Scott Dattalo wrote:

> You can rearrange the xor
>
>                                  ; now output current2
>          movf current2,w         ;
>          movwf PORTB             ; send to motor!
>
>  ;       nop                     ; let ports settle
>  ;       nop                     ; + some delay
>
>      nop
>      movf   inouts_last,w
>      xorwf  PORTA,w
>      movwf  diff
>      skpz
>       goto pwm_loop
>
> Now in the 3 cycles spare in the above loop, change 2 of the nops
> to
>
>      movf   diff,w
>      xorwf  inputs_last,f
>
> making use of the A = (A ^ B) ^ B trick.
>
> Is that clear? I doubt it...


Actually it is. You store the new pins in an XORed
form. :o) But there is a problem that your code below
still takes 9 cycles (once goto is included).

        ;-------------------------
                                ; now output current2
        movf current2,w         ;
        movwf PORTB             ; send to motor!
       nop
       movf   inputs_last,w
       xorwf  PORTA,w
       movwf  diff
       skpz
       goto pwm_loop


> What is your're trying to do?

This is supposed to:
* output byte 2 to PORTB for 8 cycles exact
* output byte 1 to PORTB for 4 cycles exact
as a form of PWM that is modulating between
2 values of PORTB, weighted so that one is output
for exactly twice as long as the other.

Somethere in there I need to see if any PORTA
pins have changed, and if so exit with the new
PORTA values in "inputs". The variable "inputs_last"
must not be destroyed.
-Roman

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