On Wed, 17 Apr 2002, Bob Ammerman wrote:

> At low PWM frequencies you can switch from using hardware PWM to doing it
> with some software assist (ie: in an interrupt handler). You'll probably end
> up with a bit of jitter in the result, but it will usually be acceptable for
> most applications.


I agree. I have a couple comments (and I haven't read the whole thread so
these comments may not be appropriate).

Varying the PIC's oscillator frequency is a perfectly reasonable approach
to obtain a lower frequency PWM. In one application I needed a constant
duty cycle, but variable frequency PWM. The frequency was controlled via a
POT as part of the R in an RC oscillator.

For the software approach, I can imagine several different solutions. One
is to manipulate the PWM registers in such a way that the oscillator
frequency can be made slower. For example, you (Jinx) could re-write TMR2
or change the mode of the capture/compare/pwm (e.g. disable the PWM
functionality until it's needed).

Another software approach is to write an interrupt driven software PWM
routine. It would be much coarser than the hardware PWM. Furthermore, it
will not transition smoothly between software - hardware - software PWM
control.

If you want smooth transitioning, then you can implement a single
cycle resolution PWM routine:

http://www.dattalo.com/technical/software/pic/pwm256.txt

But I think that may be a little overkill :). OTOH, you could ditch
hardware PWM altogether with this routine and control the PWM with any
dutycycle/frequency you'd like. The only problem with this routine is that
it's been known to induce migraines. BTW, as listed this routine is only
an 8-bit PWM. However, I've used it to generate 12-bit pwm too. Extending
it to 16 or 24-bits is, well not trivial, but doable. A 24-bit PWM driven
by a 20-MHz crystal will yield an oscillation frequency of 0.298 Hz. If
that'a not low enough, you can add 8 more bits and drop it down to 1mHz!


Scott

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