LIBRARYPower modes and sleep.

A microcontroller does not have to run flat out. Sleep modes shut down parts of the chip to cut its current from milliamps down to microamps, and on a battery that is the difference between hours and months. A board that spends almost all of its time asleep can run for a very long time on a small cell.

Active, light sleep, deep sleep

In active mode the CPU and radio run and the chip draws the most current. Light sleep pauses the CPU but keeps its state and memory alive, so it wakes quickly and picks up where it left off. Deep sleep powers down almost everything, keeping only a tiny always-on domain, which drops the current to microamps but loses most of the chip's state, so it effectively restarts on wake. Each step trades responsiveness for lower current.

What wakes it

A sleeping chip wakes on a source you configure ahead of time: a timer after a set interval, a pin changing level, or a peripheral event. A typical sensor node deep-sleeps on a timer, wakes, takes a reading, sends it over the radio, and goes back to sleep, spending a few milliseconds awake for every several seconds asleep.

Why sleep dominates battery life

When a board is awake for a few milliseconds and asleep for seconds at a stretch, the sleep current sets the average almost by itself, and the brief active current barely counts. Halving the deep-sleep current can nearly double the runtime, while shaving the active current barely moves the needle. That is why low-power design chases deeper, more frequent sleep; slowing the CPU down barely helps.