MC68HC908EY16 Datasheet, PDF (35/278 Pages) Motorola, Inc – Microcontrollers

Random Access Memory (RAM)
2.5 Random Access Memory (RAM)
Addresses $0040–$00FF and $0100–$023F are RAM locations. The location of the stack RAM is
programmable with the reset stack pointer instruction (RSP). The 16-bit stack pointer allows the stack
RAM 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 192 bytes of RAM. Because the location of the stack RAM is programmable, all page
zero RAM locations can be used for input/output (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. 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 central
processor unit (CPU) registers.
NOTE
For M6805, M146805, and M68HC05 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.
2.6 FLASH Memory (FLASH)
The FLASH memory is an array of 15,872 bytes with an additional 36 bytes of user vectors and one byte
used for block protection.
NOTE
An erased bit reads as 1 and a programmed bit reads as 0.
The program and erase operations are facilitated through control bits in the FLASH control register
(FLCR). See 2.6.1 FLASH Control Register.
The FLASH is organized internally as an 16,384-word by 8-bit complementary metal-oxide semiconductor
(CMOS) page erase, byte (8-bit) program embedded FLASH memory. Each page consists of 64 bytes.
The page erase operation erases all words within a page. A page is composed of two adjacent rows.
A security feature prevents viewing of the FLASH contents.⁽¹⁾
In the 125°C to 135°C temperature range, the FLASH is guaranteed as read only.
1. No security feature is absolutely secure. However, Freescale’s strategy is to make reading or copying the FLASH difficult for
unauthorized users.
MC68HC908EY16 • MC68HC908EY8 Data Sheet, Rev. 10
Freescale Semiconductor
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