5VLED is on when powerde by Vbat

b.a. · ‎2025-06-26

I have Nucleo-L452RE-P and want to try out battery backup for RTC.

I connected a CR2032 to Morpho connector 19 (-) and 33 (+) and when I pull the USB plug, the 5V LED LD2 remains on, although a bit less bright.

How can that be?

QSHAO.1 · ‎2025-06-30

By default, the VBAT is connected to VDD_MCU through solder bridge SB61 (closed by default). The CR2032 connected to VDD_MCU may lead a leakage current somehow on LD2 I guess. I suggest you disconnect VDD_MCU with VBAT by "open" solder bridge SB61 and try to see if any difference. thanks!

View solution in original post

Andrew Neil · ‎2025-06-26

Take a look at the schematics ...

A complex system that works is invariably found to have evolved from a simple system that worked.
A complex system designed from scratch never works and cannot be patched up to make it work.

b.a. · ‎2025-06-26

I did, even before posting...
Vbat is connected to VDD_MCU but nowhere is a connection to 5V and LD3 is connected to 5V only

QSHAO.1 · ‎2025-06-30

By default, the VBAT is connected to VDD_MCU through solder bridge SB61 (closed by default). The CR2032 connected to VDD_MCU may lead a leakage current somehow on LD2 I guess. I suggest you disconnect VDD_MCU with VBAT by "open" solder bridge SB61 and try to see if any difference. thanks!