Verify Backdoor Access Key command as valid comparison values. While the Verify
Backdoor Access Key command is active, program flash memory is not available for read
access and returns invalid data.
The user code stored in the program flash memory must have a method of receiving the
backdoor keys from an external stimulus. This external stimulus would typically be
through one of the on-chip serial ports.
If the KEYEN bits are in the enabled state, the MCU can be unsecured by the backdoor
key access sequence described below:
1. Follow the command sequence for the Verify Backdoor Access Key command as
Verify Backdoor Access Key command
2. If the Verify Backdoor Access Key command is successful, the MCU is unsecured
and the FSEC[SEC] bits are forced to the unsecure state
An illegal key provided to the Verify Backdoor Access Key command prohibits future
use of the Verify Backdoor Access Key command. A reset of the MCU is the only
method to re-enable the Verify Backdoor Access Key command when a comparison fails.
After the backdoor keys have been correctly matched, the MCU is unsecured by changing
the FSEC[SEC] bits. A successful execution of the Verify Backdoor Access Key
command changes the security in the FSEC register only. It does not alter the security
byte or the keys stored in the Flash Configuration Field (
). After the next reset of the MCU, the security state of the FTFE module
reverts back to the Flash security byte in the Flash Configuration Field. The Verify
Backdoor Access Key command sequence has no effect on the program and erase
protections defined in the program flash protection registers.
If the backdoor keys successfully match, the unsecured MCU has full control of the
contents of the Flash Configuration Field. The MCU may erase the sector containing the
Flash Configuration Field and reprogram the flash security byte to the unsecure state and
change the backdoor keys to any desired value.
16.5.12.2 Unsecuring the MCU Using Backdoor Key Access
The MCU can be unsecured by using the backdoor key access feature which requires
knowledge of the contents of the 8-byte backdoor key value stored in the Flash
Configuration Field (see
Flash configuration field description
are in the enabled state, the Verify Backdoor Access Key command (see
) can be run which allows the user to present prospective keys for
comparison to the stored keys. If the keys match, the FSEC[SEC] bits are changed to
Chapter 16 Flash Memory Module (FTFE)
Kinetis KE1xZ256 Sub-Family Reference Manual, Rev. 3, 07/2018
NXP Semiconductors
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