HCS08 datasheet(48/290 Pages) MOTOROLA

48 / 290 page

Memory

MC9S08GB/GT

MOTOROLA

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

include an 8-byte backdoor key which optionally 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 (SEC01:SEC00) to the unsecured state (1:0).

4.3 RAM

The MC9S08GB/GT includes static RAM. The locations in RAM below $0100 can be accessed using the

more efficient direct addressing mode, and any single bit in this area can be accessed with the bit

manipulation instructions (BCLR, BSET, BRCLR, and BRSET). Locating the most frequently accessed

program variables in this area of RAM is preferred.

The RAM retains data when the MCU is in low-power wait, stop2, or stop3 mode. At power-on or after

wakeup from stop1, the contents of RAM are uninitialized. RAM data is unaffected by any reset provided

that the supply voltage does not drop below the minimum value for RAM retention.

For compatibility with older M68HC05 MCUs, the HCS08 resets the stack pointer to $00FF. In the

MC9S08GB/GT, it is usually best to reinitialize the stack pointer to the top of the RAM so the direct page

RAM can be used for frequently accessed RAM variables and bit-addressable program variables. Include

the following 2-instruction sequence in your reset initialization routine (where RamLast is equated to the

highest address of the RAM in the Motorola-provided equate file).

LDHX

#RamLast+1

;point one past RAM

TXS

;SP<-(H:X-1)

Table 4-4 Nonvolatile Register Summary

Address

Bit 7

654321

Bit 0

$FFB0 –

$FFB7

NVBACKKEY

8-Byte Comparison Key

$FFB8 –

$FFBC

Reserved

—

$FFBD

NVPROT

FPOPEN

FPDIS

FPS2

FPS1

FPS0

$FFBE

Reserved(1)

NOTES:

1. This location can be used to store the factory trim value for the ICG.

—

$FFBF

NVOPT

KEYEN

FNORED

SEC01

SEC00

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