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Docs Firmware & Software Firmware Deep Sleep

Firmware Deep Sleep

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How the ESP32 enters deep sleep between beacon bursts, what stays powered, and how the device wakes on schedule.

Firmware Deep Sleep

The beacon spends most of its life asleep. Deep sleep is the difference between a battery that lasts hours and one that lasts days.

What deep sleep does

On the ESP32, deep sleep:

  • Stops the CPU and most peripherals
  • Keeps the RTC domain powered (the RTC timer that schedules the wake)
  • Drops current draw to roughly 10-150 uA depending on the board’s regulators
  • Loses RAM contents (the ULP coprocessor is not used in this project)

What is preserved

The firmware stores across sleep:

DataLocationWhy
Last GPS fixRTC RAMTransmit immediately after wake
ConfigurationNVS flashSurvives power loss too
Wake scheduleRTC timerThe only clock that runs in sleep

The wake sequence

  1. The RTC timer fires after the sleep interval.
  2. The ESP32 boots in about 50 ms.
  3. The firmware checks the wake cause; if it is the timer, it resumes the beacon cycle without a full reconfiguration of the radio.
  4. After the burst, deep sleep is re-entered.

Peripherals during sleep

The OLED, buzzer and radio are powered down. The GPS module is handled separately: either it stays powered to keep its almanac warm (higher draw) or it is cut from power and does a cold start each cycle (longer fix time). The firmware setting GPS power saving selects between these.

Measuring the sleep current

Use a multimeter in series with the battery to verify your board actually sleeps. A board with an always-on regulator and an LED that never turns off can draw 10 mA in “sleep”, which would halve the runtime. See Power Budget and Runtimes for expected numbers.