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,

Can you provide the crystal datasheet/ part number , so I check the specs against the SoC requirements? 

[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]

CRYSTAL.pdfCRYSTAL(MURUTA).pdf

I tried two different crystals, and the test results were the same.the crystals' load cap is 8pF

[ZEXU QIAN]

3051.CRYSTAL(MURUTA).pdf