> I'm just starting to experiment with the PIC16F877 using the HITECH C
> compiler. I am running the PIC at 10 Mhz and want a 50 MHz PWM period
> with a 7.5% duty cycle. However this means that I need the PR2 to be
> 3124 which is too large (max is 255 right?). Is there a way around
> this? am I doing something wrong? or do I have to use a 4MHz clock
> instead? or maybe create my own pulse using TIMER0 which would allow
> a prescalar of 256? Thanks in advance!

Are you sure you mean 50 MHz??  Running the pic at 10 Mhz, you can only
execute instructions at 10/4 = 2.5 MHz, let alone run the built-in pwm.
The formula for pwm frequency in this case is:
         pwm freq = 2.5E06 / ((PR2 + 1) * pre-scale)
if PR2 = 1 and pre-scale = 1, your pwm freq = 1.25 MHz.
However, you only have 1 bit of resolution!  (off and 50%)
The formula for resolution is:
         bits of resolution = log(2.5E06/pwm freq) / log(2)
which = 1 for pwm freq = 1.25 MHz
This also shows why PR2 = 0 won't work:  pwm freq theoretically
= 2.5 MHz, but the resolution = 0 bits!

If PR2 = 3124 and the pre-scale = 16, the pwm freq = 50 Hz, so I'm
guessing that is actually the speed you're looking for??  One problem
with the built-in pwm is that there is a definite minimum speed.  In
this case, it is with PR2 = 255 (the max as you pointed out) and
pre-scale = 16 (max available):
          pwm freq = 2.5E06 / (256 * 16) = 610 Hz
So you can't get to 50 Hz with this clock speed.
If you switch to a 4 MHz clock, the minimum pwm freq is:
          pwm_freq = 1E06 / (256 * 16) = 244 Hz
So you're still not there!
The max clock you can run = 256 * 16 * 50 * 4 = 819.2 KHz
So the ever popular 32.768 watch crystal would work, although you
wouldn't be able to hit 50 **exactly** - you could get 49.95 Hz.

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