At 10:14 PM +0100 1/30/03, Wouter van Ooijen wrote:
>  > My need is to drive an RGB LED which is more or less like driving 3
>>  discreet LEDs.  Since I want to be able to generate an arbitrary
>>  color and brightness, and still have lots of PIC left to do other
>>  things, using a part with 3 hardware PWM outputs seemed like a
>>  natural.
>
>Are you sure you won't have enough CPU left after bit-banging 3 PWMs for
>LED driving? If that is realy the case why not use a dedicated small PIC
>(12F629) to do the job?
>
>Wouter van Ooijen

yeah, further down I mention that as a possibility...

napkin calculation regarding time needed for bit banging...but
correct me if I am thinking about this the wrong way...I am still
learning...

To ensure no flicker problems lets say the basic cycle is 100hz.

Then if I use 8 bits of resolution for brightness I have to check the
duty cycle (256*100) = 25600 times per second.  At 4 Mhz I get
1000000 instructions per second.  So if the PWM calculation is in an
event loop, I get 1000000/25600  or about 39 instructions per loop.

I'd rather use 10 bits of brightness resolution.  (This *can* be seen
at low levels of brightness.  I want to be able to ramp up the LED
without "zipper noise" or obvious "steps".  I might be able to use
some kind of nonlinear transfer function so there is more resolution
at low light than bright...).

So for 10 bit resolution 10^6/(2^10*100) = 9 instructions per loop

A 20 Mhz crystal will allow about 45 instructions per loop.  Or 195
instructions at 8 bit resolution.

Given all the other stuff I expect to be doing I don't think that is
enough cycles to do it all on one PIC...frankly I'd like the "cpu" to
be entirely unburdened from PWM generation.

Phil

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