NXP Semiconductors KL25 Series - Freescale Semiconductor, Inc

27.4.10.10 Verify Backdoor Access Key Command
The Verify Backdoor Access Key command only executes if the mode and security
conditions are satisfied (see Flash Commands by Mode). Execution of the Verify
Backdoor Access Key command is further qualified by the FSEC[KEYEN] bits. The
Verify Backdoor Access Key command releases security if user-supplied keys in the
FCCOB match those stored in the Backdoor Comparison Key bytes of the Flash
Configuration Field (see Flash Configuration Field Description). The column labelled
Flash Configuration Field offset address shows the location of the matching byte in the
Flash Configuration Field.
Table 27-48. Verify Backdoor Access Key Command FCCOB Requirements
FCCOB Number FCCOB Contents [7:0] Flash Configuration Field Offset Address
0 0x45 (VFYKEY)
1-3 Not Used
4 Key Byte 0 0x0_0000
5 Key Byte 1 0x0_0001
6 Key Byte 2 0x0_0002
7 Key Byte 3 0x0_0003
8 Key Byte 4 0x0_0004
9 Key Byte 5 0x0_0005
A Key Byte 6 0x0_0006
B Key Byte 7 0x0_0007
After clearing CCIF to launch the Verify Backdoor Access Key command, the flash
memory module checks the FSEC[KEYEN] bits to verify that this command is enabled.
If not enabled, the flash memory module sets the FSTAT[ACCERR] bit and terminates.
If the command is enabled, the flash memory module compares the key provided in
FCCOB to the backdoor comparison key in the Flash Configuration Field. If the
backdoor keys match, the FSEC[SEC] field is changed to the unsecure state and security
is released. If the backdoor keys do not match, security is not released and all future
attempts to execute the Verify Backdoor Access Key command are immediately aborted
and the FSTAT[ACCERR] bit is (again) set to 1 until a reset of the flash memory module
module occurs. If the entire 8-byte key is all zeros or all ones, the Verify Backdoor
Access Key command fails with an access error. The CCIF flag is set after the Verify
Backdoor Access Key operation completes.
Table 27-49. Verify Backdoor Access Key Command Error Handling
Error Condition Error Bit
The supplied key is all-0s or all-Fs FSTAT[ACCERR]
An incorrect backdoor key is supplied FSTAT[ACCERR]
Table continues on the next page...
Chapter 27 Flash Memory Module (FTFA)
KL25 Sub-Family Reference Manual, Rev. 3, September 2012
Freescale Semiconductor, Inc. 453