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. The sleep modes shown in Fig. 1 are for the 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, Analogue 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.

Summary Table: SMCR Differences Across AVR Families

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), as presented in Fig. 2. The Power Reduction Register (PRR) is another place where AVR families differ quite a bit — sometimes even more than SMCR. If SMCR controls how deeply the CPU sleeps, then PRR controls which peripherals are powered at all.

The PRR provides a runtime method to stop the clock to select individual peripherals. The peripheral is currently 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 by clearing the bit in PRR puts it back into the state it was in before shutdown. The PRR clock shutdown can be used in Idle and Active modes to significantly reduce overall power consumption. In all other sleep modes, the clock is already stopped.

PRR Differences Across AVR Families

Tips to Minimise Power Consumption.

• Analogue to Digital Converter (ADC): Disable the ADC before entering sleep modes to save power.
• Analogue Comparator: Disable the Analogue 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, Analogue 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 minimise 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.