#include <avr/interrupt.h>
#include <util/atomic.h>

void setup() {
	TCCR0A = 0;
	TCCR0B = 5;
	TIMSK0 = 1;
	sei();

	DDRB |= 0x20;
}

volatile unsigned long count = 0;  // bad executable if initialized to 0!!!

void loop() {
    ATOMIC_BLOCK(ATOMIC_FORCEON) {
	if (count > 50) {
		PORTB ^= 0x20;
		count = 0;
	}
    }
}

ISR(TIMER0_OVF_vect) {
	++count;
}

int main() {
	setup();

	while (1) {
		loop();
	}
}