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 Aug 01 07:26:13 2020 +0000
Revision:
11:2e514d955a2b
Parent:
10:806cfec92eb6
Child:
12:8cb1f89541d1
changed RawSerial to BufferedSerial on mbed-os-6.2.0(mbed_version.h=set correct version)

Who changed what in which revision?

UserRevisionLine numberNew contents of line
kenjiArai 11:2e514d955a2b 1 https://github.com/ARMmbed/mbed-os/#a2ada74770f043aff3e61e29d164a8e78274fcd4