Calibrate internal capacitors for the 32kHz xtal

[GoodOld]

Hi

I had a device that did not start up - because it was waiting for the 32KHz clock to be stable. 

So I tried to change the setting below from 15pF to 5pF and it started fine.

&lfxo {

    load-capacitors = "internal";

    load-capacitance-femtofarad = <5000>;

};

Is there a suggested way to calibrate this value for a given Xtal?

Br

     GoodOld

[Jared]

Hi,

Can you share the full part number and datasheet of the crystal that you are using?

regards

Jared 

[GoodOld]

The part number for 32KHz Xtal is Q-SC20A03220C5AAEF, please see the datasheet in the attachment.SC-20A  12.5pF  20ppm.pdf

Br

   GoodOld

[Jared]

Hi Niels,

That crystal is out of spec, maximum internal load capacitance for external LFXO on nRF54L15 is 9 pF.

regards

Jared

[GoodOld]

So I tried to set the value to an impossible number to see what error message would come. 

I got this:

devicetree error: value of property 'load-capacitance-femtofarad' on /clocks/lfxo in C:/work/SapheZFirmware_ado_master/Lucid/build_2_9_1/mcuboot/zephyr/zephyr.dts.pre (5002) is not in 'enum' list in C:/ncs/v2.9.1/zephyr/dts/bindings\clock\nordic,nrf-lfxo.yaml ([4000, 4500, 5000, 5500, 6000, 6500, 7000, 7500, 8000, 8500, 9000, 9500, 10000, 10500, 11000, 11500, 12000, 12500, 13000, 13500, 14000, 14500, 15000, 15500, 16000, 16500, 17000, 17500, 18000])

It seems to me that it can be from 4pF to 18pF.

Please explain.

Regards 

   GoodOld