PIC16F877, 250Hz PWM

Firstly welcome to EP. Secondly, it's quite a good idea to give as much information as possible of your requirements. Unfortunately you haven't. Questions I would like answered are :

1) What language are you programing in?
2) What is the % of duty cycle you require?
3) How far have you got already yourself?

And out of interest what's the project, anything interesting?

Thanks
Adam

 

This for starters.
PWM Period = [(PR2) + 1] • 4 • TOSC •
(TMR2 Prescale Value)

http://www.micro-examples.com/public/microex-navig/doc/097-pwm-calculator.html

M.

 

Then you may have to use a low freq clock in such as external osc or crystal input, I have used the 32khz watch crystals for some low freq apps to run TMR1.
I would think the 16F877 has this feature.
M.

 

Yes of course, I was thinking of capture, you can go down to 1.2Khz with 20mhz system clk is that too high? What is the application?
M.

 

I make that a 200uS period (5kHz) with a 25% duty cycle. Or does the "1 in 50uS" and "0 in 150uS" relate to the pulse rise and fall times, respectively?

 

There should be no problem doing that using delays. 50uS is 250 instruction times.

Bob

 

Can you explain that?

 

Then you need to use interrupts. Determine when each pulse starts and stops and use the timer to interrupt at those times and toggle the outputs.

Bob

 

Then I guess you should just give up.

Seriously, though:

If the pulses are all on increments of 50uS you can set an interrupt to occur every 50uS and decide what needs to be done at each interrupt. This would be less complicated, but more overhead. How much of the processor time is needed for the other tasks?

Bob