All register writes to the protection unit must be done by a secure bus transaction as defined by TrustZone.
Once the XMPU
LOCK
[RegWrDis]
register bit is set, access to the XMPU registers is disabled and can no longer be
written to until after a POR reset. The only exception is a secure transaction can
write to the interrupt status register (ISR).
Note: Regardless of the LOCK [RegWrDis] setting, the
status registers are always writable by secure and non-secure transactions. All
XMPU registers are only writable by a secure transaction. The registers are
readable by secure or non-secure transactions.