Energy Efficiency
There are five sleep modes to select from:
- Idle Mode
- Power Down
- Power Save
- Standby
- Extended Standby
To enter any of the sleep modes, the Sleep Enable bit in the Sleep Mode Control Register (SMCR.SE) must be written to '1' and a SLEEP instruction must be executed. Sleep Mode Select bits (SMCR.SM[2:0]) select which sleep mode (Idle, Power-down, Power-save, Standby, or Extended Standby) will be activated by the SLEEP instruction. Sleep options shown are from ATmega328PB.
Idle Mode
When the SM[2:0] bits are set to '000', the SLEEP instruction puts the MCU into Idle mode, stopping the CPU but allowing peripherals like SPI, USART, Analog Comparator, 2-wire Serial Interface, Timer/Counters, Watchdog, and interrupts to continue operating. This mode halts the CPU and Flash clocks but keeps other clocks running. The MCU can wake up from both external and internal interrupts.
Power-Down Mode
When the SM[2:0] bits are set to '010', the SLEEP instruction puts the MCU into Power-Down mode, stopping the external oscillator. Only external interrupts, 2-wire Serial Interface address watch, and Watchdog (if enabled) can wake the MCU. This mode halts all generated clocks, allowing only asynchronous modules to operate.
Power-Save Mode
When the SM[2:0] bits are set to '011', the SLEEP instruction puts the MCU into Power-Save mode, similar to Power-Down but with Timer/Counter2 running if enabled. The device can wake up from Timer Overflow or Output Compare events from Timer/Counter2.
Standby Mode
When the SM[2:0] bits are set to '110' and an external clock option is selected, the SLEEP instruction puts the MCU into Standby mode, similar to Power-Down but with the oscillator running. The device wakes up in six clock cycles.
Extended Standby Mode
When the SM[2:0] bits are set to '111' and an external clock option is selected, the SLEEP instruction puts the MCU into Extended Standby mode, similar to Power-Save but with the oscillator running. The device wakes up in six clock cycles.
AVR® 8-bit microcontrollers include several sleep modes to save power. The AVR device can also lower power consumption by shutting down the clock for select peripherals via a register setting. That register is called the Power Reduction Register (PRR).
The PRR provides a runtime method to stop the clock to select individual peripherals. The current state of the peripheral is frozen, and the I/O registers cannot be read or written. Resources used by the peripheral when stopping the clock will remain committed. Hence, the peripheral should, in most cases, be disabled before stopping the clock. Waking up a module, which is done by clearing the bit in PRR, puts the module into the same state as before shutdown. PRR clock shutdown can be used in Idle mode and Active mode to significantly reduce the overall power consumption. In all other sleep modes, the clock is already stopped.
Tips to Minimize Power Consumption
- Analog to Digital Converter (ADC): Disable the ADC before entering sleep modes to save power.
- Analog Comparator: Disable the Analog Comparator in Idle and ADC Noise Reduction modes if not used.
- Brown-Out Detector (BOD): Turn off the BOD if not needed, as it consumes power in all sleep modes.
- Internal Voltage Reference: Disable it if not needed by the ADC, Analog Comparator, or BOD.
- Watchdog Timer: Turn off the Watchdog Timer if not needed, as it consumes power in all sleep modes.
- Port Pins: Configure port pins to minimize power usage, ensuring no pins drive resistive loads.
- On-chip Debug System: Disable the On-chip debug system if not needed, as it consumes power in sleep modes.
