There are conditions when the user may not want to write to the DFM. To protect against spurious DFM writes, various mechanisms have been implemented. On any Reset, the NVMEN bit is cleared. In addition, writes to the DFM are blocked during the Power-up Timer period (TPWRT).
The unlock sequence and the NVMEN bit together help prevent an accidental write during brown-out, power glitch or software malfunction.