[HW] pull-up on SWDIO

[wiesendanger2]

Posted on July 13, 2011 at 14:26

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 K

Ω

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)?

[Tesla DeLorean]

Posted on July 13, 2011 at 15:05

The internal 30-50K pull-up is more than adequate to meet ARM's suggested need, which is a minimal pull-up typical of silicon processes ~100K.

[aqueisser]

Posted on July 13, 2011 at 18:23

Our older development boards for the F1xx have pullups, some of them strong (10K). Newer boards (F2xx and Lxxx) seem to have no-loads.

I think you're probably ok without any external resistors for SWD. For JTAG it's harder to decide since it might be a bus. It may also depend on which probe you're using and whether it's the default pin. 

Andrew