nRF5340 LFXO Internal Load Cap Setting

RE: nRF5340 LFXO Internal Load Cap Setting

JONATHAN LL — Fri, 28 May 2021 06:06:29 GMT

The figure and formula is not correct if you use the internal capacitors. So the math wont add up, i belive this will be addressed in the next update of the product specification.

Regards,
Jonathan

RE: nRF5340 LFXO Internal Load Cap Setting

Akash Patel — Thu, 27 May 2021 21:55:09 GMT

Hi Jonathan,

I marked the answer as verified but then later came back to this and realized the math still isn't working out for me. So if we set I_CL = 6pF (through the register) and there is 3pF capacitance from the pad, we get a total of 9pF for both C1' and C2'. But then using the formula for C_L (figure 24 in the PS), we only get 4.5 pF. I think there is still something wrong.

Regards,

Akash

RE: nRF5340 LFXO Internal Load Cap Setting

JONATHAN LL — Thu, 27 May 2021 12:55:14 GMT

New update with some corrections here.

There will be a update in the PS that hopefully addresses this issue in a proper way as it is inconsistent at best right now.

The 6pF is correct, but a bitt difficult to understand from the docs. The real value is closer to 9pF due to stray capacitance in the XL1 and XL2 pins.

So the the internal load capacitance (I_CL) for the LF on the nrf53 has the options 6pF, 7pF and 9pF, these values will result in a load capacitances (C_L) equal to the I_CLplus the stray capacitance from the PCB (pad and routing capacitance), it is approximately 3pF.

So that gives us C_L = I_CL + 3pF, and in the case of the nrf53 DK that has a crystal that wants a max C_L of 9pF we sett the I_CL to 6pF and add the 3pF form stray PCB capacitance and we get the 9pF we want.

So the previous answer was not that great.

Regards,
Jonathan

RE: nRF5340 LFXO Internal Load Cap Setting

Akash Patel — Tue, 25 May 2021 17:35:03 GMT

Hi Jonathan,

If that is indeed the case, the firmware by default is selecting 6pF here and I don't think that is being changed with any of the overlays or files while the crystal is specifying a C_L of 9pF causing a discrepancy. Can you look into this?

Regards,

Akash

RE: nRF5340 LFXO Internal Load Cap Setting

JONATHAN LL — Tue, 25 May 2021 09:04:37 GMT

When the 9pF is selected internally then that is actually a 18pF capacitors for each pin, so C1 = 18pF and C2 = 18pF, that makes the value of CL = 9pF. So if the device spec says crystal needs CL to be 9pF then you can select internal 9pF.

There will be some updates to the docs to help clarify.
Regards,
Jonathan

RE: nRF5340 LFXO Internal Load Cap Setting

JONATHAN LL — Fri, 21 May 2021 13:55:15 GMT

Hi Akash,

I believe you are correct but have forwarded the question to the designer so will update you when we have a proper answer.

Regards,
Jonathan