16.4.2.3 VDDBU Power Switch Control

The Backup section power supply VDDBU is powered by either VBAT or VDD3V3. The voltage can be selected by configuring RSTC_MR.PWRSW. At first power-up (General reset), the backup section is powered by VDD3V3. If RSTC_MR.PWRSW is written to 1, the backup section is powered by VBAT if the voltage on VBAT is above the VBAT Power-On Reset threshold; otherwise, a General reset is performed.

The VDDBU voltage can be automatically switched from VDD3V3 to VBAT in case a VDD3V3 failure is detected by the VDD3V3 supply monitor by setting SUPC_SMMR.IOSMPWRM to 1. Refer to the section “Supply Controller (SUPC)” for details.