19.6.5.3.5 Backup and Hibernate sleep Modes

To enable the BOD33 in Backup or Hibernate Sleep mode, the Run in Backup or Hibernate Sleep mode bits in the BOD33 register (BOD33.RUNBKUP, BOD33.RUNHIB) must be written to '1'. The BOD33 is automatically set in BOD33 Ultra Low-Power mode. Additionally, the BOD33 will operate in Sampling mode if the BOD33.PSEL bit is non-null. In this state, the voltage monitored by BOD33 is always the supply of the backup domain, that is VDD or VBAT.