A math problem in the USB-SERVO2 project

[nick]

Hi I am planning to implement the USB-SERVO2 (http://www.societyofrobots.com/member_tutorials/node/25),
but I found a little problem... do the math here:

1. the ATmega8 uses a 12MHz external oscillator;
2. and the Timer1 for PWM generation sets prescaler at 256 (from source code);
3. the PWM should be at 50Hz;
4. Then the ICR1 (aka topcount) is 486.75 = 12MHz/(2*256*50) -> Decimal?!
what goes wrong??

[bens]

I don't understand what problem you're having.  Are you surprised that you can't acheive exactly 50 Hz when using a timer that's clocked at 46875 Hz?

If you want an exact 50 Hz signal, use Timer1 in phase-correct PWM mode with a prescaler of 8 and a top count of 15000 (or maybe 14999?).

- Ben

[nick]

Thanks Ben.
I am just wondering why it uses 12MHz and 256 prescaler to generate a not-so-accurate top count.
AVR-USB runs fine with 12,16..MHz.

[dunk]

AVR-USB runs fine with 12,16..MHz.

hobby servos don't need to be at exactly 50Hz either. the actual pulse duration has far more effect than the exact frequency.

dunk.

[nick]

Thanks dunk.