Probleme mit ISR bei AT90CAN128

#include <io.h>

#include <interrupt.h>

#define DEBUG

int main(){

SREG =0x80;//all interruots are availabe

sei();

TCCR0A = 0x85; //1 00 00 101b PWM Mode=0 wave=normal non PWM Precaler=1024

TCNT0  = 0x00; // timer startet bei diesem wert

TIMSK0 = 0x01; //Interrupts enabled

TIFR0  = 0x00; //Interrupt Flag is disabled

DDRA   = 0xFF; // Port A is output

for(;;){

}//end for endlosschleife

ISR(TIMER0_OVF_vect){//Interrrupt Service Routine

  if (PORTA == 0x00){

      PORTA = 0xFF;

    }//end if 

    else

    PORTA=0xFF;

}//end isr

return(0);

}//end main