13.11.6.5.1 Initialization

Before a Brown-Out Detector (BOD33) is enabled, it must be configured, as outlined by the following:

Set the BOD threshold level (BOD33.LEVEL)
Set the configuration in Active, Standby, Backup modes (BOD33.ACTCDG, BOD33.STDBYCFG, BODVDD.BKUP)
Set the prescaling value if the BOD will run in sampling mode (BOD33.PSEL)
Set the action and hysteresis (BOD33.ACTION and BOD33.HYST)

The BOD33 register is Enable-Protected, meaning that it can only be written when the BOD is disabled (BOD33.ENABLE = 0 and SYNCBUSY.BOD33EN = 0). As long as the Enable bit is '1', any writes to Enable-Protected registers will be discarded, and an APB error will be generated. The Enable bits are not Enable-Protected.