UART on STM32F411E-DISCO

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I’m toying with this nice evaluation board from STMicroelectronics and ran into a couple gotchas. Finding the solutions was harder than I anticipated, so I’d like to document them here.

You can’t connect to UART over USB

Some other boards provide a virtual COM port: you connect them to the computer and use screen, minicom or even echo to communicate with USART1. Not so with STM32F411E, because its ST-LINK debug probe is only V21, which does not support a virtual COM port2. This is a hardware limitation; you can’t fix it by upgrading ST-LINK firmware or re-installing the drivers.

The solution is to get an external USB-to-UART dongle and connect it to the board’s pins. That’s when you run into the second problem…

PA9 and PA10 don’t work

The first UART peripheral, USART1, can be connected to two sets of MCU pins: (PA9, PA10) and (PB6, PB7). Unfortunately, the Discovery board re-uses the first set for the USB peripheral. If you try to use UART with PA9 and PA10, you’ll see the green LED at the bottom of the board light up — but no signals on UART.

That’s why you should use the second set of pins, or one of the other two USART peripherals (USART2 or USART6).

  1. see UM1842, User Manual, Discovery kit with STM32F411VE MCU, in ST’s documentation section↩︎

  2. see TN1235, Technical Note, Overview of ST-LINK derivatives, in ST’s documentation section↩︎

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