If the external AND gate has one GPIO connected to each input, and 
 assuming it's a 6 input AND gate, ALL pins would have to be HIGH for 
 the output to go HIGH. If there are any unused inputs on the AND gate,
 ie an 8 input AND gate, pull each of them HIGH with a 10K or so ohm 
 resistor.  

 The reason it works for the case of writing -1 is that this makes the PORT
 take the value of '255', or all pins HIGH (0-1=255).

 Try using an OR gate in place of the AND gate, and see if that works.


                                           Regards,

                                             Jim
 



> 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/
> -- 
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist


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