Data Sheet

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ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
Atmel-8271H-AVR- ATmega-Datasheet_08/2014
INT0 or INT1, or a pin change interrupt 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. ”External Interrupts” on page 71. The start-up
time is defined by the SUT and CKSEL Fuses as described in ”System Clock and Clock Options” on page 27.
When waking up from 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, as described in ”Clock
Sources” on page 28.
10.6 Power-save Mode
When the SM2...0 bits are written to 011, the SLEEP instruction makes the MCU enter Power-save mode. This
mode is identical to Power-down, with one exception:
If Timer/Counter2 is enabled, it will keep running during sleep. The device can wake up from either Timer
Overflow or Output Compare event from Timer/Counter2 if the corresponding Timer/Counter2 interrupt enable
bits are set in TIMSK2, and the Global Interrupt Enable bit in SREG is set.
If Timer/Counter2 is not running, Power-down mode is recommended instead of Power-save mode.
The Timer/Counter2 can be clocked both synchronously and asynchronously in Power-save mode. If
Timer/Counter2 is not using the asynchronous clock, the Timer/Counter Oscillator is stopped during sleep. If
Timer/Counter2 is not using the synchronous clock, the clock source is stopped during sleep. Note that even if
the synchronous clock is running in Power-save, this clock is only available for Timer/Counter2.
10.7 Standby Mode
When the SM2...0 bits are 110 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.