The Battery Backup Power Switch (BBPS) allows the device to switch the backup domain supply (VDDBU) between main power (VDD) and battery backup power (VBAT), see Figure 1. The BBPS can be forced to, or automatically switch between, either VDD or VBAT.
Forced Power Switch
After any reset the BBPS is configured with no action (BBPS.CONF written to 0x0). This means that the backup domain is supplied by VDD. The BBPS can also be forced (BBPS.CONF written to 0x2) to supply the backup domain from the VBAT supply pin.
Automatic Power Switch
As mentioned, the BBPS can also be configured to automatically switch from main power to battery backup power and vice-versa. The Automatic Power Switch (BBPS.CONF written to 0x1) is the only configuration allowing switching both ways. If VDD decreases below a certain threshold level the Automatic Power Switch switches to VBAT and the device enters BACKUP. If VDD is restored, the device can either stay in BACKUP or leave BACKUP, depending on the Wake Enable (BBPS.WAKEEN) configuration.
BOD33 Power Switch
BOD33 Power Switch (BBPS.CONF written to 0x3) enables automatic switching from VDD to VBAT. The Threshold Level bit group in the BOD33 Control register (BOD33.BKUPLEVEL) holds the VDD threshold when the device is in Backup sleep mode, and the VBAT threshold level in Active mode. The device will enter BACKUP when VDD threshold value is violated if BOD33 Action is to enter Backup sleep mode (BOD33.ACTION written to 0x3) and Voltage Monitor is set to monitor VDD (BOD33.VMON written to zero).
Main Power Supply OK Pin Enable
Main Power Supply OK Pin Enable is a configuration enabling automatic switching from VBAT to VDD using the PSOK pin (PB00). If the Automatic Power Switch is not enabled (BBPS.CONF is not 0x1) when VDD is restored and Main Power Supply OK Enable is set (BBPS.PSOKEN written to one), a low-to-high transition on the PSOK pin will switch VDDBU back to VDD. The device can either stay in BACKUP or leave BACKUP, depending on the Wake Enable (BBPS.WAKEEN) configuration.