Datasheet

Chapter 4 Memory
MC9S08EL32 Series and MC9S08SL16 Series Data Sheet, Rev. 3
Freescale Semiconductor 45
Nonvolatile FLASH registers, shown in Table 4-4, are located in the FLASH memory. These registers
include an 8-byte backdoor key, NVBACKKEY, which can be used to gain access to secure memory
resources. During reset events, the contents of NVPROT and NVOPT in the nonvolatile register area of
the FLASH memory are transferred into corresponding FPROT and FOPT working registers in the
high-page registers to control security and block protection options.
Provided the key enable (KEYEN) bit is 1, the 8-byte comparison key can be used to temporarily
disengage memory security. This key mechanism can be accessed only through user code running in secure
memory. (A security key cannot be entered directly through background debug commands.) This security
key can be disabled completely by programming the KEYEN bit to 0. If the security key is disabled, the
only way to disengage security is by mass erasing the FLASH if needed (normally through the background
debug interface) and verifying that FLASH is blank. To avoid returning to secure mode after the next reset,
program the security bits (SEC) to the unsecured state (1:0).
Table 4-4. Nonvolatile Register Summary
AddressRegister NameBit 7654321Bit 0
0xFFAE
Reserved for
FTRIM storage
— — — — — — —FTRIM
0xFFAF
Reserved for
ICSTRM storage
TRIM
0xFFB0 –
0xFFB7
NVBACKKEY
8-Byte Comparison Key
0xFFB8 –
0xFFBC
Reserved
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
0xFFBD
NVPROT
EPS FPS FPOP
0xFFBE
Reserved
— — — — — — — —
0xFFBF
NVOPT
KEYEN FNORED EPGMOD
— — — SEC