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Sleep modes and waking up

A battery device spends most of its time asleep. What stays on, what wakes it up, and what is lost while it sleeps.

Core

Read firstWriting a power budget
Deeper sleep saves power but wakes more slowly Three sleep depths: each deeper one switches off more clocks, regulators and memory, so it draws less power but takes longer to wake, and the deepest loses RAM contents. An RTC alarm, a pin change from a sensor's interrupt line, or a peripheral event wakes the microcontroller. power drawn wake-up time Not to scale Light sleep some clocks off Deeper sleep more clocks and regulators off Deepest sleep memory off too: RAM contents lost What triggers a wake-up RTC alarm Digital sensor watches its own threshold Peripheral event interrupt line: a pin change so the chip can stay asleep until something actually changes (a button can raise a pin change too) such as an incoming byte Microcontroller asleep
Each deeper sleep mode draws less power but takes longer to wake, and the deepest loses RAM; an RTC alarm, a pin change or a peripheral event wakes the chip.

Key points

  1. Microcontrollers offer several sleep depths; deeper modes switch off more clocks, regulators and memory, saving power at the cost of longer wake-up times and, in the deepest modes, lost RAM contents.12
  2. Common wake sources are an RTC alarm, a pin change from a button or sensor interrupt, and a peripheral event such as an incoming byte.12
  3. Many digital sensors can watch a threshold themselves and raise an interrupt line, letting the main processor stay asleep until something actually changes.3

Where to go next

Sources

  1. ESP-IDF: Sleep Modes · Espressif · Project docs
  2. RP2350 datasheet · Raspberry Pi · Datasheet
  3. LIS2DW12 datasheet (ST DocID029682 Rev 4) · STMicroelectronics · Datasheet