// All values/comments are valid with ATmega8 running at Fosc = 4.000MHz

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

// TIMER0 with prescaler clkI/O/1024
#define TIMER0_PRESCALER      (1 << CS02) | (1 << CS00)


void main()
{
   // ... do something ...

   // init Timer0
   TIMSK |= (1 << TOIE0);        // interrupt enable - here overflow
   TCCR0 |= TIMER0_PRESCALER;    // use defined prescaler value
   sei();                        // global interrupt enable

   // ... do something ...

   while (1)
   {
      // ... do something ...

   } /* end of while(1)  */
}

// *** Interrupt Service Routine *****************************************

// Timer0 overflow interrupt handler (~65ms 4MHz@1024 precale factor)
ISR(TIMER0_OVF_vect)
{
   // handle interrupt
}