Unable to retain RTC contents using Vbat (STM32 NUCLEO-F401RE)

[bhondoo]

I am using the internal RTC and powering the board through Vin, also connected is a Li ion battery to Vbat. When disconnecting Vin supply, I am observing the following:

1. The Vbat is actually powering the entire board and draining quickly, contrary to my expectations of the board entering Vbat mode and powering only the RTC and LSE.

2. Contents of the RTC are lost.

Please suggest what can be done. I have dug into the reference manual and application notes, however I seem to be missing something. Note that I am using a Nucleo board. Thanks.

[Peter BENSCH]

Welcome @bhondoo, to the community!

The NUCLEO-F401RE uses the board with the  MB1136 as a basis. If you look at the schematics of this board, you will notice that the jumper SB45 is set by default, which connects VBAT to VDD. Unsolder it, connect your battery to VBAT/VLCD/VDD (CN7.33) and your problem should be solved:

Good luck!
/Peter

[waclawek.jan]

With VBAT not connected to VDD, I've seen similar symptoms when VDDA was not connected to VDD.

Explanation above by @Peter BENSCH sounds more likely in this case, though.

JW

[Tesla DeLorean]

Check and install LSE to maintain time keeping and don't reset RTC in your startup process