Do I need an external pull-up resistor on the SWDIO line for Serial Wire Debug?
The STM32 reference manual (Doc ID 13902 Rev 13) is not quite on this matter…
Section 31.4.3: Internal pull-up and pull-down on JTAG pins
JTMS/SWDIO: Internal pull-up
Having embedded pull-ups and pull-downs removes the need to add external resistors.
Section 31.8: SW protocol introduction
For SWDIO bidirectional management, the line must be pulled-up on the board (100 KΩ recommended by ARM).
So I am getting mixed messages; besides I could not find any info on the value of the internal pull up resistor – is it the usual weak pull-up resistor found on the other pins (30-50kohms)?