About SWD signals

[v_tamakov]

Posted on June 14, 2018 at 16:00

Hello, please tell me, what is the minimum number of signals required for the SWD interface: SWDIO, SWCLK? NRST is also required or possible without it?

[Vitaliy Chernobay]

Posted on June 14, 2018 at 21:14

NRST is desirable to have, but it works without it

[Geoffrey1]

Posted on June 15, 2018 at 03:05

You definitely want nRST.   It is possible to configure the stm32 components so that the debugger interface is disabled or the pins are reconfigured (e.g. st32l4 parts in standby or shutdown).  In this situation, the only way to connect the debugger is to reset the part.  It may not be enough to simply have a reset button -- the debugger may need to 'connect under reset'.   My strong recommendation is to provide all three signals.

Geoffrey

[v_tamakov]

Posted on June 15, 2018 at 12:49

Thanks for your advice, I will follow them

[Tesla DeLorean]

Posted on June 15, 2018 at 18:05

SWO (PB3) is also generally desirable, although not helpful on CM0(+) device, and currently broken on H7.

Some debuggers need VTarget to power buffers in the pod, and obviously you need a common ground.

If you don't have room for headers or test points, consider edge castellations, or break-off portions of the PCB

Consider your needs for manufacturing test and product development when scoping what is in fact 'minimal'