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/

Committer:
kenjiArai
Date:
Sat Jul 18 22:46:14 2020 +0000
Revision:
10:806cfec92eb6
run on mbed-os6.2.0 (mbed_version.h = os5.15.4)

Who changed what in which revision?

UserRevisionLine numberNew contents of line
kenjiArai 10:806cfec92eb6 1 {
kenjiArai 10:806cfec92eb6 2 "requires": ["bare-metal"],
kenjiArai 10:806cfec92eb6 3 "target_overrides": {
kenjiArai 10:806cfec92eb6 4 "*": {
kenjiArai 10:806cfec92eb6 5 "target.printf_lib": "std"
kenjiArai 10:806cfec92eb6 6 }
kenjiArai 10:806cfec92eb6 7 }
kenjiArai 10:806cfec92eb6 8 }