Match capacitance for teh crystall oscillator nrf5340 and nrf Connect SDK

[dario.sortino]

Hello everyone,

I'm developing my own board using the nRF5340. I am quite lost because of the capacitance matching that I need to perform with the 32M Crystal and the 32k Crystal, and I would like to do it with external capacitors.

Following both the PS pdf (https://infocenter.nordicsemi.com/pdf/nRF5340_PS_v1.2.pdf) and this additional resource that I found online https://infocenter.nordicsemi.com/pdf/nwp_015.pdf?cp=12_12 I am trying to figure out what capacitance should I use.

Note that the load capacitance of the 32M crystal in the nrf5340DK is the same of our crystal (8 pF)

Assuming we work with internal capacitors disabled, then Cpin = 5.5 pF

We know from the link above that

C' = Ct + Cpcb + Cpin = 2 * C

Ct + Cpcb + 5.5 pF = 2*8 pF --> Ct + Cpcb = 11.5 pF

Is this calculation correct? Also, IF we enable and keep default settings (from the blinky example) about the internal capacitors, what is this default value? I couldn't find anything with respect to the 32M Crystal.

Instead, I know that for the 32k crystal (load capacitance = 8 pF) the internal set capacitance Ct is by default at 7 pF.

Assuming we work with internal capacitors enabled and to default, then Cpin = 7 pF (typical from the datasheet)

We know from the link above that

C' = Ct + Cpcb + Cpin = 2 * C

Ct + Cpcb + 7 pF = 2*8 pF --> Ct + Cpcb = 9 pF --> Cpcb = 2 pF

Are these calculations correct? Is it reasonable to assume to have a Cpcb so small? I know that it depends by many thing, but I 'd like to have a reasonable dimensioning for the capacitors that I need to place.

[ketiljo]

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.

[dario.sortino]

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?

[ketiljo]

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

[dario.sortino]

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?

[ketiljo]

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.

[dario.sortino]

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?

[Bendik Heiskel]

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

[dario.sortino]

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