Datasheet

13.2. Idle Mode
When the SM2:0 bits are written to 000, the SLEEP instruction makes the MCU enter Idle mode, stopping
the CPU but allowing SPI, USART, Analog Comparator, ADC, Two-wire Serial Interface, Timer/Counters,
Watchdog, and the interrupt system to continue operating. This sleep mode basically halts clk
CPU
and
clk
FLASH
, while allowing the other clocks to run.
Idle mode enables the MCU to wake up from external triggered interrupts as well as internal ones like the
Timer Overflow and USART Transmit Complete interrupts. If wake-up from the Analog Comparator
interrupt is not required, the Analog Comparator can be powered down by setting the ACD bit in the
Analog Comparator Control and Status Register – ACSR. This will reduce power consumption in Idle
mode. If the ADC is enabled, a conversion starts automatically when this mode is entered.
13.3. ADC Noise Reduction Mode
When the SM2:0 bits are written to 001, the SLEEP instruction makes the MCU enter ADC Noise
Reduction mode, stopping the CPU but allowing the ADC, the external interrupts, the Two-wire Serial
Interface address watch, Timer/Counter2 and the Watchdog to continue operating (if enabled). This sleep
mode basically halts clk
I/O
, clk
CPU
, and clk
FLASH
, while allowing the other clocks to run.
This improves the noise environment for the ADC, enabling higher resolution measurements. If the ADC
is enabled, a conversion starts automatically when this mode is entered. Apart form the ADC Conversion
Complete interrupt, only an External Reset, a Watchdog Reset, a Brown-out Reset, a Two-wire Serial
Interface address match interrupt, a Timer/Counter2 interrupt, an SPM/EEPROM ready interrupt, an
External level interrupt on INT0 or INT1, or an external interrupt on INT2 can wake up the MCU from ADC
Noise Reduction mode.
13.4. Power-down Mode
When the SM2:0 bits are written to 010, the SLEEP instruction makes the MCU enter 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 an External Reset, a
Watchdog Reset, a Brownout Reset, a Two-wire Serial Interface address match interrupt, an External
Level Interrupt on INT0 or INT1, or an External Interrupt on INT2 can wake up the MCU. This sleep mode
basically halts all generated clocks, allowing operation of asynchronous modules only.
Note that if a level triggered interrupt is used for wake-up from Power-down mode, the changed level
must be held for some time to wake up the MCU. Refer to External Interrupts for details.
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.
Related Links
External Interrupts on page 69
Clock Sources on page 40
13.5. 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:
Atmel ATmega32A [DATASHEET]
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