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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