RTC Clock delay

[Vignesh_S]

We have tested RTC in our IMXRT1172 custom board and we observed some delay with respect to actual time. Below are the test cases. Delay was observed with/without setting the date & time.

1. Setup was running continuously for 70 hours and we observed 50 seconds delay with respect to actual time.

2. Setup was tested for 5 hours run to reproduce the issue and we are able to see around 3 secs delay.

We are using 32.768KHz crystal with 10pF capacitors. What can be the reason for the delay? Do we need to calibrate the RTC to solve this delay?

[Vignesh_S]

Hi @PabloAvalos 

Thanks for the response. Yes, we are using the Internal RTC of IMXRT1172 with external Oscillator.

For the above mentioned deviation in seconds, How can i calculate the offset value for the calibration? Is there any calculation or documents available to refer the calibration.

[PabloAvalos]

Hi @Vignesh_S 

Please take a look into the Secure Reference Manual for RT1170. On chapter 5.6.1.16 you will find the registers for the calibration of RTC.

About your external crystal oscillator, may I have the part number you are using for that? just to see if it agrees with the hardware requirements and compatibility with the internal RTC.

Thanks a lot.

BR.
Pablo Avalos.

[Vignesh_S]

Hi,

I checked the Safety Reference Manual but How can I select the count value for the observed RTC delay? Do we have to any calculations to arrive this value?

External Crystal Part Number : EXS00A-MU00929

[PabloAvalos]

Hi @Vignesh_S 

Thanks a lot for your patience on this case.

Regarding your question about the count value, on Secure Reference Manual is not specified and I am afraid that this is not accessible. Then, I think I am not getting your question "Do we have to any calculations to arrive this value?", you mean like some formula to calculate the RTC counter value?

Hope you may clarify a little bit your questions, because I am not catching it. I appreciate it!

Thanks a lot.

BR.
Pablo Avalos.

[Vignesh_S]

Hi Pablo,

Yes. Only the Calibration registers are mentioned in the Security Reference Manual but the Calibration procedure or formula to calculate the calibration value (LPCALB_VAL) is not available.

Do we have any formula or procedure to calculate the LPCALB_VAL for the RTC calibration?

[PabloAvalos]

Dear @Vignesh_S 

I am still in the process of having an answer from my apps engineer to know if this information is available. If it is, I will be providing you this information by private e-mail as it is sensible content.

Also remember you have a local DFAE that can help to get this information.

Thanks a lot for the patience.

BR.
Pablo Avalos.

[Vignesh_S]

Hi @PabloAvalos 

Please share the information once you got it.

Thank You.

[PabloAvalos]

Hi @Vignesh_S 

Thanks a lot for reaching our technical support.

RTC calibration is always necessary, I handled a similar case than yours and I would like to know first of all, is it your RTC the internal one RT1172 ? Crystal oscillator is external, right? as you are using custom board. Which part number is that one? There are several factors where RTC cannot work with such a precision to a real-life clock, so my suggestion also is to reach your local DFAE or representative to reach out better results on your tests.

Please let me know if you have some additional questions so I can help with everything that is within my hands.

Thanks a lot!

BR.
Pablo Avalos.