Match capacitance for teh crystall oscillator nrf5340 and nrf Connect SDK

RE: Match capacitance for teh crystall oscillator nrf5340 and nrf Connect SDK

dario.sortino — Thu, 15 Sep 2022 12:27:43 GMT

Thanks. Everything is clear now.
I really appreciate your answers!

RE: Match capacitance for teh crystall oscillator nrf5340 and nrf Connect SDK

Bendik Heiskel — Thu, 15 Sep 2022 12:24:11 GMT

Hi,

The traces from the XL1 and XL2 pin to the LF crystal on the DK is very long, which will increase the capacitance. By setting the internal capacitor to 7pF the extra capacitance from the traces are compensated for. There is not need to change the internal load caps to 9pF when using the DK.

Best regards,

Bendik

RE: Match capacitance for teh crystall oscillator nrf5340 and nrf Connect SDK

dario.sortino — Thu, 15 Sep 2022 07:20:53 GMT

Thank you, I'm finally understanding how the whole system is working.

Just one last question: in the DK the 32K crystal has CL = 9 pF, but in the SDK examples (like blinky), the internal match is set to 7 pF. I would say that this is not wrong because blinky doesn't make any use of the 32K crystal, so in that context, everything works. But I guess if I want to use some RF functionalities the internal match needs to be set at 9 pF (or matched with an external capacitor).

Do you confirm this?

RE: Match capacitance for teh crystall oscillator nrf5340 and nrf Connect SDK

ketiljo — Wed, 14 Sep 2022 14:46:13 GMT

Default _register_ settings are external caps. But the SDK examples will probably have this set to match a 32 MHz crystal with CL = 8 pF.

12.5 pF is out of spec, max is 9 pF: https://infocenter.nordicsemi.com/topic/ps_nrf5340/chapters/oscillators/doc/oscillators.html#unique_63981518. Hence, you can set only 7, 8 or 9 pF for the LFXO.

RE: Match capacitance for teh crystall oscillator nrf5340 and nrf Connect SDK

dario.sortino — Wed, 14 Sep 2022 14:29:56 GMT

Hi ketiljo,

I don't get why in the DK there are no external load caps, the blinky example works fine but the default settings are external loads.

Also, if my LF crystal have 12.5 pF, I should use both the _CL_ value and external capacitors?

RE: Match capacitance for teh crystall oscillator nrf5340 and nrf Connect SDK

ketiljo — Wed, 14 Sep 2022 14:21:50 GMT

The default setting of both the LF and HF XOSC caps are external load caps.

Note that the cap settings are done differently for the LF and HF oscillators. For the HF clock, what you write to the register is the actual cap setting you want, but for the LF clock, you select the _CL_ value of the crystal. So if you have a crystal with CL ) 9 pF, you need to write 3 to the XOSC32KI.INTCAP register: https://infocenter.nordicsemi.com/topic/ps_nrf5340/chapters/oscillators/doc/oscillators.html#register.XOSC32KI.INTCAP

RE: Match capacitance for teh crystall oscillator nrf5340 and nrf Connect SDK

dario.sortino — Tue, 13 Sep 2022 07:42:27 GMT

Also, according to the calculation and the datasheet, for the 32k crystal, I should have in the nrf5340DK

Ccap = 9 pF * 2 - (7 pf + 1 pf) = 10 pf

But the internal capacitor by default is set at 7 pF, or am I missing something?

RE: Match capacitance for teh crystall oscillator nrf5340 and nrf Connect SDK

dario.sortino — Mon, 12 Sep 2022 13:29:31 GMT

Hi Ketiljo, thank for your confirmation.

In case I want to keep the internal capacitor on for the 32Mhz crystal, what is the default setting for its value?

RE: Match capacitance for teh crystall oscillator nrf5340 and nrf Connect SDK

ketiljo — Mon, 12 Sep 2022 12:49:38 GMT

You can calculate the cap values like this:

Ccap = CL * 2 - (Cpin + Cpcb).

You can assume ~1 pF for Cpcb and 5.5 pF for Cpin. So for a crystal with Cl = 8 pF, you need 8 *2 - (5.5 +1) = 10 pF.

Both crystals on the DK have a CL value of 8 pF so any example that uses this will work with your crystal as well.