ST Micro M41T11M6 RTC Drift Issue
I’m currently having an issue w/ ST Micro P/N M4T11M6 RTC w/ clock drift. Specifically, I’ve performed verification testing on 4 units to this point and I’m seeing over 8 seconds/day (Extrapolating to a value of 4.25 minutes/month) of drift from NIST time. The RTC clock calibration register only allows for a maximum of -2.75 minutes/month which isn’t enough range to allow for the drift observed to be calibrated out. This makes me question my PCA schematic and perhaps layout design. The XTAL used meets the required 32.768 KHz XTAL w/ 12.5 pF load capacitance. I have cut-away the GND plane under the XTAL as well (However, it turns-out the +3.3V plane isn’t cut-away, but is in the middle of the 8 layer stack-up and the RTC and XTAL are on the top layer).
What am I potentially doing wrong that would cause such a drift issue?
We are in the middle of verification on a high-priority project w/ a tight schedule, so your urgency on this matter is truly appreciated. Thanks in advance for your assistance w/ resolving this RTC issue.