MC68HC908JW32_09 Datasheet, PDF (35/232 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Random-Access Memory (RAM)
2.4 Random-Access Memory (RAM)
Addresses $0060 through $045F are RAM locations. The location of the stack RAM is programmable.
The 16-bit stack pointer allows the stack to be anywhere in the 64k-byte memory space.
NOTE
For correct operation, the stack pointer must point only to RAM locations.
Within page zero are 160 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.
2.5 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 and erase operations are enabled
through the use of an internal charge pump.
2.5.1 Functional Description
The FLASH memory consists of an array of 32,768 bytes for user memory plus a block of 48 bytes for
user interrupt vectors and one byte for the mask option register. An erased bit reads as logic 1 and a
programmed bit reads as a logic 0. The FLASH memory page size is defined as 512 bytes, and is the
minimum size that can be erased in a page erase operation. Program and erase operations are facilitated
through control bits in FLASH control register (FLCR). The address ranges for the FLASH memory are:
• $7000–$EFFF; user memory, 32,768 bytes
• $FFD0–$FFFF; user interrupt vectors, 48 bytes
Programming tools are available from Freescale. Contact your local Freescale representative for more
information.
NOTE
A security feature prevents viewing of the FLASH contents.⁽¹⁾
1. No security feature is absolutely secure. However, Freescale’s strategy is to make reading or copying the FLASH difficult for
unauthorized users.
MC68HC908JW32 Data Sheet, Rev. 6
Freescale Semiconductor
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