mc9s08qd4 Freescale Semiconductor, Inc, mc9s08qd4 Datasheet - Page 45

• Manufacturer Part Number: mc9s08qd4
• Description: Hcs08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Chapter 4 Memory Map and Register Definition
One use for block protection is to block protect an area of flash memory for a bootloader program. This
bootloader program then can be used to erase the rest of the flash memory and reprogram it. Because the
bootloader is protected, it remains intact even if MCU power is lost in the middle of an erase and
reprogram operation.
4.5.7
Vector Redirection
Whenever any block protection is enabled, the reset and interrupt vectors will be protected. Vector
redirection allows users to modify interrupt vector information without unprotecting bootloader and reset
vector space. Vector redirection is enabled by programming the FNORED bit in the NVOPT register
located at address 0xFFBF to 0. For redirection to occur, at least some portion but not all of the flash
memory must be block protected by programming the NVPROT register located at address 0xFFBD. All
of the interrupt vectors (memory locations 0xFFC0–0xFFFD) are redirected, though the reset vector
(0xFFFE:FFFF) is not.
For example, if 512 bytes of flash are protected, the protected address region is from 0xFE00 through
0xFFFF. The interrupt vectors (0xFFC0–0xFFFD) are redirected to the locations 0xFDC0–0xFDFD. For
example, vector redirection is enabled and an interrupt occurs, the values in the locations 0xFDE0:FDE1
are used for the vector instead of the values in the locations 0xFFE0:FFE1. This allows the user to
reprogram the unprotected portion of the flash with new program code including new interrupt vector
values while leaving the protected area, which includes the default vector locations, unchanged.
4.6
Security
The MC9S08QD4 series includes circuitry to prevent unauthorized access to the contents of flash and
RAM memory. When security is engaged, flash and RAM are considered secure resources. Direct-page
registers, high-page registers, and the background debug controller are considered unsecured resources.
Programs executing within secure memory have normal access to any MCU memory locations and
resources. Attempts to access a secure memory location with a program executing from an unsecured
memory space or through the background debug interface are blocked (writes are ignored and reads return
all 0s).
Security is engaged or disengaged based on the state of two register bits (SEC01:SEC00) in the FOPT
register. During reset, the contents of the nonvolatile location NVOPT are copied from flash into the
working FOPT register in high-page register space. A user engages security by programming the NVOPT
location, which can be performed at the same time the flash memory is programmed. The 1:0 state
disengages security and the other three combinations engage security. Notice the erased state (1:1) makes
the MCU secure. During development, whenever the flash is erased, it is good practice to immediately
program the SEC00 bit to 0 in NVOPT so SEC01:SEC00 = 1:0. This would allow the MCU to remain
unsecured after a subsequent reset.
The on-chip debug module cannot be enabled while the MCU is secure. The separate background debug
controller can be used for background memory access commands, but the MCU cannot enter active
background mode except by holding BKGD/MS low at the rising edge of reset.
A user can choose to allow or disallow a security unlocking mechanism through an 8-byte backdoor
security key. If the nonvolatile KEYEN bit in NVOPT/FOPT is 0, the backdoor key is disabled and there
MC9S08QD4 Series MCU Data Sheet, Rev. 3
Freescale Semiconductor
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