HCS08 datasheet(54/316 Pages) NXP

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Chapter 4 Memory

MC9S08SG8 MCU Series Data Sheet, Rev. 8

Freescale Semiconductor

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 4-4. 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.

4.5.7

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 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 SPI 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.

4.6

Security

The MC9S08SG8 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

FPS7

FPS6

FPS5

FPS4

FPS3

FPS2

FPS1

A15

A14

A13

A12

A11

A10

A7 A6 A5 A4 A3 A2 A1 A0

111

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