Well I can't find the exact chip, but it seems to be some sort of "uP Supervisor" IC
Thank you for your suggestions and the datasheets that look indeed very similar. Although it sounds logical now, I still doesn't fully get how that power switching works.
It looks like there are at least three different power sources:
+ 5 V (pin 8 ) ?
+ 5V trickle power or standby power (pin 2) coming straight from the PSU
+ 3.6 V (pin 7) from the backup battery
If I'm reading the schematics correctly, the MCU (U17) is powered directly by U12 whose pin 1 must be therefore the output voltage connected to MCU's V
DD (pin 13).
I wonder what that strange S1 switch attached to pin 1 of U12 is supposed to do?