#include <avr/sleep.h>

void setup () 
{
  set_sleep_mode (SLEEP_MODE_PWR_DOWN);  
  noInterrupts ();           // timed sequence follows
  sleep_enable();
 
  // turn off brown-out enable in software
  MCUCR = bit (BODS) | bit (BODSE);  // turn on brown-out enable select
  MCUCR = bit (BODS);        // this must be done within 4 clock cycles of above
  interrupts ();             // guarantees next instruction executed
  sleep_cpu ();              // sleep within 3 clock cycles of above
}  // end of setup

void loop () { }