22.6.3.3 Automatic Battery Backup Power Switch
The supply of the backup domain can be switched automatically to VBAT supply pin by the Automatic Power Switch or by using the BOD33.
The supply of the backup domain can be switched automatically to VDD supply pin either by the Automatic Power Switch or the Main Power Pin when VDD and VDDCORE are restored.
Automatic Power Switch (APWS)
When the Configuration bit field in the Battery Backup Power Switch register (BBPS.CONF) is selecting the APWS, the Automatic Power Switch will function as Battery Backup Power Switch.
The Automatic Power switch allows to switch the supply of the backup domain from VDD to VBAT power and vice-versa.
When the Automatic Power Switch configuration is selected, the Automatic Power Switch Ready bit in the Status register (STATUS.APWSRDY) is set when the Automatic Power Switch is ready to operate. The Automatic Power Switch Ready bit in the Interrupt Flag Status and Clear (INTFLAG.APSWRDY) will be set at the same time.
BOD33 Power Switch
When the Configuration bit field in the Battery Backup Power Switch register (BBPS.CONF) are selecting the BOD33, BOD33 will function as Battery Backup Power Switch. In this case, when the VDD voltage is below the BOD33 threshold, the backup domain supply is switched to VBAT.
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.
Backup Battery Power Switch Status
The Battery Backup Power Switch bit in the Status register (STATUS.BBPS) indicates whether the backup domain is currently powered by VDD or VBAT.