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Docs Firmware & Software Boot Loop Troubleshooting

Boot Loop Troubleshooting

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The beacon that restarts endlessly: brownouts, watchdog resets, flash corruption, and how to catch the loop in the act.

Boot Loop Troubleshooting

A beacon that reboots forever looks dead and is often one measurement from fixed. This page is the boot-loop isolation sequence.

The loop types

PatternLikely cause
Rebooting during TX burstsPower sag (see Battery Troubleshooting)
Rebooting on a scheduleWatchdog timeout: firmware bug (see Firmware Watchdog)
Rebooting at randomBrownout detection, or marginal power
Rebooting after flashingFlash corruption or wrong partition table

Catch it in the act

  1. Connect the serial monitor before power-on (see Serial Debug System).
  2. Power on and watch the boot log.
  3. The last line before the reset names the cause: “Brownout detector was triggered”, “Watchdog timer expired”, “Guru Meditation Error”, or a clean reset with no message.

Brownout

The ESP32 has a built-in brownout detector that resets the chip when the supply dips. A “Brownout detector was triggered” line means the 3.3 V rail collapsed, almost always from the TX burst on a weak cell or a thin supply wire. Fixes: fresh cell, thicker power wires, decoupling capacitor at the board input.

Watchdog

“Task watchdog got triggered” or a reset at a fixed interval means a firmware loop overran its window. This is a firmware bug: update to the latest firmware and report the scenario (see Contributing).

Flash corruption

Resets immediately after a flash, or “invalid header” lines, mean the flash contents are wrong. Reflash completely (erase first), then factory-reset the configuration (see Factory Reset and Recovery).

The last-resort loop

If the beacon loops too fast to flash, hold BOOT while connecting so it stays in flash mode, erase the flash, and flash fresh. See Flash Troubleshooting for the mode entry.