#include <avr/io.h>
#include <avr/interrupt.h>

void initialization()
{
    // set up timer with prescaler = 64 and Normal mode
    TCCR1A = 0;
    TCCR1B |= (1 << CS12)|(1 << CS10);
    TCNT1 = 2^6-25000;  // initialize counter
    TIMSK1 |= (1 << TOIE1);    // enable timer overflow interrupt
    sei();                      // enable global interrupt
}

int main(void)
{
    DDRC |= (1 << PC0); // connect led to pin PC0
    initialization();
    while(1) {};
}

ISR (TIMER1_OVF_vect)
{
    TCNT1 = 2^6-25000;  // initialize counter
    PORTC ^= (1 << PC0); // toggle led here
}