RTC with Battery Switchover function loses time during reset

[Central SUPPORT]

We use the M41T93 RTC as shown in the attached schematic snippet. Occasionally, the RTC will report 0's for time and date after a power cycle. More often than not, the system retains time and date w/o issue. We are investigating possible SPI communication as well as hardware issues. For example, I checked Vcc and battery voltages at the clock and they are within spec. 

Can you identify other possible issues I can check? For example, can a spike at power up cause this? 

NOTE: This question and answer originated from a customer support case which was handled by us. We regularly review support cases and add any helpful ones here for all to benefit from.

[Central SUPPORT · Accepted solution]

The source of your problem is probably related to the presence of D1 and R44. In fact, our M41T93 integrates these components internally already. Their addition to the external circuitry reduces the voltage at the VBAT pin when in backup-mode thus causing the device to run out of power. You probably found the OF bit set to 1 which tells you that the oscillator have been stopped.

Please remove these components and connect the battery directly to the VBAT pin and the problem should disappear.

See also AN3060 Application Note, page 5 which describes battery connection in detail.

Note: This answer is valid for M41T93 and also for these components: M41T82, M41T83, M41T93, M41T00S, M41T11, M41T56, M41T94, M41T81S