HCS08 datasheet(47/316 Pages) NXP

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Chapter 4 Memory

MC9S08SG8 MCU Series Data Sheet, Rev. 8

Freescale Semiconductor

Nonvolatile FLASH registers, shown in Table 4-4, are located in the FLASH memory. These registers

include an 8-byte backdoor key, NVBACKKEY, which can be used to gain access to secure memory

resources. During reset events, the contents of NVPROT and NVOPT in the nonvolatile register area of

the FLASH memory are transferred into corresponding FPROT and FOPT working registers in the

high-page registers to control security and block protection options.

Provided the key enable (KEYEN) bit is 1, the 8-byte comparison key can be used to temporarily

disengage memory security. This key mechanism can be accessed only through user code running in secure

memory. (A security key cannot be entered directly through background debug commands.) This security

key can be disabled completely by programming the KEYEN bit to 0. If the security key is disabled, the

only way to disengage security is by mass erasing the FLASH if needed (normally through the background

debug interface) and verifying that FLASH is blank. To avoid returning to secure mode after the next reset,

program the security bits (SEC) to the unsecured state (1:0).

Table 4-4. Nonvolatile Register Summary

Address

Bit 7

654321

Bit 0

0xFFAE

Reserved for 

storage of FTRIM

—

—FTRIM

0xFFAF

Reserved for 

storage of ICSTRM

TRIM

0xFFB0 –

0xFFB7

NVBACKKEY

8-Byte Comparison Key

0xFFB8 –

0xFFBC

Reserved

—

0xFFBD

NVPROT

FPS

FPDIS

0xFFBE

Reserved

—

0xFFBF

NVOPT

KEYEN

FNORED

—

SEC

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