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  • Case ID: 345372
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nRF5340 DK not powering correctly over USB — only works on coin cell (dim/blink behavior).

Peter C

I have an nRF5340 DK that exhibits unusual power behavior. When I plug in the IMCU USB port (USB connected to onboard debugger), the board briefly flashes all LEDs, including LED5 and the four user LEDs — but only for a split second. The flash is very dim, and all LEDs then remain off. Flipping the power switch ON/OFF while USB remains connected causes the same brief dim impulse.

I've verified the following:

  • White check mark USB cable is known good (verified for both power and data with my phone)

  • White check mark Behavior is identical using both PC USB ports and a high-current USB wall adapter

  • White check mark Flipping the 3-way power selector switch between USB, VDD, and Li-Po makes no difference in this case — USB power remains non-functional

However, when I connect USB to the nRF5340 USB port (the target SoC's USB peripheral), and set the 3-way selector to USB, the 4 user LEDs begin blinking — but again, they are extremely dim. LED5 remains off.

Here's the interesting part:

When I removed the plastic tab from the coin cell, set the 3-way selector to VDD, and powered the board from the coin cell, the four user LEDs cycle at normal brightness, and the blinking behavior appears stable. This seems to confirm that the board and MCU are functional, but the power circuitry from USB and Li-Po inputs may not be working correctly.

I also tested:

  • White check mark Attaching 3.8V to the Li-Po header (P27) and selecting Li-Po power = same brief dim impulse blink

  • X PC does not detect the board in Device Manager when connected via IMCU USB (no J-Link or COM port)

  • X No mass storage device (e.g., JLINK drive) appears

  • X LED5 (power/status LED) never stays on when using USB or Li-Po sources

This suggests to me that the onboard power regulation or the debugger section may not be functioning correctly. Am I missing a required jumper or switch for enabling USB power regulation or debugger power path?

TL;DR:

Condition Result
USB into IMCU (PowerSelector=USB) All LEDs blink dimly once, then dead
USB into nRF5340 USB (PowerSelector=USB) User LEDs cycle dimly, LED5 off
Wall power into either USB Same as above
3.8V to P27 (PowerSelector=Li-Po) One dim blink on all LEDs, then dead
Coin cell (PowerSelector=VDD) White check mark User LEDs blink bright and properly, LED5 still off (expected?)
PC never detects J-Link/debugger No device shows in Device Manager
  • 0

    Follow-Up (Power Issue Diagnosis):

    Quick update based on voltage measurements:

    • VDD rail measures only 1.5–1.8V under all power sources (USB, Li-Po, coin cell).

    • USB 5V input is present, but LED5 never stays on and the board does not enumerate via IMCU USB.

    • On coin cell (PowerSource = VDD), the board runs normally — 4 user LEDs blink at full brightness. The cell drops briefly to ~2.7–2.8V under load, then recovers to 3.0V when removed, suggesting no excess draw.

    • USB_VDD line near the nRF5340 USB port reads ~1.8V, possibly due to backfeed while on coin cell?

    This points to a likely power regulation or switching fault on the board, preventing proper VDD. Please advise if further steps are recommended or if this warrants replacement.

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