The datasheet is unclear about the fuse required for clock failure detection on the ATMega328PB.
The only mention anywhere of the fuse is in 13.2, which refers to “FEB” bit 4. I don’t know from FEB, but EFB is the Extended Fuse Byte (Table 33-4). However, this table is unchanged from the 328P datasheet. Bit 4 is undefined.
AT15007 doesn't talk about fuse changes at all.
So what changed in the fuses between the 328P and 328PB? In particular, Is EFB bit 4 the CFD enable fuse?