void setup() {
  DDRB = 0xFF;
  TCCR0B &= ~(1 << WGM02);
  TCCR0A &= ~(1 << WGM01);
  TCCR0A &= ~(1 << WGM00);
  TCCR0B |= (1 << CS00) | (1 << CS01) | (1 << CS02);
  TIMSK0 |= (1 << TOIE0);
  sei();
}

void loop() {
  
}

ISR(TIMER0_OVF_VECT){
  static cnt = 0;
  cnt++;
  if(cnt == 90){
    PINB ^= (1 << PB0); cnt=0;
  }
}