How did you know those were the pins to use, BTW? I did some grepping around, but couldn’t find anything that seemed relevant. If it weren’t for you, I would have been stuck.
I’m glad I got that sorted. It has emboldened me to see if I can migrate a separate Arduino Uno project I have over to RIOT.
Part 1. If you use google you will find a pinout diagram for the BluePill.
Part 2. All the used targets for RIOT-OS have to endup linking to resources that are physically present on the chip.
This is defined for every possible board.
So for BluePill this is: …/RIOT/boards/common/blxxxpill/include There you will find the actual used device. In board_common.h you will see that the STDIO output is assiged for UART 2 - resources start counting from 0 - on the Bluepill. The pinout diagram tells you which physical pins.
Keep this location in mind if you plan to use more of the Bluepill internal devices!