When the SMCR.SM is written to '0x010', the
SLEEP instruction makes the MCU enter Power-Down mode. In this mode, the external
Oscillator is stopped, while the external interrupts and the Watchdog continue operating
(if enabled).
Only an these events can wake up the MCU:
Watchdog System Reset
External level interrupt on INT0
Pin change interrupt
This sleep mode basically halts all generated clocks, allowing operation of
asynchronous modules only.