Is it safe to use USB dp/dm pins for SWD interface?

[stephan]

We are building a new version of a small device with the new nRF52832. Its only external connection is a micro USB plug that we use for charging - we do not need it as USB data interface. But would like to (mis-)use the two data pins for Serial Wire Debug (SWCLK/SWDIO) for flashing, debugging, and RTT. As long as charging is done with our standard charger, everything is fine, because we do not connect the USB data lines.

Is it safe, if the device is connected/charged via a USB host, e.g. a PC, or is it possible that an USB host causes mal-functioning of the nRF52832, i.e. stops/crashes the application code?

EDIT: Assume that signals are clipped to protect the SWD-pins electrically from over-voltage.

Has anyone experience with a similar setting?

[Alex]

Just an idea. To protect nRF chip from being erased or put in debug mode, put switches on D+ and D- lines and deactivate them if 5V present on USB connector. You can go further and enable them if 3V is on power pin meaning that it is your debugger connected rather than USB source, but that is extras.

[Luis]

Ok. So maybe when it is connected thru USB, chip is in debug mode or even reseting itself or erases firmware... We will have to test. Anyway it does not look like a problem for our application, where that USB is only connected when programming/debuging the device.

[Luis]

Alex, could you expand a bit more? What kind of switches can be enabled only when 3.3V is present and disabled when 5V is present?

[Luis]

I am not the guy who made the original question, but we followed this approach in two devices we are testing right now: a small helicopter drone and a beacon. Programming/debugging is working fine in both, and we connected them directly to an smartphone charger (5V) and nothing happened, they just turned on and worked. Still we have to do extra tests connecting them to PCs, other devices, etc, To see if anything rare happens, or firmware is erased, etc. But looking good so far!

[Alex]

Sorry for not being specific. I don't know what switch you found but most of them have low level input logic at around 0.5V. The idea is to shift it up to 3V so if Vdd has 0-3V then switch enable lines, if above 3V lines disconnected.