The main loop stood still for half a second

What ArduPilot says:

• Internal errors 0x8000 (main_loop_stuck)

The main loop reads the sensors and drives the motors hundreds of times a second. A separate monitor looks every 100 ms whether it is still running. After 200 ms without it, the monitor starts writing what the loop was doing into the log (MON messages); after 500 ms, unless ArduPilot expected the delay (erasing flash, a calibration), it records main_loop_stuck. The l: number is then the source line where the main loop was waiting for a lock. If the loop stays stuck, the watchdog restarts the board.

What to do, most likely first

1. Lua scripts run at a raised priority and keep the processor from the main loop.

   CheckSCR_THD_PRIORITY is not 0. Stop the scripts and see whether the error still comes.

   FixSet SCR_THD_PRIORITY back to 0 (Normal) and reboot.

2. Two parts of the firmware wait for each other (a deadlock): a bug, more likely in a beta or custom build.

   Check“CRITICAL Deadlock …” may appear with it. Note the l: number and what you were doing: a parameter download, a log download, a calibration.

   FixUse the stable release and report the line, the log and the steps to the ArduPilot developers.

3. A sensor or storage device answers so slowly that the loop waits on it.

   Checkspi_fail or i2c_isr is in the same mask, or the error stops when a device is unplugged.

   FixReplace that device or its cable.

Half a second without control is enough to lose a copter. Do not fly until you can run a whole battery on the bench, props off, without it.

Parameters to look at

Where to do it:Setup & Tuning GuideLua Applet Catalog