On Wed, 5 Jun 2002, Bob Ammerman wrote:

> Gordon,
>
> You can do this with a 1-bit output direct-digital synthesis technique.
>
>
> Given the following variables:
>
> unsigned long increment_value;
> unsigned long phase_accumulator;
>
> for (some number of iterations)
> {
>     phase_accumulator += increment_value;
>
>     // copy hi bit of phase_accumulator to output pin
>     // code could look something like:
>     movf    output_port,W               ; get existing port bit
>     andlw    ~(1 << output_bit)       ; turn off current value
>     btfsc     phase_accumulator;      ; is high bit on?
>     iorlw     1 << output_bit            ; yes: turn the bit on
>     movwf   output_port                 ; send it back to the hardware
> }
>
> The output frequency is directly proportional to 'increment_value'.


A simpler way is to toggle the output everytime the phase accumulator
rolls over:

     movf    increment_value,w
     addwf   phase_accumulator,f     ; or better, a 16-bit phase acc.
     movlw   1<<output_bit
     skpnc
      xorwf  output_port,f           ;rmw, oh well.


If you're worried about read-modify-writes to an I/O pin, then do this:

     movf    increment_value,w
     addwf   phase_accumulator,f     ; or better, a 16-bit phase acc.

     movlw   1<<output_bit
     xorwf   output_port,w
     skpnc
      movwf  output_port


Note that monitoring the carry gives you an extra bit too. (But, you're
counting on the I/O line to accurately retain the state.)

Scott

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