13.11.6.3.3.3 Main Power Supply OK (PSOK) Pin Enable

The state of the Main Power VDD can be used to switch between supply sources as long as the Battery Backup Power Switch is not configured as Automatic Power Switch (i.e., BBPS.CONF not set to APWS): when the Main Power Supply OK Pin Enable bit in the BBPS register is written to '1' (BBPS.PSOKEN), restoring VDD will form a low-to-high transition on the PSOK pin. This low-to-high transition will switch the Backup Power Supply back to VDD.