AVR PWM timer frequency

Question

I'm learning how to use an Attiny84. I've generated a fast pwm signal. I managed to understand how to do this:

#include <avr/io.h>
int main(void)
{
 // setup timer0
 TCCR0B |= _BV(CS00); // use clk_i/O without prescalers as clock source.
 // also enables TIMER0
 // set fast PWM mode
 TCCR0A |= _BV(WGM01) | _BV(WGM00); // Mode 3
 TCCR0A |= _BV(COM0A1) | _BV(COM0A0); // set OC0A on compare match, clear on BOTTOM
 OCR0A = 0xff / 2; // 50% PWM
 // set PB2 as output. This should enable OC0A
 DDRB |= _BV(PB2);
 for (;;) {
 }
}

However, I don't understand why I got only ~30kHz on the scope. I disabled the CKDIV8 fuse and sets my CKSEL to "Calibrated Internal RC Oscillator 8MHz".

I though I could have at least 8MHz PWM frequency. I can't find in the documentation how clk_i/o and clk_cpu are related. Is there any way to gain a higher PWM frequency ?

N.B.: I don't need to have a particular PWM frequency, I'm just testing the chip and trying to understand its boundaries.

Answer 1

You have a 8MHz clock, but 8-bit PWM. This results in a 8MHz/256=31.25kHz output frequency. If you need a higher output frequency then you can use mode 7 instead, at the cost of PWM signal depth.