In the past, you need modify rtc_api.c in mbed-dev source code. From this revision, you can just use RTC function all of conditions (Normal, Reset, Stand-by, Power OFF).

Note

From now on, you do NOT need any modification for mbed-dev library because STM team updates rtc_api.c source code and support RTC function under reset & standby condition includes power off condition (You need additional VBAT back-up hardware).

Please refer following NOTE information.
/users/kenjiArai/notebook/nucleo-series-rtc-control-under-power-onoff-and-re/

mbed_app.json

Committer:
kenjiArai
Date:
2021-03-06
Revision:
14:4b03106ece1f
Parent:
10:806cfec92eb6

File content as of revision 14:4b03106ece1f:

{
    "requires": ["bare-metal"],
    "target_overrides": {
        "*": {
            "target.printf_lib": "std"
        }
    }
}