Title: Custom FMUv4 board on PX4 v1.14.0: USB works in bootloader, but MAVLink does not start in app firmware

PX4 Autopilot
1

Ảnh chụp màn hình 2026-04-08 093517
Ảnh chụp màn hình 2026-04-08 093517399×274 11 KB

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Hi everyone,

I am working on a custom flight controller based on FMUv4 and I would like to ask for help with a USB issue in PX4 v1.14.0.

My board is based on the FMUv4 design, but the USB VBUS hardware is different from the reference board.

On the FMUv4 reference design, the VBUS sense line is connected to PA9.
On my custom board, however, VBUS is tied directly to 5V5, so it is not routed to PA9 in the same way as the original FMUv4 board.

Because of this hardware difference, I disabled VBUS sensing in the bootloader by setting:

#define BOARD_USB_VBUS_SENSE_DISABLED 1

After doing that, I can still flash firmware normally, and the bootloader side works.

However, after the PX4 application firmware starts, USB MAVLink does not come up, so QGroundControl cannot connect over USB. As a workaround, I connected QGC through TELEM1, and after reducing the MAVLink data rate there, parameter download and setup became stable.

So at the moment:

  • SD card works

  • parameter saving works

  • telemetry MAVLink on TELEM1 works

  • but USB MAVLink does not start in the PX4 app

One thing I noticed is that in PX4 v1.14.0, I cannot find any cdcacm_autostart or mavlink start -d /dev/ttyACM0 section in ROMFS/px4fmu_common/init.d/rcS. It seems that in this version rcS only sources rc.serial, so I suspect the problem may be in the app-level board config, NuttX USB config, or generated serial startup rather than in rcS itself.

My main question is:

Which file should I modify in the PX4 application firmware to make USB MAVLink work on this custom board?

Should I be looking at:

  • the board-specific app config,

  • the NuttX board config,

  • USB/VBUS-related board definitions,

  • or the generated serial startup for v1.14.0?

To help narrow it down, I can provide:

  • my default.px4board

  • board-specific config files related to USB/VBUS

  • NuttX defconfig

  • generated rc.serial

  • ls /dev

  • mavlink status

  • schematic of the USB section

I also attached the USB schematic snippet showing that VBUS is tied to 5V5 on my custom board.

Any guidance on which file or subsystem I should patch would be greatly appreciated.

Thank you very much.