MC9S12HZ256 Datasheet, PDF (663/692 Pages) Freescale Semiconductor, Inc – HCS12 Microcontrollers

Appendix E ROM Description
Table E-2. ROPT Field Descriptions
Field
Description
7:6
Backdoor Key Mechanism Enable — Determines the state of the ROM backdoor key access. If the backdoor
KEYEN[1:0] key mechanism is enabled, user firmware can write a 4-words value that matches the non-volatile backdoor key
(NVBACKKEY through NVBACKKEY+7 in that order) to temporarily disengage security until the next reset. The
backdoor key mechanism is accessible only from user (secured) firmware. BDM commands cannot be used to
write key comparison values that would unlock the backdoor key. If disabled, no backdoor key access is allowed.
The backdoor keywords 0x0000 and 0xFFFF are invalid and will effectively disable the backdoor key access
when used.
00 Disabled
01 Disabled (preferred KEYEN state to disable backdoor key access)
10 Enabled
11 Enabled
5:2
Non-volatile flag bits — This bits are available to the user.
NV[5:2]
1:0
SEC[1:0]
Security State Code — Determines the security state of the MCU.
00 Secured
01 Secured
10 Unsecured
11 Secured
E.1.2 ROM Configuration Register (RCNFG)
7
6
5
4
3
2
1
0
R
0
0
0
0
0
0
0
KEYACC
W
Reset
R
R
R
R
R
R
R
R
= Unimplemented or Reserved
Figure E-2. ROM Configuration Register (RCNFG)
Table E-3. RCNFG Field Descriptions
Field
Description
5
KEYACC
Security Access Key Write Enable — Enables writes to the backdoor comparison key to disable security. This
bit can not be set unless the KEYEN1:KEYEN0 bits are 10. This bit can not be written through BDM.
0 Backdoor key access enabled
1 Backdoor key access disabled
MC9S12HZ256 Data Sheet, Rev. 2.04
Freescale Semiconductor
663