Ok, sorry to dig up this old thread, but now that I actually go to
integrate it into a project, I seem to have forgotten that perfect view
of the solution I had a month ago :)

I am looking to use the code at
http://www.dattalo.com/technical/software/pic/pwm8.asm
to do multiple PWMs on one chip. My basic question is...where and when
do I update the PWM counters? I plan on putting the timer checking and
updating in an ISR, and triggering it with a TMR0 interrupt. This will
work on the first iteration, but once a counter has hit zero, doesn't it
just start again at 255? Therefore, I would need to reload the value I
want in the counter so that it will start to count down again. What if
after:
         DECFSZ  pwm0,F          ;If the first counter has not reached 0
         IORLW  00000001b       ;then we don't want to turn it off.
I add something to the effect of:
        check pwm0 for zero
        if zero, reload pwm0 with proper value
This would still be in the ISR. How does this sound? Plausible? Am I on
the right track? I'm using an 18f252 chip, so I was thinking about using
the BZ (Branch if Zero) operand. I guess I would have to do a movf
pwm0,f in order to trigger the zero bit. If only there was an
instruction similar to TSTFSZ (Test f, skip if 0), but skip if not zero.
Can having the branch operand in an ISR cause a stack overflow if I run
too many of them?

Anyway, as per usual, any and all help/advice/assistance is appreciated.

Thanks,

Josh
--
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
        -Douglas Adams


Scott Dattalo wrote:
> Well, as Olin suggested, the PWM routine can be placed in a Timer
> interrupt routine. You could run
>
> http://www.dattalo.com/technical/software/pic/pwm8.asm
>
> For 23 cycles / 8 PWM's. Or 46 cycles for 16 PWMs. For the last two PWMs
> you could shrink the routine to just this:
>
>         CLRW                    ;Build the bit mask for turning
>                                 ;off the PWM outputs. Assume that
>                                 ;all of the outputs will be turned
>                                 ;off.
>                                 ;
>         DECFSZ  pwm0,F          ;If the first counter has not reached 0
>          IORLW  00000001b       ;then we don't want to turn it off.
>                                 ;
>         DECFSZ  pwm1,F          ;Same for the second one
>          IORLW  00000010b       ;
>
>         ANDWF   pwm_state,W     ; Clear all of those pwm outputs
>                                 ;that have reached zero.
>                                 ;
>         XORLW   00000011b       ;Toggle the current state.
>         INCFSZ  rising_edge,F   ;If the rising edge counter has not
>          XORLW  00000011b       ;rolled over then toggle them again.
>                                 ;Double toggle == no effect. However,
>                                 ;if the rising edge counter does roll
>                                 ;over then a single toggle will turn
>                                 ;the pwm bits on, unless of course the
>                                 ;pwm counter has just rolled over too.
>                                 ;
>         MOVWF   pwm_state       ;Save the state
>         MOVWF   PWM_PORT        ;update the outputs
>
> That'd be 58 cycles for 18 outputs. If TMR0 is the interrupt source and
> you don't select the prescaler, then you'd have ~75% of the CPU time
> available for other things. Those "other things" would be updating all
> those PWMs! (BTW, there's a way to save a few cycles by combining the
> checks for "rising_edge").

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.