Issues with External 32 MHz Crystal Oscillator Startup for nRF54Lm20A

[ZEXU QIAN]

Hello,

I am developing a mouse product using the nRF54LM20A (QFN52). Several prototype boards have been built and are currently under debugging. The hardware uses an external 32 MHz crystal with internal‑configured load capacitors.

I have observed that on some boards, the crystal fails to start oscillating when powered by battery only. However, once USB power is plugged in (USB power detection triggered), the crystal starts normally. After that, if the USB cable is unplugged and USB power is removed, the crystal keeps oscillating properly.

To summarize: the crystal cannot start up without USB power, but it can start normally when USB power is present. Could you please advise which configuration setting may be causing this behavior? Thank you!

[ZEXU QIAN]

the attached is schmatic 

[Ressa]

Hi,

For USB VBUS pin of SoC , the connection is not as it is recommended in reference design. The RC components are meant to create a low pass filter:


Your schematic:




reference design: 




* Also there is a LC tank on DECRF pin which does not exist on your schematic. This is mainly related to RF performance issue so I do not expect cause crystal issue. 



*I do not see crystal part number on schematic, if you could provide it or its datasheet, I can check the spec to see if it is a good candidate for nRF54LM20. 


Br,
Ressa

[ZEXU QIAN]

I submitted the schematic to the Nordic FAE team, and their feedback is that this LC tank is not required for devices in the QFN52 package. This LC circuit should only affect the power‑supply quality for the RF section, right?

When the crystal fails to start oscillating, injecting a certain voltage into pin VBUS_5V0 (PIN17) will trigger crystal oscillation. This suggests that external conditions may be invoking a different crystal configuration.