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.