unsecure the MCU. The entire 8-byte key cannot be all 0s or all 1s, i.e.
0x0000_0000_0000_0000 and 0xFFFF_FFFF_FFFF_FFFF are not accepted by the
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.
16.6 Reset Sequence
On each system reset the FTFE module executes a sequence which establishes initial
values for the flash block configuration parameters, FPROT, FDPROT, FEPROT, FOPT,
FSEC, FXACC, FSACC, and FACNFG registers and the FCNFG[RAMRDY, EEERDY]
bits.
Reset Sequence
Kinetis KE1xZ256 Sub-Family Reference Manual, Rev. 3, 07/2018
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NXP Semiconductors
Summary of Contents for Kinetis KE1xZ256
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