
Memory
MC9S08RC/RD/RE/RG Data Sheet, Rev. 1.11
Freescale Semiconductor
51
4.6.2
FLASH Options Register (FOPT and NVOPT)
During reset, the contents of the nonvolatile location NVOPT are copied from FLASH into FOPT. Bits 5
through 2 are not used and always read 0. This register may be read at any time, but writes have no meaning
or effect. To change the value in this register, erase and reprogram the NVOPT location in FLASH memory
as usual and then issue a new MCU reset.
4.6.3
FLASH Conguration Register (FCNFG)
Bits 7 through 5 may be read or written at any time. Bits 4 through 0 always read 0 and cannot be written.
76543210
R
KEYEN
FNORED
0000
SEC01
SEC00
W
Reset
This register is loaded from nonvolatile location NVOPT during reset.
= Unimplemented or Reserved
Figure 4-6. FLASH Options Register (FOPT)
Table 4-7. FOPT Field Descriptions
Field
Description
7
KEYEN
Backdoor Key Mechanism Enable — When this bit is 0, the backdoor key mechanism cannot be used to
disengage security. The backdoor key mechanism is accessible only from user (secured) rmware. BDM
commands cannot be used to write key comparison values that would unlock the backdoor key. For more detailed
0
No backdoor key access allowed.
1
If user rmware writes an 8-byte value that matches the nonvolatile backdoor key (NVBACKKEY through
NVBACKKEY+7 in that order), security is temporarily disengaged until the next MCU reset.
6
FNORED
Vector Redirection Disable — When this bit is 1, then vector redirection is disabled.
0
Vector redirection enabled.
1
Vector redirection disabled.
1:0
SEC0[1:0]
Security State Code — This 2-bit eld determines the security state of the MCU as shown below. When the
MCU is secure, the contents of RAM and FLASH memory cannot be accessed by instructions from any
unsecured source including the background debug interface. For more detailed information about security, refer
00 Secure
01 Secure
10 Unsecured
11 Secure
SEC01:SEC00 changes to 1:0 after successful backdoor key entry or a successful blank check of FLASH.