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:

Below table is what Xtal vendor change C1/C2 to get frequency deviation

You can see 15/15 is min. deviation but the value is negative

Since 32.768K xtal characteristic at room temp(25 degree). has fastest speed (that mean when go higher or lower temp. the speed will slower than room temp.)

So vendor propose 15/12 with positive deviation, that's why we select 15/12

So do you think we can use 15/12? 

Thank you~~

Status	C1[pF]	C2[pF]	Frequency deviation[x10-6] CL= 9.0 [pF]	DL [uW]	|-R| [kohm]
Present	15(15.1)	15(15.2)	-3.6	0.005	500
	12	15	18.4	0.004	630
Proposal	15	12	13.8	0.004	630
	12	12	29.4	0.004	630

Poki

[JONATHAN LL]

Hi Poki,


Example:

Say you want to transmit at 2.4400 GHz, then the following offset will be the actual transmit frequency. 

2440*10^6 + (13.8*2440) = 2440033672
Delta offset =  33672 Hz

2440*10^6 + (-3.6*2440) = 2439991216
Delta offset = 8784 Hz

Following the recommendations from the vendor should work fine. Its good to have some headroom for drift, aging, ect for the crystal, but 15/12 is looks to be ok.

Regards,
Jonathan 

[Poki Huang]

Hi Jonathan:

I have question about 32.768K xtal cap, not 32M xtal

So I don't understand why you mention 2.44GHz?

And we have both external 32.768K and 32M Xtal, so BLE 2.44G shouldn't use 32.768K right?

So if 32.768K no related with 2.44G, 15/12PF can be used?

Thank you~~

Poki

[JONATHAN LL]

Hi

Poki Huang said:

So if 32.768K no related with 2.44G, 15/12PF can be used?

 Yes.

Sorry my previous response was confusing. It was a bad example for showcasing the accuracy.
For the 32KHz xtal it is perfectly fine to use 15/12pF.  

Regards,
Jonathan