@RWB
You can solder in a wire on pin 1 of the STM32 and power it via a coin cell (add a diode in series) - this is untested but might work.
The second option seems doable by putting the Core into deep sleep and letting only the RTC run.
@RWB
You can solder in a wire on pin 1 of the STM32 and power it via a coin cell (add a diode in series) - this is untested but might work.
The second option seems doable by putting the Core into deep sleep and letting only the RTC run.