nRF52833 Unable to Use External 32 MHz Crystal

[ge90114b]

Hewy there:

I have a custom nRF52833 board with NCS v3.1.0, using ESB and USB CDC ACM. The external 32 MHz crystal is oscillating fine — we see a clean 32 MHz sine wave at 700 mVpp on the scope. Crystal is X201632MOB4SI (YSX211SL, CL=12 pF), with 12 pF caps on both pins.

But EVENTS_HFCLKSTARTED never fires. HFCLKSTAT still shows SRC=0 (internal RC). We also tried setting HFXODEBOUNCE to 0x40 before starting the task, no change.

The app hangs in lfclk_spinwait() when using CONFIG_CLOCK_CONTROL_NRF_K32SRC_SYNTH=y. With RC everything runs but USB fails (Windows error code 43) and ESB doesn't work.

Why would EVENTS_HFCLKSTARTED not trigger when the crystal is clearly oscillating at the right frequency and amplitude?

Thank you.

[JONATHAN LL]

Is this a custom board and if so have you had a review don on this ? If not we can have a look at the design if you want, either here or in a new ticket(private if so needed). 


From our design guide here, General PCB design guidelines for nRF52 series :  

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The load capacitors, Ccap, are connected to the crystal and the values of the Ccap are given by the formula:

Ccap = 2*CL – C_pcb – C_pin

CL is the load capacitance of the crystal in use and can be found in the datasheet of the crystal.

C_pcb + C_pin is approximately 4 pF for the HFXO pins and approximately 5 pF for the LFXO pins. 

More details can be found in the whitepaper "Crystal Oscillator Design Considerations". Please note that this paper is written for older chips (not nRF51/52) and that the input capacitance is given by: C_pcb + C_pin and not 1 pF, as it's stated in the whitepaper.

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So for you 12pF CL you would want a value of around 20pF 

Regards,
Jonathan