/*
 * ATtiny85 Timer0 Output COMPA mode
 */
#include <avr/io.h>
#include <avr/interrupt.h>
#define ledPin PB0

ISR(TIMER0_COMPA_vect)
{
  PORTB ^= _BV(ledPin);		// toggle led
}

int main(void)
{
  // setup
  DDRB |= _BV(ledPin);		// led pin as output
  PORTB &= ~(_BV(ledPin));	// turn off LED
  
  // timer setup, CTC mode
  TCCR0A |= _BV(WGM01);		// CTC mode
  TCCR0B |= _BV(CS02) | _BV(CS00);	// prevscaler 1024
  OCR0A = 255;	// set counter to max value
  TIMSK |= _BV(OCIE0A);		// enable CTC interrupt
  sei();		// enable global interrupt
  
  // loop
  while (1);
}