#include <mega8535.h>

unsigned int timecount = 0; // global counter variable

interrupt [TIM0_OVF] void timer0_ovf_isr(void)
{
    TCNT0 = 131;    // reload value for 0.004s ticks
    if (++timecount == 250)
        {
        PORTA ^= 0x01;   // toggle PORTA.0
        timecount = 0;  // clear for next 500ms interval
        }
}

void main(void)
{
    PORTA=0x00; // PORTA LOW
    DDRA=0x01;  // PORTA.0 AS OUTPUT

    TCCR0=0x05; // set system clock/1024 for Timer 0 input
    TCNT0=0x00; // start timer with 0 in timer

    TIMSK=0x01; // unmask Timer 0 overflow interrupt

    #asm("sei") // global enable interrupts

    while (1)
        ; // Do nothing (yet)

}