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-os.lib

Committer:
kenjiArai
Date:
2020-08-01
Revision:
11:2e514d955a2b
Parent:
10:806cfec92eb6
Child:
12:8cb1f89541d1

File content as of revision 11:2e514d955a2b:

https://github.com/ARMmbed/mbed-os/#a2ada74770f043aff3e61e29d164a8e78274fcd4