AVR-Chat

On Oct 10, 2011, at 9:11 PM, Jim Wagner wrote:

> 
> On Oct 10, 2011, at 8:34 PM, Philippe Habib wrote:
> 
>> Thanks to all of the help I got a couple of weeks ago with timers, I  
>> have been able to get my PWM stuff (mostly) working.
>> 
>> I assume it is possible to tune the PWM period beyond the blunt  
>> instrument of the div by registers but I don't see how to do it.
>> 
>> Here are my settings, which give me a 500Hz frequency with an 8M  
>> clock. I would like to get that to between 200 and 300 Hz if I can.  
>> Am I missing a setting that lets me do that, or is it not possible.
>> 
>> DDRD |= (1 << PD6);	 // enable PWM (OC0A, PD6 ) output pin
>> OCR0A = 0;	 // set PWM % 0= off 255 = full
>> TCCR0A = (1 << COM0A1) | (0 << COM0A0) | // OC0A in PWM mode
>> (0 << COM0B1) | (0 << COM0B0) | // OC2B is not used
>> (1 << WGM01) | (1 << WGM00); // PWM mode
>> TCCR0B = (0 << CS00) | (1 << CS01) | (0 << CS02); // div 8 on clock
>> TCCR0B |= (0 << WGM02); // more PWM mode
>> 
>> Thank you.
>> 
> There are modes where the timer resets based on the value in one  
> compare register or, perhaps, the capture register. Most of these  
> timers have two compare registers. This allows you to use one to set  
> the period and the other to set the PWM time.

For the Mega 328 it looks like you get this mode with WGM02 = 1. I've never used this mode (I think I will though.  Note that this means the PWM value will vary from 0 - OCR0A or in this case, 0 - 100 to 125 roughly.

Clark Martin
Redwood City, CA, USA
Macintosh / Internet Consulting

"I'm a designated driver on the Information Super Highway"