Power-Down Mode

When the SM[2:0] bits are written to '010', the SLEEP instruction makes the MCU enter the Power-Down mode. In this mode, the external oscillator is stopped, while the external interrupts, the two-wire serial interface address watch, and the Watchdog continue operating (if enabled).

Only one of these events can wake up the MCU: This sleep mode basically halts all generated clocks, allowing operation of asynchronous modules only.
Note: If a level triggered interrupt is used for wake-up from power-down, the required level must be held long enough for the MCU to complete the wake-up to trigger the level interrupt. If the level disappears before the end of the start-up time, the MCU will still wake up, but no interrupt will be generated. The start-up time is defined by the SUT and CKSEL Fuses.

When waking up from the Power-Down mode, there is a delay from the wake-up condition occurs until the wake-up becomes effective. This allows the clock to restart and become stable after having been stopped. The wake-up period is defined by the same CKSEL fuses that define the Reset time-out period.