Hi I did something like this, ages ago (Z-80, 8085 era). I just use one
byte with a circular  counter. You just have to rotate and output one
particular bit (B0 ie.). I couldn't find nothing faster than this, Z80 has
rotates os shifts, I don't remember now, where the carry bit don't interfere
with operation, it makes the things even easier ( b0->b7 , b1 ->b2,....
b7->b6).
    You just have to perform the rotation and output a particular bit. Hope
it helps.


Regards
Horta


> I've spent some time playing with an RC car controller and needed a very
> fast 2 channel PWM routine.  Resolution was not important, so 8 steps (3
> bits) was fine.
>
> My first attempt which I'm sure can easily be bettered:
>
>         clrf    outport         ; clear shadow register
>         incf    pwmcount,w      ; increment pwm counter
>         andlw   0x07            ; 8 step PWM, so roll counter over after 7
>         movwf   pwmcount
>
>         movf    pwmset1,w
>         subwf   pwmcount,w      ; subtract setpoint from counter
>         skpc
>         bsf     outport,0       ; result negative so turn on PWM output in
> shadow register
>
>         movf    pwmset2,w
>         subwf   pwmcount,w      ; subtract setpoint from counter
>         skpc
>         bsf     outport,1       ; result negative so turn on PWM output in
> shadow register
>
> So this gives a 4 cycle overhead for the counter and clearing the shadow
> register, and 4 cycles for each additonal channel. This actually gives
nine
> steps; 0 gives 0%, 8 or greater gives 100%.
>
> I have seen Scott Dattalos very fast PWM routines, but they seem to incur
> too much overhead if the duty cycle needs to be varied.
>
> Regards
>
> Mike
>
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