MC68908GZ8MFJE Datasheet, PDF (38/314 Pages) Freescale Semiconductor, Inc – Microcontrollers

Memory
2.5 Random-Access Memory (RAM)
Addresses $0040 through $043F are RAM locations. The location of the stack RAM is programmable.
The 16-bit stack pointer allows the stack 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 192 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 out of page zero, direct addressing mode instructions can efficiently
access all page zero RAM locations. 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 M6805 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 may overwrite data in
the RAM during a subroutine or during the interrupt stacking operation.
2.6 FLASH Memory (FLASH)
This subsection describes the operation of the embedded FLASH memory. This memory can be read,
programmed, and erased from a single external supply. The program, erase, and read operations are
enabled through the use of an internal charge pump. It is recommended that the user utilize the FLASH
programming routines provided in the on-chip ROM, which are described more fully in a separate
application note.
2.6.1 Functional Description
The FLASH memory is an array of 15,872 bytes with an additional 44 bytes of user vectors and one byte
of block protection. An erased bit reads as logic 1 and a programmed bit reads as a logic 0. Memory in
the FLASH array is organized into two rows per page basis. For the 16-K word by 8-bit embedded FLASH
memory, the page size is 64 bytes per page and the row size is 32 bytes per row. Hence the minimum
erase page size is 64 bytes and the minimum program row size is 32 bytes. Program and erase operation
operations are facilitated through control bits in FLASH control register (FLCR). Details for these
operations appear later in this section.
MC68HC908GZ16 • MC68HC908GZ8 Data Sheet, Rev. 4
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