topic Re: KL17 RTC high drift in Kinetis Microcontrollers

KL17 RTC high drift

AndreVB — Wed, 17 Aug 2022 18:57:09 GMT

Hi,

I have a 32,768kHz +/-20ppm 12pF load cap crystal in my two custom boards. In both PCB the RTC drifts aproximatelly +120s/week. That yields +200ppm if I am not mistaken, which is quite high.

SC8P and SC4P fields of the OSC_CR register are enable using the Clocks Config tool on MCUXpresso.

I noticed RTC_CR is disable. Is that correct?

I understand that enabling SC8P and SC4P the oscillator block will add 24pF on EXTAL and 24pF on XTAL pins, correct? Besides, the crystal is 1mm far from the XTAL/EXTAL pins of the MCU (see next).

below and to the right of the crystal is GND, above is RESET pin.

PCB is at room temperature all the time. That said, if the OSC_CR setup is correct, any idea what could be causing such high drift?

Thank you very in advance.

Best regards,

Andre VB

Re: KL17 RTC high drift

_Leo_ — Tue, 23 Aug 2022 17:31:34 GMT

Thank you so much for your interest in our products and for using our community.

For that we have an application note that could help you. Please refer to "Crystal Oscillator Troubleshooting Guide" (AN3208).

Hope it helps you. Please let me know if you have more questions about it.

Have a nice day!

Re: KL17 RTC high drift

AndreVB — Thu, 25 Aug 2022 12:06:55 GMT

Thank you for your reply. The AN describes many potential problems but for Kinetics KL devices, as log as the low frequency crystal is close to the I/Os, the load capacitance is correct, and the crystal far from other noisy signals, it seems there is not much to do about it. I was also very careful in soldering the crystal.

The drift is very high. I have used a RTL code example from KL27 as starting point.

If you have any clue of what could cause the drift, please share.

Thank you,

Andre

Re: KL17 RTC high drift

_Leo_ — Fri, 26 Aug 2022 22:34:04 GMT

I must admit that I can't think of another clue that could cause the drift that isn't mentioned in the aforementioned application note and the recommendations in section 28.6 External Crystal / Resonator Connections of the reference manual.

Now, if you can no longer optimize the oscillator design, you could try to increase reliability through software, for more information about this, please refer to section 32.4.3 Compensation of the reference manual. Also application note AN4949 can serve as a reference.