// Enable watchdog interrupt, set prescaling to 1 sec
void init_wdt()
{
    // Disable interrupts
    cli();
    // Start timed sequence
    // Set Watchdog Change Enable bit
    WDTCR |= (1<<WDCE);
    // Set new prescaler (1 sec), unset reset enable
    // enable WDT interrupt
    WDTCR = (1<<WDTIE)|(1<<WDP2)|(1<<WDP1);
    sei();
}