I have a Pico W running some app, and I use a Zero W to log (via UART) what is going on for debugging purposes.

The Zero W is configured so that UART0 is the primary UART, and the UART pins are connected to the Pico W's UART pins:

  • Zero W pin 6 (GND) – Pico W pin 3 (GND)
  • Zero W pin 8 (UART TX) – Pico W pin 2 (UART0 RX)
  • Zero W pin 10 (UART RX) – Pico W pin 1 (UART0 TX)

Both devices are connected to a small USB hub with a power switch for each port.

Now, if the Zero W is turned on, and I switch on the power for the Pico W, the Zero W resets (crashes?) and starts booting again.

Why could this be happening?

Is there some way this comes through the linked GND wire? Is there some way to resolve this issue?

1 Answer 1


You might be creating a ground loop with that extra gound wire between the two boards. They already share a ground via the shared USB power supply so it's not needed.

  • This is nonsense. Ground loops can impact sensitive devices e.g. audio but will never affect DC circuitry.
    – Milliways
    Mar 25 at 21:44
  • Your comment is nonsense. There is an inherent AC element generated by the debounce of the mechanical switch on the USB hub when switched on/off. And the fact still remains that the extra GND wire is superfluous.
    – hexbugger
    Mar 26 at 16:20
  • Thanks for the suggestion. I checked without the ground wire, and the problem still occurs. In fact, it still occurs without any wiring between the two Raspberrys. So it is most likely just a problem with the (cheap) USB hub.
    – cheersmate
    Mar 31 at 11:22

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