topic Re: Kinetis K65 RTC crystal selection in Kinetis Microcontrollers

Kinetis K65 RTC crystal selection

GordyCarlson — Mon, 02 Nov 2015 15:18:57 GMT

My customer is having issues with the Kinetis K65 RTC resetting itself to 1987 (default values) when they do environmental testing on their end product prototypes for several hours at -40C. We initially reviewed the VBat supply integrity, and it appears OK. Our attention is now on the external RTC crystal + circuit he uses. Perhaps it is having oscillation issues at the low temp.

We have reviewed the K65 data sheet and it's crystal requirements/specifications. And would like to run this by you for additional comment...

See circuit and attached crystal data sheet used for the 32KHz RTC oscillator, EPSON PN MC-306-32.7680K-A0:ROHS.

The crystal is designed for a 12.5pF load capacitance.

Presently we have (2) 18pF caps as shown in schematic for a load capacitance (CL) of 9pF plus any stray capacitance (Cs). (CL = (C42*C41)/(C42+C41) + Cs).

The K66 data sheet lists Cpara = 5pF typical, where Cpara is the parasitical capacitance of XTAL32 and EXTAL32.

Would I use 5pF for Cs, resulting in CL = 9pF + 5pF = 13pF or would it be the actual load capacitance be calculated differently?

I wondering if I should considered adding 2pF using K66 internal capacitance registers or leave as is based on Cpara value.

-Gordy Carlson

Freescale FAE

Upstate NY

Re: Kinetis K65 RTC crystal selection

egoodii — Mon, 02 Nov 2015 21:58:36 GMT

I would remove your external R and Cs in this circuit -- the R is internal, and you should use the internal Cs based entirely on loading to 'nominal accuracy' at 25C. For my 12.5pF XTAL ABS07-32.768KHZ-1-T (+/-10ppm tol), I chose 20pf (16+4 selections) per pin, but I have virtually NO other PCB parasitics. Together these hold me to about +/-25ppm overall accuracy (+/-2s/day) over 0 to 50C. I test accuracy by synchronizing to a 'timeanddate' web site, then checking it the next day.

These changes may keep the oscillator running at -40C, but I would also watch for condensation in that test -- if such is a concern within the product, then you may need to coat this extremely high-impedance area.

Re: Kinetis K65 RTC crystal selection

egoodii — Wed, 04 Nov 2015 18:05:52 GMT

By the way, if you are expecting any significant ACCURACY over extended temperature ranges, like If one is TRULY trying to hit 'about 1s/day' (+/-11.6ppm), IMHO the cleanest option is an external 'compensated oscillator' from SiTime: SIT1552AC-JE-DCC-32.768D. Takes a little more battery, but insures +/-10ppm 0-70C (extended temp versions available -40+85C, holds +/-13ppm). Also AVOIDS these very issues of high-impedance/low-signal-level connections on the PCB.

Re: Kinetis K65 RTC crystal selection

GordyCarlson — Thu, 05 Nov 2015 21:26:54 GMT

Thanks for the comments Earl, I'm going to review these suggestions and see if they resolve or improve the issues with RTC operation at -40c.

Gordy Carlson