M68HC08 datasheet(21/124 Pages) FREESCALE

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MC68HC05C9A Advance Information Data Sheet, Rev. 5.1

Freescale Semiconductor

Chapter 2

Memory

2.1 Introduction

The microcontroller unit (MCU) has a 16-Kbyte memory map. The memory map consists of:

•

Input/output (I/O), control, and status registers

•

User random-access memory (RAM)

•

User read-only memory (ROM)

•

Self-check ROM

•

Reset and interrupt vectors

See Figure 2-1 and Figure 2-2.

Two control bits in the option register ($3FDF) allow the user to switch between RAM and ROM at any

time in two special areas of the memory map, $0020–$004F (48 bytes) and $0100–$017F (128 bytes).

2.2 RAM

The main user RAM consists of 176 bytes at $0050–$00FF. This RAM area is always present in the

memory map and includes a 64-byte stack area. The stack pointer can access 64 bytes of RAM in the

range $00FF down to $00C0.

NOTE

Using the stack area for data storage or temporary work locations requires

care to prevent it from being overwritten due to stacking from an interrupt

or subroutine call.

Two additional RAM areas are available at $0020–$004F (48 bytes) and $0100–$017F (128 bytes) (see

Figure 2-1 and Figure 2-2.) These may be accessed at any time by setting the RAM0 and RAM1 bits,

respectively, in the option register.

Refer to 1.4 Software-Programmable Options for additional information.

2.3 ROM

The user ROM consists of 48 bytes of page zero ROM from $0020 to $004F, 15,872 bytes of ROM from

$0100 to $3EFF, and 16 bytes of user vectors from $3FF0 to $3FFF.

2.4 ROM Security

A security feature has been incorporated into the MC68HC05C9A to help prevent external access to the

contents of the ROM in any mode of operation.

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