MC9S08AC16_08 Datasheet, PDF (49/344 Pages) Freescale Semiconductor, Inc – 8-Bit HCS08 Central Processor Unit (CPU)

Table 4-4. Nonvolatile Register Summary
Address Register Name Bit 7
6
5
4
3
0xFFB0 – NVBACKKEY
0xFFB7
8-Byte Comparison Key
0xFFB8 – Reserved
0xFFBC
—
—
—
—
—
—
—
—
—
—
0xFFBD NVPROT
0xFFBE Reserved¹
FPS7
—
FPS6
—
FPS5
—
FPS4
—
FPS3
—
0xFFBF NVOPT
KEYEN FNORED
0
0
0
1 _{This location can be used to store the factory trim value for the ICG.}
2
—
—
FPS2
—
0
Chapter 4 Memory
1
Bit 0
—
—
FPS1
—
SEC01
—
—
FPDIS
—
SEC00
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 MC9S08AC16 Series 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.
For compatibility with older M68HC05 MCUs, the HCS08 resets the stack pointer to 0x00FF. In the
MC9S08AC16 Series, it is usually best to re-initialize 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-provided equate file).
LDHX
TXS
#RamLast+1
;point one past RAM
;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 4.5, “Security” for a detailed
description of the security feature.
MC9S08AC16 Series Data Sheet, Rev. 6
Freescale Semiconductor
49