Code to make a very low power delay by setting the watchdog timer and then sleeping.

AVR watchdog delay

// Goto bed, will only wake up on button press interrupt (if enabled) or WDT

static void deepSleep(void) {
	set_sleep_mode( SLEEP_MODE_PWR_DOWN );
    sleep_enable();
    sleep_cpu();        // Good night    
}  


// Don't do anything, just the interrupt itself will wake us up
// TODO: If we ever need more code space, this could be replaced by an IRET in the VECTOR table. 

EMPTY_INTERRUPT( WDT_vect );

#define HOWLONG_16MS   (_BV(WDIE) )
#define HOWLONG_32MS   (_BV(WDIE) | _BV( WDP0) )
#define HOWLONG_64MS   (_BV(WDIE) | _BV( WDP1) )
#define HOWLONG_125MS  (_BV(WDIE) | _BV( WDP1) | _BV( WDP0) )
#define HOWLONG_250MS  (_BV(WDIE) | _BV( WDP2) )
#define HOWLONG_500MS  (_BV(WDIE) | _BV( WDP2) | _BV( WDP0) )
#define HOWLONG_1S     (_BV(WDIE) | _BV( WDP2) | _BV( WDP1) )
#define HOWLONG_2S     (_BV(WDIE) | _BV( WDP2) | _BV( WDP1) | _BV( WDP0) )
#define HOWLONG_4S     (_BV(WDIE) | _BV( WDP3) )
#define HOWLONG_8S     (_BV(WDIE) | _BV( WDP3) | _BV( WDP0) )

// Goto sleep - get woken up by the watchdog timer or other interrupt like button
// This is very power efficient since chip is stopped except for WDT
// Note that if the timer was on entering here that it will stay on, so LED will still stay lit.

static void sleepFor( uint8_t howlong ) {
    
    wdt_reset();
    WDTCR =   howlong;              // Enable WDT Interrupt  (WDIE and timeout bits all included in the howlong values)
    
    sei();
    deepSleep();
    cli();
    
    WDTCR = 0;                      // Turn off the WDT interrupt (no special sequence needed here)
                                    // (assigning all bits to zero is 1 instruction and we don't care about the other bits getting clobbered
    
}