ADC Noise Reduction Mode

When the SM[2:0] 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 2-wire Serial Interface address watch, Timer/Counter2⁽¹⁾, 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 from the ADC Conversion Complete interrupt, only these events can wake up the MCU from ADC Noise Reduction mode:

External Reset
Watchdog System Reset
Watchdog Interrupt
Brown-out Reset
2-wire Serial Interface address match
Timer/Counter2 interrupt
SPM/EEPROM ready interrupt
External level interrupt on INT
Pin change interrupt

Note: 1. Timer/Counter2 will only keep running in asynchronous mode.