/*
 * ATtiny85 Blink w/Timer Overflow
 */
#include <avr/io.h>
#include <avr/interrupt.h>
#define ledPin PB0

ISR(TIMER0_OVF_vect)
{
  static byte cnt=0;

  if (++cnt > 10){
    PORTB ^= _BV(ledPin); // toggle led pin
    cnt = 0;
  }
}

int main(void)
{
  // setup
  DDRB |= (1<<ledPin);
  PORTB &= ~(1<<ledPin);

  TCCR0B |= _BV(CS02) | _BV(CS00);  // set prescale to 1024
  TIMSK |= _BV(TOIE0); // enable timer overflow interrupt
  sei();  // enable global interrupt

  // loop
  while (1);
}