MC68HC908GT16_07 Datasheet, PDF (40/292 Pages) Freescale Semiconductor, Inc – Microcontrollers

Memory
2.5 Random-Access Memory (RAM)
Addresses $0040 through $023F 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
This sub-section 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.
2.6.1 Functional Description
The Flash memory is an array of 15,872 bytes (7,680 bytes on MC68HC908GT8) with an additional
36 bytes of user vectors, one byte of block protection and two bytes of ICG user trim storage. An erased
bit reads as 1 and a programmed bit reads as a 0. Memory in the Flash array is organized into two rows
per page basis. 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.
The address ranges for the user memory and vectors are:
• $C000–$FDFF; user memory ($E000–$FDFF on MC68HC908GT8)
• $FE08; Flash control register
• $FF7E; Flash block protect register
• $FF80; ICG user trim register (ICGTR5)
• $FF81; ICG user trim register (ICGTR3)
• $FFDC–$FFFF; these locations are reserved for user-defined interrupt and reset vectors
MC68HC908GT16 • MC68HC908GT8 • MC68HC08GT16 Data Sheet, Rev. 5.0
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