On Fri, 9 Jul 2004, Russell McMahon wrote:

> [EE:] Design Challenge - Lowest cost PWM

I've written an all-software version that has single instruction cycle
resolution and can run on the 12-bit cores:

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

It has an 8-bit dynamic range (though in one instance I boosted this to
12-bits). The PWM frequency is fosc/4/256, or about 20kHz for an
fosc=20Mhz.

The algorithm leaves available roughly 50% of the CPU time for other
tasks. So in your case where you need to receive either an analog or
digital control, you'd have CPU time to implement it.

I promise this routine will make your head explode! It's completely
isochronous and uses the program counter as a control variable. But with
the new PICs dropping below $0.50, I think this implementation may be
optimal in the cost * flexibility solution space. If flexibility is not a
desirable feature, then your hardware-only solutions will most probably be
cheaper.

Scott

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