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Chapter 4 Memory

MC9S08SG8 MCU Series Data Sheet, Rev. 8

Freescale Semiconductor

4.4

RAM

The MC9S08SG8 includes static RAM. The locations in RAM below 0x0100 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 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 (VRAM).

For compatibility with M68HC05 MCUs, the HCS08 resets the stack pointer to 0x00FF. In the

MC9S08SG8, 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 Freescale Semiconductor-provided equate file).

LDHX

#RamLast+1

;point one past RAM

TXS

;SP<-(H:X-1)

When security is enabled, the RAM is considered a secure memory resource and is not accessible through

BDM or through code executing from non-secure memory. See Section 4.6, “Security”, for a detailed

description of the security feature.

4.5

FLASH

The FLASH memory is intended primarily for program storage. In-circuit programming allows the

operating program to be loaded into the FLASH memory after final assembly of the application product.

It is possible to program the entire array through the single-wire background debug interface. Because no

special voltages are needed for FLASH erase and programming operations, in-application programming

is also possible through other software-controlled communication paths. For a more detailed discussion of

in-circuit and in-application programming, refer to the HCS08 Family Reference Manual, Volume I,

Freescale Semiconductor document order number HCS08RMv1/D.

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