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Kinetis K65 RTC crystal selection

Question asked by Gordy Carlson Employee on Nov 2, 2015
Latest reply on Nov 5, 2015 by Gordy Carlson

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

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