MC68HC08AS32A Datasheet, PDF (40/296 Pages) Motorola, Inc – Microcontrollers

Memory
Freescale Semiconductor, Inc.
2.3 Random-Access Memory (RAM)
Addresses $0050–$044F are RAM locations. The location of the stack RAM is
programmable. The 16-bit stack pointer allows the stack to be anywhere in the
1024-byte memory space.
NOTE: For correct operation, the stack pointer must point only to RAM locations.
Within page zero are 176 bytes of RAM. Because the location of the stack RAM is
programmable, all page zero RAM locations can be used for input/output (I/O)
control and user data or code. When the stack pointer is moved from its reset
location at $00FF, direct addressing mode instructions can access all page zero
RAM locations efficiently. Page zero RAM, therefore, provides ideal locations for
frequently accessed global variables.
Before processing an interrupt, the CPU uses five bytes of the stack to save the
contents of the CPU registers.
NOTE: For M68HC05, M6805, and M146805 compatibility, the H register is not stacked.
During a subroutine call, the CPU uses two bytes of the stack to store the return
address. The stack pointer decrements during pushes and increments during pulls.
NOTE: Be careful when using nested subroutines. The CPU could overwrite data in the
RAM during a subroutine or during the interrupt stacking operation.
2.4 Read-Only Memory (ROM)
The user ROM consists of 32,256 bytes of ROM from addresses $8000–$FDFF.
The monitor ROM and vectors are located from $FE20–$FEFF. See Figure 2-1.
Thirty-six of the user vectors, $FFDC–$FFFF, are dedicated to user-defined reset
and interrupt vectors.
Security has been incorporated into the MC68HC08AS32A to prevent external
viewing of the ROM contents. This feature ensures that customer-developed
software remains proprietary.
2.5 Electrically Erasable Programmable ROM (EEPROM)
This subsection describes the 512 bytes of electrically erasable programmable
read-only memory (EEPROM) residing at address range $0800 to $09FF.
See Figure 2-1.
EEPROM features include:
• 512 bytes non-volatile memory
• Byte, block, or bulk erasable
• Non-volatile EEPROM configuration and block protection options
Data Sheet
40
Memory
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MC68HC08AS32A — Rev. 1
MOTOROLA