// **************************************************************************
// *                                                                        *
// *                   ATtiny85 1Hz-Blinker mit timer 0                     *                                         *
// *                                                                        *
// **************************************************************************


#define F_CPU 8000000L
int systick = 0;
int n = 0;

ISR(TIMER0_COMPA_vect) // 1 ms Timer ISR
{
        static uint16_t count=0;
        if ( ++count > 999 ){
        systick = 1;
        count =0;                       
        }
}

void init_timer()
{
  PORTB = 0xff;
  DDRB = 0xff;
  TCCR0A = (1<<CS01);         // Prescaler 8
  OCR0A = (F_CPU/1024)-1;     // 1 ms at 1 MHz cpu clock
  TIMSK |= 1<<OCIE0A;         // Timer/Counter0 Output Compare Match A Interrupt Enable
  sei();                      // globale Interruptfreigabe
}

void setup(){
   init_timer();
   }

void loop(){
  if ( systick ){ // do something every 1 second
     PORTB ^= 1;
     systick = 0;
     }
  }