Test the accuracy of external LFCLK.

[victor_CHINA]

Hi Dear nordic employee

I want to test the accuracy of my external LFCLK,    

1、I enable the nordic RTC

2、set the prescaler 31（32768/32=1024），1/1024 second  per step

3、I enable a gpiote with high accuracy 

4、connect a pps （GPS） with the nordic gpio

5、In gpiote interupt envent ，I print the RTCcounter when the interrupt happen

the result is ：when I measure 20 minutes，the test value is 109ms more than 200000，so the accuracy of CFCLK is ≈ 100ppm

My question ：does the result is trustworthy？

thanks and regards

[awneil]

victor_CHINA said:

Hi Dear nordic employee

You realise that this is a public, user forum - not staffed purely by Nordic employees?

https://devzone.nordicsemi.com/backstage/f/welcome-to-the-devzone/30115/how-to-know-the-respondent-is-nordic-employee-or-not

[victor_CHINA]

Hi owneil，I know that,  thanks for your remind

[Sigurd]

victor_CHINA said:

so the accuracy of CFCLK is ≈ 100ppm

Is this the accuracy you expected? What is the accuracy of the external LF crystal you are using?

[victor_CHINA]

The datasheet shows the accuracy of the external LF is 20ppm

So ，I am not sure  wether my test result is trustworthy or not

[Sigurd]

If I were to test this, I would have used an logic analyzer(e.g. from Saleae). Configured the RTC to generate a compare event after e.g. 100 seconds, then used PPI and connected the GPIOTE to toggle a GPIO together with the RTC START task (PPI fork), and GPIOTE to toggle the same GPIO on the RTC compare event. Then measured the time using the logic analyzer tool.