NXP Semiconductors MC9S08QL4, Reference Manual

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Chapter 4 Memory
MC9S08QL8 MCU Series Reference Manual, Rev. 1
NXP Semiconductors 43
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).
4.4 RAM
The MC9S08QL8 series include 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.
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 (V
RAM
).
For compatibility with M68HC05 MCUs, the HCS08 resets the stack pointer to 0x00FF. In the
MC9S08QL8 series, 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 NXP Semiconductors-provided equate file).
LDHX #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
background debug mode (BDM) or through code executing from non-secure memory. See
Section 4.6,
Security for a detailed description of the security feature.
Table 4-4. Reserved Flash Memory Addresses
Address Register Name Bit 7 6 5 4 3 2 1 Bit 0
0xFFAE Reserved for
Storage of
FTRIM
0 0 0 0 0 0 0 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 0 0 0 0 SEC