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:
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USB cable is known good (verified for both power and data with my phone) -
Behavior is identical using both PC USB ports and a high-current USB wall adapter
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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:
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Attaching 3.8V to the Li-Po header (P27) and selecting Li-Po power = same brief dim impulse blink
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PC does not detect the board in Device Manager when connected via IMCU USB (no J-Link or COM port) -
No mass storage device (e.g., JLINK drive) appears
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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) | User LEDs blink bright and properly, LED5 still off (expected?) |
| PC never detects J-Link/debugger | No device shows in Device Manager |
