I have, what was to be a simple controller, that suddenly got an additional
job of controlling an LED backlight.  Dimming is done via PWM, and the spec
states a max frequency around 250 Hz.  It's a PIC18LF45K22, and seems to be
like the rest of the 18F family.  Hardware PWM runs from a timer--such as
TMR2--with an optional prescale.  If we let the timer run to a full count
(256) and invoke the maximum prescale (16), then at 8 MHz we get a period o=
f
488 Hz.  If I drop the clock down to 4 MHz that puts us under the 250 Hz ma=
x
requirement.  But now I'm running at 4MHz on a processor that can go as hig=
h
as 64 (it has a PLL inside).   Now, a typical processor that reads
pushbuttons, blinks lights, and occasionally sequences some power supplies
probably doesn't need to run faster than 4MHz.  But it's also going to be a=
n
i2c slave, and it would be nice to have some extra speed when handling i2c
commands.

It may or may not turn out to be that important, but I wanted to explore th=
e
options when it comes to generating low-rate PWM.  I'm always surprised at
how limited the dividers are with this.  If there's anything that would be
easy to do inside the chip, it would be to divide some slow clocks down to
even slower clocks.  So, here are the options I came up with.

1. PWM hardware, CPU at 4 MHz, and live with it.
2. PWM in software.  The can work at really low PWM rates, but if you put
125 steps into a 250 Hz signal you now need a precision of 32 microseconds,
and if any latency stretches your pulses, that really shows up, especially
at low brightness.
3. Fix the Windows driver that was supposed to do the PWM in the first
place. Definitely out of my area :)
4. Go on the PICList and find out I missed something and that it really can
go slower.  They do keep adding features to these things.
5. Cascade two timers somehow.  Some seem to have external inputs, but not
sure I see any outputs.  Maybe someone's been through this before.  I can't
do it this time because the boards are already being made, and the pins are
used up.
--=20
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