MC68HC908AS32A Datasheet, PDF (40/280 Pages) Freescale Semiconductor, Inc – Microcontrollers

Memory
2.7 Random-Access Memory (RAM)
The 1024 bytes of random-access memory (RAM) are located at address $0050–$044F is the RAM
location. The 16-bit stack pointer allows the stack RAM to be anywhere in the 64 Kbyte 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 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. Therefore, page zero RAM 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.
2.8 Electrically Erasable Programmable Read-Only Memory (EEPROM)
This subsection describes the 512 bytes of electrically erasable programmable read-only memory
(EEPROM) residing at address range $0800–$09FF.
Features include:
• 512 bytes nonvolatile memory
• Byte, block, or bulk erasable
• Nonvolatile EEPROM configuration and block protection options
• On-chip charge pump for programming/erasing
• Security option⁽¹⁾
• AUTO bit driven programming/erasing time feature
2.8.1 Functional Description
The 512 bytes of EEPROM are located at $0800–$09FF and can be programmed or erased without an
additional external high voltage supply. The program and erase operations are enabled through the use
of an internal charge pump. For each byte of EEPROM, the write/erase endurance is 10,000 cycles.
1. No security feature is absolutely secure. However, Freescale’s strategy is to make reading or copying the EEPROM difficult
for unauthorized users.
MC68HC908AS32A Data Sheet, Rev. 2.0
40
Freescale Semiconductor