topic Re: PCA 21125 Xtal load capacitor impact to RTC accuracy in Other NXP Products

PCA 21125 Xtal load capacitor impact to RTC accuracy

GregB_in_Michigan — Thu, 18 Apr 2024 18:50:22 GMT

For PCA 21125 RTC...

Data Sheet says "The frequency tolerance depends on the quartz crystal tolerance, the
capacitor tolerance and the device-to-device tolerance (on average 5  106). An
average deviation of 5 minutes per year can easily be achieved."

If Xtal load capacitance goes up or down 10pF, what is the impact to RTC accuracy?

Clock speed Hz goes up or down if capacitance varies +10pF? Varation impact or measurement difference between 32768Hz and 32778Hz is not easy to differentiate with typical lab equipment.

gbober@lgensol.com

Re: PCA 21125 Xtal load capacitor impact to RTC accuracy

guoweisun — Fri, 19 Apr 2024 03:11:22 GMT

For your question:

If Xtal load capacitance goes up or down 10pF, what is the impact to RTC accuracy?

[gw]without the suitable CL matching, the clock also has the accuracy issue.

Re: PCA 21125 Xtal load capacitor impact to RTC accuracy

GregB_in_Michigan — Fri, 19 Apr 2024 14:32:58 GMT

For many years we had 10pf 5% Xtal load cap with the crystal. Then it was recommended to

change to 18pF 5% tol. What would have been the normal RTC time counting impact of such a change?

Is there an equation to help me determine how the 8pf shift and +/-5% variation impacts the RTC time accuracy?

Re: PCA 21125 Xtal load capacitor impact to RTC accuracy

guoweisun — Mon, 22 Apr 2024 11:00:40 GMT

You can use below equation to calculate the Cext value to match crystal requirement CL value, whatever it's 18pF or 10pf

Re: PCA 21125 Xtal load capacitor impact to RTC accuracy

GregB_in_Michigan — Mon, 22 Apr 2024 13:26:49 GMT

thanks... but for PCA21125 what is delta_Frequency/delta_Cload sensitivity? no sensitivity identified in Tables 41 to 43 of component specification. The Cload equation is from Table 42. Section 14.2 only mentions adjusting the pin load capacitance as a method to improve accuracy... thus there is an impact to accuracy...the +/-Hz or seconds over a span of time impact is not stated.

Re: PCA 21125 Xtal load capacitor impact to RTC accuracy

guoweisun — Tue, 23 Apr 2024 05:51:58 GMT

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Section 14.2 only mentions adjusting the pin load capacitance as a method to improve accuracy... thus there is an impact to accuracy...the +/-Hz or seconds over a span of time impact is not stated.

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I comprehend that the error between calculated CL and actual CL requirement will impact the RTC accuracy.