NXP Semiconductors K22F series, Reference Manual, Page: 627 / 1407

28.5.4.2.6 FAC application tips
In one use case, the NVSACC1 and NVXACC1 locations are programmed by NXP and
they protect NXP libraries that have been programmed into associated flash segments in a
device. Later, the NVSACC2 and NVXACC2 NVM locations can optionally be
programmed by a third-party vendor who wants to program their proprietary software and
to extend the protection of protected flash segments to include their software libraries
before supplying it all to their customers.
Their customer would then develop their own code to use the available libraries, and
program their code into the remaining available on-chip flash. The device continues to
support the end user with standard security features that further limit external access to
flash resources.
28.6 Initialization and application information
The FMC does not require user initialization. Flash acceleration features are enabled by
default.
The FMC has no visibility into flash memory erase and program cycles because the Flash
Memory module manages them directly. As a result, if an application is executing flash
memory commands, the FMC's cache might need to be disabled and/or flushed to prevent
the possibility of returning stale data. Use the PFB0CR[CINV_WAY] field to invalidate
the cache in this manner.
Chapter 28 Flash Memory Controller (FMC)
K22F Sub-Family Reference Manual, Rev. 4, 08/2016
NXP Semiconductors 627