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We would like to program several (6) PCB successively, each one working with a Nordic nRF52 µC. The idea is to have only one J-Link with the SWD lines (SWDIO, SWCLK and VTREF) common to my six boards.

I have seen some people multiplexing SWDIO, SWCLK and VTERF (for example here: https://devzone.nordicsemi.com/f/nordic-q-a/35094/what-is-the-swd-driver-impedance-of-swdio-line -- you need to scroll a bit) but I was wondering if only multiplexing VTREF could be enough. I mean: if the VTREF pin is used as supply pin (by providing 5V from the J-Link and converting it into 3.0V through a LDO), can the SWD work correctly if only one board is supplied while the others are not?

I do not see any problem to that solution but maybe I have missed something (pullup/down resistors causing problems, etc.).

Best regards,

Nil

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You probably know a MCU digital IO pin features ESD protection diodes that typically look like this:

Classical ESD protection

(from IC protection diodes confusion)

If VDD is actually floating and there is 3.3 V on IO, VDD will be supplied though D1 and you'll get 3.3 V minus diode's forward voltage on VDD (i.e. ~2.8 V). Decoupling capacitors of target chip will get charged through IC pins. If chip consumption is low enough and VDD in range, it will run properly.

In your situation, all MCUs will be connected on the SWD lines. All of them will get powered through protection diodes, they will most probably wake up, and all of them will conflict on the SWD lines. Expect bad results.

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