nRF54L15 power on crystal load capacitor selection

[JerrySJ]

Hi

I have some questions regarding the 32 MHz crystal on the nRF54L15:

1. If my firmware is configured to use external capacitors, will the very first crystal startup waveform captured after power-on already be using the external capacitors? Or is it still using the internal capacitors at that moment?

2. The reason I’m asking is that I used the same PCB design but tested it with different firmware settings (external vs. internal capacitors). The measured frequency offset did not match my expectations. My external load capacitors are 12 pF, and the internal setting is the default 15 pF. However, the result I observed is that when using only the external capacitors, the frequency offset is actually 2 ppm slower compared to using the internal capacitors in firmware (with the external capacitors still mounted).

[Einar Thorsrud]

JerrySJ said:

After reading the calculation explanation, I understand that the declared 15 pF is adjusted by slope and offset to produce the final value written to INTCAP (which we read as 0x2B).

Yes, you are correct. And this slope and offset are trim values from FICR, that vary from device to device.

JerrySJ said:

Or is it just a setting that cannot be directly converted to pF, and only indicates that after applying slope and offset the MCU’s CL will be 15 pF?

Yes, that is correct. So it is important to remember that this can vary from device to device. Therefore I see no point in calculating this externally to the device. If you want to do this calculations none the less out of interest, you need to read out the FICR values. I do understand the need for this, as the code that does this is well tested, but if you want to that is no problem. You can do that using for instance nrfutil like this:

nrfutil device read --address 0x00FFC620 --direct

See the XOSC32MTRIM register documentation for details on where you find the SLOPE and OFFSET field. If you drop this into the formula together with the target CAPACITANCE, you will et the INTCAP register value that represents the target capacitance.