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Power-Down Mode When the SM[1:0] bits are written to 10, the SLEEP instruction makes the MCU enter Powerdown mode. In this mode, the Oscillator is stopped, while the external interrupts, and the Watchdog continue operating (if enabled). Only an External Reset, a Watchdog Reset, a Brown-out Reset, an external level interrupt on INT0, or a pin change interrupt can wake up the MCU. This sleep mode halts all generated clocks, allowing operation of asynchronous modules only. ... Standby Mode When the SM[1:0] bits are 11 and an external crystal/resonator clock option is selected, the SLEEP instruction makes the MCU enter Standby mode. This mode is identical to Power-down with the exception that the Oscillator is kept running. From Standby mode, the device wakes up in six clock cycles. ... External Interrupts are triggered by the INT0 pin or any of the PCINT[11:0] pins. Observe that, if enabled, the interrupts will trigger even if the INT0 or PCINT[11:0] pins are configured as outputs. This feature provides a way of generating a software interrupt. Pin change 0 interrupts PCI0 will trigger if any enabled PCINT[7:0] pin toggles. The PCMSK0 and PCMSK1 Registers control which pins contribute to the pin change interrupts. Pin change interrupts on PCINT[11:0] are detected asynchronously, which means that these interrupts can be used for waking the part also from sleep modes other than Idle mode.