system bootloader via USB - DFU

[rohoog]

This is not really an issue, but it might be useful information for users of this board that want to use the system bootloader.

I have noticed that the usb/dfu functionality does not work reliably if the Rx of USART1 is left floating. If you want to use the bootloader to load the flash using USB and you don't use the USART1, please ground the Rx (PA10) pin.

ST knowledge base article https://community.st.com/s/article/FAQ-DFU-mode-with-system-bootloader-is-not-working-while-USB-does is not the answer for this board.

This article https://electronics.stackexchange.com/questions/586676/stm32f401-fails-to-enter-dfu-mode describes my finding too.

This board uses an 8 MHz crystal, while the ST discovery boards use 25 MHz for the STM32F4xx. If the 8 MHz crystal is replaced with a 25 MHz one, the ST bootloader usb functionality is also much more reliable.