I've built a small cable-tester. In a loop, I have these lines:

        movlw   b'00000001'         ; run cable test on GP0
        movwf   GPIO
        call    delay_500ms

        movlw   b'00000010'         ; run cable test on GP1
        movwf   GPIO
        call    delay_500ms

        movlw   b'00000100'         ; run cable test on GP2
        movwf   GPIO
        call    delay_500ms

        movlw   b'00010000'         ; run cable test on GP4
        movwf   GPIO
        call    delay_500ms

        movlw   b'00100000'         ; run cable test on GP5
        movwf   GPIO
        call    delay_500ms

Each of the corresponding GPs are connected to an LED and an
(external) AND-gate. The problem is, none of the LEDs are being lit
when using the above code.

Even more weird, if I insert this piece of test-code (light all LEDs):

        movlw   -1
        movwf   GPIO
        call    delay_500ms
        clrf    GPIO
        call    delay_500ms

between the GP0/GP1 tests above, all LEDs are being lit.
Step-debugging in MPLAB and watching the GPIO, I clearly see
"00000001", "00000010" etc in GPIO for the various tests, but none of
them are being lit.

I also tried this:

    movlw b'11111100'
    movwf GPIO
    call delay_500ms

Then the LEDs are instantly flashing (for like 0.1 ms, just barely
noticeable) before turned off.

I've been scratching my head for a couple of hours now,
double-checking the datasheets but I just can't see what's wrong. It
works for all the LEDs at the same time, but not one at a time.

Can anyone explain what's happening? Could it be a RMW-problem? I
can't think of anything right now.


-- 
- Rikard - http://bos.hack.org/cv/
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