// sketch_05_06_sleep_external_wake
#include <avr/sleep.h>

const int ledPin = 13;
const int inputPin = 2;

volatile boolean flag;

void setup() 
{
  pinMode(ledPin, OUTPUT);
  pinMode(inputPin, INPUT_PULLUP);
  goToSleep();
}

void loop() 
{
  if (flag)
  {
    doSomething();
    flag = false;
    goToSleep();
  }
}

void setFlag()
{
  flag = true; 
}

void goToSleep()
{
   set_sleep_mode(SLEEP_MODE_PWR_DOWN);
   sleep_enable();
   attachInterrupt(0, setFlag, LOW); // pin D2
   sleep_mode(); // sleep now
   // Now asllep until LOW interrupt, then..
   sleep_disable();
   detachInterrupt(0);
}

void doSomething()
{
  for (int i = 0; i < 20; i++)
  {
    digitalWrite(ledPin, HIGH);
    delay(200);
    digitalWrite(ledPin, LOW);
    delay(200);
  }
}