#include <avr/wdt.h>
  #include <avr/interrupt.h>
  #include <util/delay.h>
  volatile int f_wdt=0;
  int counter=0;
  ISR(WDT_vect)
    {
      if(f_wdt == 0)
        {
        f_wdt=1;
        }
      else
        {
        } 
    }
  int main(void) 
    {
    DDRB |= (1<<7); //portB.7 output
    DDRA=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)
    {
      if(f_wdt == 1)
        {
          /* Toggle the LED */
          PORTB ^=(1<<7);
          /* Don't forget to clear the flag. */
          f_wdt = 0;
          /* Re-enter sleep mode. */
          //enterSleep();
        }
      else
      {
        /* Do nothing. */
        _delay_ms(300);
        PORTA=~counter++;
      }
    }
    }