I'm Trying to get the RTC time saved when my board is no more powered. So I added a lithium battery on the VDDIO pins with diodes so that VDDIO is powered by my 3v3 regulator when the board is powered, and by the 3v lithium battery when it's not.
This works well, and time is preserved.
The problem is that there is way too much current used by VDDIO. I measured 7.8mA at the output of the battery. I checked my design, and the VBat only goes to a BAT54C diode, then to my VDDIO pins. Nothing more.
I also see something strange: on the Vout pin, there is about 250mV instead of nothing, making me think the internal voltage regulator is not really switched off. I tried enabling the brownout detector and brownout reset, but that didn't help. What I expect is the MCU to consume only a few µA as stated in the datasheet, when it is in backup mode. Maybe I missed something and it doesn't go to backup mode?