System Startup

The PX4 boot is controlled by shell scripts in the ROMFS/px4fmu_common/init.d folder.

All files starting with a number and underscore (e.g. 10000_airplane) are canned airframe configurations. They are exported at build-time into an airframes.xml file which is parsed by QGroundControl for the airframe selection UI. Adding a new configuration is covered here.

The remaining files are part of the general startup logic, and the first executed file is the rcS script, which calls all other scripts.

Debugging the System Boot

A failure of a driver of software component can lead to an aborted boot.

An incomplete boot often materializes as missing parameters in the ground control stations, because the non-started applications did not initialize their parameters.

The right approach to debug the boot sequence is to connect the system console and power-cycle the board. The resulting boot log has detailed information about the boot sequence and should contain hints why the boot aborted.

Common boot failure causes

  • A required sensor failed to start
  • For custom applications: The system was out of RAM. Run the free command to see the amount of free RAM.
  • A software fault or assertion resulting in a stack trace

Replacing the System Startup

In most cases customizing the default boot is the better approach, which is documented below. If the complete boot should be replaced, create a file /fs/microsd/etc/rc.txt, which is located in the etc folder on the microSD card. If this file is present nothing in the system will be auto-started.

Customizing the System Startup

The best way to customize the system startup is to introduce a new airframe configuration. If only tweaks are wanted (like starting one more application or just using a different mixer) special hooks in the startup can be used.

The system boot files are UNIX FILES which require UNIX LINE ENDINGS. If editing on Windows use a suitable editor.

There are three main hooks. Note that the root folder of the microsd card is identified by the path /fs/microsd.

  • /fs/microsd/etc/config.txt
  • /fs/microsd/etc/extras.txt
  • /fs/microsd/etc/mixers/NAME_OF_MIXER

Customizing the Configuration (config.txt)

The config.txt file is loaded after the main system has been configured and before it is booted and allows to modify shell variables.

Starting additional applications

The extras.txt can be used to start additional applications after the main system boot. Typically these would be payload controllers or similar optional custom components.

Calling an unknown command in system boot files may result in boot failure. Typically the system does not stream mavlink messages after boot failure, in this case check the error messages that are printed on the system console.

The following example shows how to start custom applications:

  • Create a file on the SD card etc/extras.txt with this content:
    custom_app start
  • A command can be made optional by gating it with the set +e and set -e commands:

    set +e
    optional_app start      # Will not result in boot failure if optional_app is unknown or fails
    set -e
    mandatory_app start     # Will abort boot if mandatory_app is unknown or fails

Starting a custom mixer

By default the system loads the mixer from /etc/mixers. If a file with the same name exists in /fs/microsd/etc/mixers this file will be loaded instead. This allows to customize the mixer file without the need to recompile the Firmware.


The following example shows how to add a custom aux mixer:

  • Create a file on the SD card, etc/mixers/gimbal.aux.mix with your mixer content.
  • Then to use it, create an additional file etc/config.txt with this content:
    set MIXER_AUX gimbal
    set PWM_AUX_OUT 1234
    set PWM_AUX_DISARMED 1500
    set PWM_AUX_MIN 1000
    set PWM_AUX_MAX 2000
    set PWM_AUX_RATE 50

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