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SWD EMC issues (pullup SWDIO)

Question asked by Werner Meier on May 18, 2017

AN2586 (Getting started with STM32F10xxx hardware development) recommends

JTMS/SWDIO: Internal pull-up

TCK/SWCLK: Internal pull-down

in an older STM32F100 Project where we had some field issues probably due to EMC (serial interface 3 seems to stop working after a few days or weeks), I realised, that through the use of the standard peripheral library both pins are set to pull-down**. After soldering a 10k pull up to the SWDIO, the problems seemed to stop to occur.

Has anyone experienced similar issues?

BTW the PCB traces to the debug interface are only about 2cm long, GND plane on other side of the PCB.





** this is shown when you observe GPIOA CNF13 / 14 through the debugger, but I just did a measurement with a DMM, where the correct Pullup on SWDIO and Pulldown to SWCLK can be confirmed