PIC: EXAMPLE 8

EXAMPLE 8

Using timer TMR1 and using interrupt

16-bit timer TMR1 is used in this example. By occupying its registers TMR1L and TMR1H, an interrupt occurs and the number on port B is incremented. This has already been seen in the previous examples. The difference is in the program delay which is a bit longer this time because the prescaler rate is 1:8.

Source Code

;*************************** Header *****************************************
;************* DEFINING VARIABLES *******************************************

       cblock      0x20            ; Block of variables starts at address 20h
       w_temp                      ; Variable at address 20h
       pclath_temp                 ; Variable at address 21h
       status_temp                 ; Variable at address 22h
       endc
       
;************************ PROGRAM START ************************
       org         0x0000          ; Address of the first program instruction
       goto        main            ; Jump to label "main"

;************************ INTERRUPT ROUTINE **********************************

       org         0x0004          ; Interrupt vector
       movwf       w_temp          ; Save register W

       movf        STATUS          ; Save register STATUS
       movwf       status_temp
       
       movf        PCLATH          ; Save register PCLATH
       movwf       pclath_temp
       
       banksel     PORTB           ; Selects bank containing PORTB
       incf        PORTB           ; Register PORTB is incremented by 1
       
       movf        pclath_temp,w   ; PCLATH is given its original content
       movwf       PCLATH
       movf        status_temp,w   ; STATUS is given its original content
       movwf       STATUS
       swapf       w_temp,f        ; W is given its original content
       swapf       w_temp,w
  
       banksel     PIR1            ; Selects bank containing PIR1
       bcf         PIR1,TMR1IF     ; Clears interrupt flag TMR1IF
       
       bsf         INTCON,GIE      ; Global interrupt enabled
       retfie                      ; Return from interrupt routine
       
;************************ MAIN PROGRAM ***************************************

main                               ; Start of main program
       banksel     ANSEL           ; Selects bank containing register ANSEL
       clrf        ANSEL           ; Clears registers ANSEL and ANSELH
       clrf        ANSELH          ; All pins are digital
       
       banksel     TRISB           ; Selects bank containing register TRISB
       clrf        TRISB           ; All port B pins are configured as outputs
       
       banksel     T1CON           ; Selects bank containing register T1CON
       bcf         T1CON,TMR1CS    ; TMR1 counts pulses generated by oscillator
       
       bsf         T1CON,T1CKPS0   ; Prescaler rate is 1:8
       bsf         T1CON,T1CKPS1
       bsf         T1CON,TMR1ON    ; Turns on timer TMR1
       
       banksel     PIE1            ; Selects bank containing register PIE1
       bsf         PIE1,TMR1IE     ; TMR1 interrupt overflow enabled
       bsf         INTCON,PEIE     ; Peripheral modules interrupt enabled       
                                   ; Timer TMR1 belongs to peripheral modules
       bsf         INTCON,GIE      ; Global interrupt enabled
       
       banksel     PORTB           ; Selects bank containing register PORTB
       clrf        PORTB           ; Clears port B
loop
       goto        loop            ; Remain here
       end                         ; End of program