end_isr
end_isr:bcf INTCON,0   ;Clear PORTB INTERRUPT FLAG
bcf INTCON,1          ;Clear TIMER OVER FLOW FLAG
bcf INTCON,2          ;Clear EXTERNAL INTERRUPT FLAG
retfie                       ;Return from ISR enabling GLOBAL INTERRUPT FLAG
END                       ;END OF THE PROGRAMM