sc9s08mz16 Freescale Semiconductor, Inc, sc9s08mz16 Datasheet - Page 51

• Manufacturer Part Number: sc9s08mz16
• Description: Hcs08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Chapter 4 Memory
memory. In addition to programming the FPS bits to the appropriate value, FPDIS (bit 0 of NVPROT)
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.
FPS7 FPS6 FPS5 FPS4 FPS3 FPS2 FPS1
1
1
1
1
1
1
1
1
1
A15
A14
A13
A12
A11
A10
A9
A8
A7 A6 A5 A4 A3 A2 A1 A0
Figure 4-4. 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.
4.4.7
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 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 an RTI interrupt is taken for instance, the values in the locations 0xFDCC:FDCD are used for the vector
instead of the values in the locations 0xFFCC:FFCD. 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.
4.5
Security
The SC9S08MZ16 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 nonvolatile 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 done 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
SC9S08MZ16 MCU Data Sheet, Rev. 0 Draft C
Freescale Semiconductor
51