demo.c

#include <avr/io.h>

#include <avr/interrupt.h>

volatile uint8_t min      = 0x00;

volatile uint8_t overflow   = 0x00;

int main(void) 

{

    DDRB    = 0x00;

    PORTB   = 0x00;

    TCCR1B  |= (1<<CS12) ;

    TIMSK   |= (1<<TOIE1);

    sei();

    for(;;) {

  PORTB = min;

    }

    return 0;

}

ISR(TIMER1_OVF1_vect)

{

    ++overflow;

    if (overflow == 5) {

      ++min;

      overflow = 0;

    }

}