Delay loop PIC assembly code not working on breadboard

Question

I tried to program the PIC 12F675 so it would blink an LED diode connected to its GPIO 0 pin (pin number 7).

I have set the pins to digital mode:

clrf ANSEL

I have set them all as outputs:

clrf TRISIO

and finally this is the loop I used:

                     ;this delay loop should consume 1 000 000 ticks
                     ;which is 1 second approximately
movlw 0xff           ;assigned the highest 8bit value to accumulator
movwf 0x20           ;moved the value to general purpose register 20
movwf 0x21           ;moved the value to GPR 21
clrw                 ;cleared the working register (accumulator)
movlw 0x15           ;assigned the value 15 to accumulator
movwf 0x22           ;moved the value to GPR 22

bsf GPIO,0       ;set the first pin high

loop1

decfsz 0x20, 1
goto loop1
decfsz 0x21, 1
goto loop1
decfsz 0x22, 1
goto loop1        ;nested loops to delay further action

bcf GPIO,0         ;set the first pin low

And finally this is the code I used to configure the chip in the fist place:

__CONFIG _FOSC_INTRCIO & _WDTE_OFF & _PWRTE_ON & _MCLRE_OFF & _BOREN_OFF & _CP_OFF & _CPD_ON

My issue is: when compiling the program in MPLAB X and debugging it step-by-step, there is no problem in program execution, the bit GPIO 0 indeed changes its state according to the loop. But when I connect the chip to breadboard the LED fails to blink, just stays on.

My question is: could the issue be caused by the faulty oscillator configuration? (I also use 100 nF decoupling ceramic capacitor)

I already tried quite a number of code variations. Any advice is welcome.

The whole .asm code that i use:

          list p=12F675
#include <p12F675.inc>

    __CONFIG _FOSC_INTRCIO & _WDTE_OFF & _PWRTE_ON & _MCLRE_OFF & _BOREN_OFF & _CP_OFF & _CPD_ON

    OSCCAL equ 0x90
    TRISIO equ 0x85
    ANSEL equ 0x9f
    GPIO equ 0x05

    org 0x00

    bsf 0x03,5      ;bank1

    movlw b'00111100'   ;osccal value
    clrf OSCCAL
    movwf OSCCAL    ;osccal set

    movlw b'11111110'   ;ansel value
    movwf ANSEL     ;set pin AN0 as digital I/O

    movlw b'11111110'   ;trisio value
    movfw TRISIO    ;set pin GPIO0 as output
    clrf TRISIO


    bcf 0x83,5      ;bank0

    clrf GPIO       ;clear GPIO port

start

    movlw 0xff
    movwf 0x20
    movwf 0x21
    clrw
    movlw 0x15
    movwf 0x22


    bsf GPIO,0

loop1
    decfsz 0x20, 1
    goto loop1
    decfsz 0x21, 1
    goto loop1
    decfsz 0x22, 1
    goto loop1


    bcf GPIO,0

    movlw 0xff
    movwf 0x20
    movwf 0x21
    clrw
    movlw 0x15
    movwf 0x22


loop2
    decfsz 0x20, 1
    goto loop2
    decfsz 0x21, 1
    goto loop2
    decfsz 0x22, 1
    goto loop2

    goto start

    end

I generated this piece of code: _WDTE_OFF, I think it is off. All generated Coniguration bits code: __CONFIG _FOSC_INTRCIO & _WDTE_OFF & _PWRTE_ON & _MCLRE_OFF & _BOREN_OFF & _CP_OFF & _CPD_ON — Avo

GJ. GJ. · Accepted Answer · 2016-07-11T11:37:03

Everything looks OK, except the comparator configuration! Insert the folowing code to disable comparator (default is on) to use GP0, GP1 and GP2 as digital I/O pins.

bcf      STATUS,RP0 ;Bank 0    
movlw    07h        ;Set GP<2:0> to digital IO
movwf    CMCON      ;digital IO

Delay loop PIC assembly code not working on breadboard

1 Answers