> -----Original Message-----
> From: PY2NI [SMTP:py2ni@UOL.COM.BR]
> Sent: Thursday, February 21, 2002 12:10 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [PIC]: Ultra fast low res PWM challenge
>
>     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
>
Thanks for the tip, by coincidence I coded a rouine that does exactly that
last night.

        clrf            outport         ; clear all output bits in the
shadow register

        rrf             pwm1,w          ; put bit 0 into carry
        rrf             pwm1,f          ; rotate byte
        skpnc
        bsf             outport,0       ; if carry set and set output bit

        rrf             pwm2,w          ; put bit 0 into carry
        rrf             pwm2,f          ; rotate byte
        skpnc
        bsf             outport,1       ; if carry set, then set output bit

        movf    outport         ; update the PORT from the shadow reg
        movwf   PORTB

This is actually no faster than the original method, but has a couple of
advantages, and one disadvantage.  The disadvantage is that the duty cycle
can no longer be set directly, you have to translate to a bit pattern which
adds some overhead:

setpwm: ; call with duty cycle in W.  Need to add any bank switching, and
limit maximum W!!!
        addwf   PCL,f
        retlw   b'00000000'
        retlw   b'00000001'
        retlw   b'00010001'
        retlw   b'01001001'
        retlw   b'01010101'
        retlw   b'10110101'
        retlw   b'11101101'
        retlw   b'01111111'
        retlw   b'11111111'


To set a pwm channel you'd simply use the following which (I think) takes 8
cycles.

        movlw   5
        call            setpwm
        movwf   pwm1

One advantage of this method is that one byte less RAM is needed (no PWM
base counter), the other (possible) advantage is that the PWM frequency for
mid range settings can be increased which can make filtering easier, or
maybe reduce motor whine.  This is quite a nice way to do things if you have
the PWM running under an interrupt timer and it is not updated very
frequently.

Regards

Mike

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