AVR® Low Power Sleep Modes

AVR® 8-bit microcontrollers include several sleep modes that enable the application to shut down unused modules in the MCU, thereby saving power. The AVR device provides various sleep modes allowing you to tailor the power consumption to the application requirements.

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.

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Interrupts

If an enabled interrupt occurs while the MCU is in a sleep mode, the MCU wakes up. The MCU is then halted for four cycles in addition to the start-up time, executes the interrupt routine, and resumes execution from the instruction following SLEEP. The contents of the Register File and SRAM are unaltered when the device wakes up from sleep.

Reset

If a reset occurs during sleep mode, the MCU wakes up and executes from the Reset Vector.

Brief AVR Sleep Mode Summary



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Idle Mode

When the SM[2:0] bits are written to '000', the SLEEP instruction makes the MCU enter Idle mode, stopping the CPU but allowing the Serial Peripheral Interface (SPI), Universal Synchronous/Asynchronous Receiver/Transmitter (USART), Analog Comparator, 2-wire Serial Interface, Timer/Counters, Watchdog, and the interrupt system to continue operating. This sleep mode basically halts the CPU clock (clkCPU) and Flash clock (clkFLASH), while allowing the other clocks to run.
Idle mode enables the MCU to wake up from external 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.

Power-Down Mode

When the SM[2:0] 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 2-wire Serial Interface address watch, and the Watchdog continue operating (if enabled). Only one of these events can wake up the MCU:

  • External Reset
  • Watchdog System Reset
  • Watchdog Interrupt
  • Brown-out Reset
  • 2-wire Serial Interface address match
  • External level interrupt on INT
  • Pin change interrupt

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. The start-up time is defined by the Start Up Timer (SUT) and Clock Select (CKSEL) Fuses.

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.

Power-save Mode

When the SM[2:0] 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 Timer Interrupt Mask Register (TIMSK2), and the Global Interrupt Enable bit in Status Register (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. Even if the synchronous clock is running in Power-save, this clock is only available for Timer/Counter2.

Standby Mode

When the SM[2:0] bits are written to '110' and an external 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.

Extended Standby Mode

When the SM[2:0] bits are written to '111' and an external clock option is selected, the SLEEP instruction makes the MCU enter Extended Standby mode. This mode is identical to Power-Save mode with the exception that the Oscillator is kept running. From Extended Standby mode, the device wakes up in six clock cycles.

Tips and Tricks to Minimize Power Consumption

There are several possibilities to consider when trying to minimize the power consumption in an AVR controlled system. In general, sleep modes should be used as much as possible, and the sleep mode should be selected so that as few as possible of the device’s functions are operating. All functions not needed should be disabled.

In particular, the following modules may need special consideration when trying to achieve the lowest possible power consumption.

Analog to Digital Converter

If enabled, the ADC will be enabled in all sleep modes. To save power, the ADC should be disabled before entering any sleep mode. When the ADC is turned off and on again, the next conversion will be an extended conversion.

Analog Comparator

When entering Idle mode, the Analog Comparator should be disabled if not used. When entering ADC Noise Reduction mode, the Analog Comparator should be disabled. In other sleep modes, the Analog Comparator is automatically disabled. However, if the Analog Comparator is set up to use the Internal Voltage Reference as input, the Analog Comparator should be disabled in all sleep modes. Otherwise, the Internal Voltage Reference will be enabled, independent of sleep mode.

Brown-Out Detector

If the Brown-Out Detector (BOD) is not needed by the application, this module should be turned off. If the BOD is enabled by the BODLEVEL Fuses, it will be enabled in all sleep modes, and hence, always consume power. In the deeper sleep modes, this will contribute significantly to the total current consumption.

Internal Voltage Reference

The Internal Voltage Reference will be enabled when needed by the Brown-Out Detection, the Analog Comparator or the Analog-to-Digital Converter. If these modules are disabled as described in the sections above, the internal voltage reference will be disabled and it will not be consuming power. When turned on again, the user must allow the reference to start up before the output is used. If the reference is kept on in
sleep mode, the output can be used immediately.

Watchdog Timer

If the Watchdog Timer is not needed in the application, the module should be turned off. If the Watchdog Timer is enabled, it will be enabled in all sleep modes and hence always consume power. In the deeper sleep modes, this will contribute significantly to the total current consumption.

Port Pins

When entering a sleep mode, all port pins should be configured to use minimum power. The most important is then to ensure that no pins drive resistive loads. In sleep modes where both the I/O clock (clkI/O) and the ADC clock (clkADC) are stopped, the input buffers of the device will be disabled. This ensures that no power is consumed by the input logic when not needed. In some cases, the input logic is needed for detecting wake-up conditions, and it will then be enabled. Refer to the section Digital Input Enable and Sleep Modes for details on which pins are enabled. If the input buffer is enabled and the input signal is left floating or have an analog signal level close to VCC/2, the input buffer will use excessive power.

For analog input pins, the digital input buffer should be disabled at all times. An analog signal level close to VCC/2 on an input pin can cause significant current even in active mode. Digital input buffers can be disabled by writing to the Digital Input Disable Registers (DIDR0 for ADC, DIDR1 for AC).

On-chip Debug System

If the On-chip debug system is enabled by the DWEN Fuse and the chip enters sleep mode, the main clock source is enabled and hence always consumes power. In the deeper sleep modes, this will contribute significantly to the total current consumption.

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