nRF52840 32.768KHz capacitor value different

[Poki Huang]

Hi

Since we need use different C1/C2 (15PF/12PF) for 32.768K XL1/XL2

I check "https://devzone.nordicsemi.com/f/nordic-q-a/37448/asymmetric-crystal-load-capacitors" , and it mention 13PF/15PF is ok but 10PF/15PF is not recommend

So could you help check if 15PF/12PF for 32.768K XL1/XL2 is ok or not?

Thank you~~

Poki

[JONATHAN LL]

Hi Poki,

What Crystal are you using? can you link to the data sheet of the crystal you have in mind?
Regards,
Jonathan

[Poki Huang]

Ｈi Jonathan:

We use Epson crystal --> FC1610AN_32.768kHz_9pF+-20ppm_X1A0001210006_E180604_R00a

Sorry I don't know how to attached file, please ref. below link

https://www.digikey.tw/product-detail/zh/epson/FC1610AN-32.7680KA-AC/FC1610AN32.7680KA-AC-ND/7727613

Thank you

[JONATHAN LL]

Hi,
Is there any specific reason to why you want different copastor values? It is recommended to keep C1 and C2 the same value, ref. 
When calculating the C1 and C2 for the crystal you can use this formula:
C1 = C2 = CL * 2 -C_pin

Where CL is the Load capacitance, in this case I believe it is 9pF as it is what you use given he link and component name, and C_pin is the input capacitance on XL1 and XL2 pads. For the nrf52840 it is given the C_pin is 4pF. 

This gives the value C1 and C2 = 9pF * 2 - 4pF = 14pF


If it is the case that you have problem sourcing the 14pF capacitors then 12pF/15pF could work fine to. There might be some more jitter then normal but it is hard to say exactly and it doesn't take much before the RF-carrier is to much off to not be within the spec. 

Regards,
Jonathan