MC9S08JM16 Datasheet, PDF (54/386 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 4 Memory
is no way to disengage security without completely erasing all flash locations. If KEYEN is 1, a secure
user program can temporarily disengage security by:
1. Writing 1 to KEYACC in the FCNFG register. This makes the flash module interpret writes to the
backdoor comparison key locations (NVBACKKEY through NVBACKKEY+7) as values to be
compared against the key rather than as the first step in a flash program or erase command.
2. Writing the user-entered key values to the NVBACKKEY through NVBACKKEY+7 locations.
These writes must be done in order starting with the value for NVBACKKEY and ending with
NVBACKKEY+7. STHX must not be used for these writes because these writes cannot be done
on adjacent bus cycles. User software normally would get the key codes from outside the MCU
system through a communication interface such as a serial I/O.
3. Writing 0 to KEYACC in the FCNFG register. If the 8-byte key that was just written matches the
key stored in the flash locations, SEC01:SEC00 are automatically changed to 1:0 and security will
be disengaged until the next reset.
The security key can be written only from secure memory (RAM or flash), so it cannot be entered through
background commands without the cooperation of a secure user program.
The backdoor comparison key (NVBACKKEY through NVBACKKEY+7) is located in flash memory
locations in the nonvolatile register space, so users can program these locations exactly as they would
program any other flash memory location. The nonvolatile registers are in the same 512-byte block of flash
as the reset and interrupt vectors, so block protecting that space also block protects the backdoor
comparison key. Block protects cannot be changed from user application programs, so if the vector space
is block protected, the backdoor security key mechanism cannot permanently change the block protect,
security settings, or the backdoor key.
Security can always be disengaged through the background debug interface by taking these steps:
1. Disable any block protections by writing FPROT. FPROT can be written only with background
debug commands, not from application software.
2. Mass erase flash if necessary.
3. Blank check flash. Provided flash is completely erased, security is disengaged until the next reset.
To avoid returning to secure mode after the next reset, program NVOPT so SEC01:SEC00 = 1:0.
4.7 Flash Registers and Control Bits
The flash module has nine 8-bit registers in the high-page register space, three locations in the nonvolatile
register space in flash memory which are copied into three corresponding high-page control registers at
reset. There is also an 8-byte comparison key in flash memory. Refer to Table 4-3 and Table 4-4 for the
absolute address assignments for all flash registers. This section refers to registers and control bits only by
their names. A Freescale-provided equate or header file normally is used to translate these names into the
appropriate absolute addresses.
MC9S08JM16 Series Data Sheet, Rev. 2
54
Freescale Semiconductor