#include <Sleep.h>

#define DEBUG 1

int INTpin = 2;

volatile boolean wdt_expired=0;

// Interrupt Serive Routine that will be executed when the Watchdog Timer expired.
ISR(WDT_vect) {
  wdt_expired=1;
}

//****************************************************************
// 0=16ms, 1=32ms,2=64ms,3=128ms,4=250ms,5=500ms
// 6=1 sec,7=2 sec, 8=4 sec, 9= 8sec
void setup_watchdog(int ii) {

  byte bb;
  int ww;
  if (ii > 9 ) ii=9;
  bb=ii & 7;
  if (ii > 7) bb|= (1<<5);
  bb|= (1<<WDCE);
  ww=bb;

  MCUSR &= ~(1<<WDRF);
  // start timed sequence
  WDTCSR |= (1<<WDCE) | (1<<WDE);
  // set new watchdog timeout value
  WDTCSR = bb;
  WDTCSR |= _BV(WDIE);
}

void setup(void)
{
  pinMode(INTpin, INPUT);
#if DEBUG  
  Serial.begin(9600);
#endif  
}

void loop(void)
{
#if DEBUG    
  Serial.print(millis(), DEC);
  Serial.print(" I'm awake, caused by ");
#endif  
  if (wdt_expired==1){
    wdt_expired=0;
#if DEBUG    
    Serial.println("the WDT");
#endif    

  // Do something, like sending a heatbeat
  // sometimes

  }
  else
  {
    // External interrupt triggered!
#if DEBUG    
    Serial.println("INT0");
#endif    

  Serial.print("Doing some work...");
  delay(400);
  Serial.println("finished!");
  }
  
  //
#if DEBUG    
  Serial.print(millis(), DEC);
  Serial.println(" Sleeping...");
  delay(100);     // wait for serial to finish  
#endif                        
  setup_watchdog(9);
  Sleep.powerDownAndWakeupExternalEvent(0);     // sleep function called here
}