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[HW] pull-up on SWDIO

Question asked by markus on Jul 13, 2011
Latest reply on Jul 13, 2011 by Andrew Queisser

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)?