mc68hc908gr8b Freescale Semiconductor, Inc, mc68hc908gr8b Datasheet - Page 36

• Manufacturer Part Number: mc68hc908gr8b
• Description: M68hc08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Memory
2.5 Random-Access Memory (RAM)
Addresses $0040 through $01BF 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.
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 $00FF out of page zero, direct addressing mode instructions can efficiently
access all page zero RAM locations. Page zero RAM, therefore, provides ideal locations for frequently
accessed global variables.
Before processing an interrupt, the CPU uses five bytes of the stack to save the contents of the CPU
registers.
During a subroutine call, the CPU uses two bytes of the stack to store the return address. The stack
pointer decrements during pushes and increments during pulls.
2.6 FLASH Memory (FLASH)
This subsection describes the operation of the embedded FLASH memory. This memory can be read,
programmed, and erased from a single external supply. The program, erase, and read operations are
enabled through the use of an internal charge pump.
2.6.1 Functional Description
The FLASH memory is an array of 7,680 bytes with an additional 36 bytes of user vectors and one byte
of block protection. An erased bit reads as 1 and a programmed bit reads as a 0. Memory in the FLASH
array is organized into two rows per page basis. The page size is 64 bytes per page and the row size is
32 bytes per row. Hence the minimum erase page size is 64 bytes and the minimum program row size is
32 bytes. Program and erase operation operations are facilitated through control bits in FLASH control
register (FLCR). Details for these operations appear later in this section.
The address ranges for the user memory and vectors are:
1. No security feature is absolutely secure. However, Freescale’s strategy is to make reading or copying the FLASH difficult for
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unauthorized users.
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$E000–$FDFF; user memory
$FE08
$FF7E; FLASH block protect register
$FFDC–$FFFF; these locations are reserved for user-defined interrupt and reset vectors
;
FLASH control register
For correct operation, the stack pointer must point only to RAM locations.
Be careful when using nested subroutines. The CPU may overwrite data in
the RAM during a subroutine or during the interrupt stacking operation.
A security feature prevents viewing of the FLASH contents.
For M6805 compatibility, the H register is not stacked.
MC68HC908GR8B Data Sheet, Rev. 3.0
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Freescale Semiconductor