Freescale Semiconductor HCS08 Series, Reference Manual

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Chapter 4 Memory
MC9S08JS16 MCU Series Reference Manual, Rev. 4
Freescale Semiconductor 55
must be programmed to logic 0 to enable block protection. Therefore the value 0xF8 must be programmed
into NVPROT to protect addresses 0xFA00 through 0xFFFF.
Figure 4-6. Block Protection Mechanism
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.
4.6.7 Flash Block Protection Disabled
Protected flash section can be disabled by back to back write 0x55 and 0xAA to the address of FPROTD
(flash protection defeat register) first, then setting bit FPDIS = 1 in FPROT.
4.6.8 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 zero. 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. Now,
if a TPM1 overflow interrupt is taken for instance, 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.
4.7 Security
The MC9S08JS16 series include 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).
FPS7 FPS6 FPS5 FPS4 FPS3 FPS2 FPS1
A15 A14 A13 A12 A11 A10 A9 A8
1
A7 A6 A5 A4 A3 A2 A1 A0
1 1 1 1 1 1 1 1