STM32F4: USB HID Keyboard not working in BIOS (before Linux start)

[dariogoetz]

MCU: STM32F401CC

I have written an embassy-based keyboard firmware for a keyboard. The firmware works well in Linux. However, the keyboard does not produce anything when the laptop asks for a system password when booting (before Linux starts).

I could reproduce the issue with the code from the examples directory examples/stm32f4/src/bin/usb_hid_keyboard.rs (adjusted for clocks of the STM32F401CC, otherwise vanilla): Within my Linux system, it produces an a when pulling PC13 up. In the password prompt before starting Linux, however, nothing happens.

I have another firmware based on rtic, stm32f4xx-hal, and usb-device that does not have this issue, so I would exclude it being a hardware issue.

Any ideas, what the issue could be?