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%, which is actually quite handy.

 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.  The pwm duty cyle
is set every loop, so this must be taken into consideration.

Regards

Mike

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu