#include <avr/wdt.h>
#include <avr/interrupt.h>
#include <util/delay.h>
volatile int f_wdt=0;
int counter=1;
ISR(WDT_vect)
{
  PORTA=~counter++;
}
int main(void) {
  DDRA=0xFF;
  PORTA = 0xFF;
  /* Clear the reset flag. */
  MCUSR &= ~(1<<WDRF);
  /* In order to change WDE or the prescaler, we need to
  * set WDCE (This will allow updates for 4 clock cycles).
  */
  WDTCSR |= (1<<WDCE) | (1<<WDE);
  /* set new watchdog timeout prescaler value */
  WDTCSR = 1<<WDP2 | 1<<WDP1  | 1<<WDP0; /* 2.0 seconds */
  // wdt_enable(WDTO_4S); // don't used, cause system reset.
  /* Enable the WD interrupt (note no reset). */
  WDTCSR |= _BV(WDIE);
  sei(); // don't forget to activate global interrupt
  while(1)
  {
  }
}