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The 8-byte comparison key can be used to temporarily disengage memory security
provided the key enable field, NV_FSEC[KEYEN], is 10b. 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 NV_FSEC[KEYEN] bit to 0b. 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,
NV_FSEC[SEC], to the unsecured state (10b).
3.4 Random-access memory (RAM)
The locations in RAM below 0x0100 can be accessed using the more efficient direct
addressing mode. Any single bit in this area can be accessed with the bit manipulation
instructions (BCLR, BSET, BRCLR, and BRSET).
The RAM retains data when the MCU is in low-power wait, or stop 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 this series, re-initialize the stack pointer to the top of the RAM so that 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-provided equate file).
LDHX
#RamLast+1
TXS
When security is enabled, the RAM is considered a secure memory resource and is not
accessible through BDM or code executing from non-secure memory.
3.5 Flash memory
This device includes flash memory. The flash memory is ideal for single-supply
applications that allow for field reprogramming without requiring external high voltage
sources for program or erase operations. The flash module includes a memory controller
that executes commands to modify flash memory contents. The user interface to the
memory controller consists of the indexed flash common command object (FCCOB)
NXP Semiconductors
;point one past RAM
;SP<-(H:X-1)
MC9S08SU16 Reference Manual, Rev. 5, 4/2017
Chapter 3 Memory
51
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