External Crystal selection for nrf52810 (Laird BL651)

[chrismoi]

Hello DevZone,

I am using Laird BL651 which incorporates nrf52810.

I am currently at the development stage of my project and I'm experimenting with lowering the power consumption using an external crystal. All recommended crystal packages I have seen are SMD type and I was wondering If I could use a THT crystal which I found for a cheaper price. We are trying to minimize the cost for mass production.

Actually I have tried in my custom board https://gr.mouser.com/datasheet/2/3/WC-Series-1847816.pdf and particularly WC-26, THT crystal, which is within the recommended parameters given by nordic and laird. Since it is a 12.5pF crystal, I checked the BL651 datasheet:

Input capacitance on XTAL pins is 4pf and I added +1 maybe for the traces, hence I calculated the value for the capacitors as follow (12.5 - 4 - 1) * 2 = 15pF.

I made the necessary changes in the sdk_config.h file as follows:

Though when I turn on the device, nothing happens and the output from the crystal pins is this:

about 500mV output with small spikes each ~300us.

Any thoughts on what could be wrong?

[ketiljo]

If you have have a crystal with CL = 12 5 pF, the caps should be as follows: 

Ccap = CL * 2 - Cpin, where Cpin = 4 pF.  

Ccap = 12.5 * 2 - 4 = 21 pF. 22 pF is a standard value so you can use that. 

The scope probe will load the crystal too much so it's better to test it with a timer clocked from the 32 kHz crystal.