Datasheet

MC68HC812A4 Data Sheet, Rev. 7
Freescale Semiconductor 55
Chapter 5
Operating Modes and Resource Mapping
5.1 Introduction
The MCU can operate in eight different modes. Each mode has a different default memory map and
external bus configuration. After reset, most system resources can be mapped to other addresses by
writing to the appropriate control registers.
5.2 Operating Modes
The states of the BKGD, MODB, and MODA pins during reset determine the operating mode after reset.
The SMODN, MODB, and MODA bits in the MODE register show the current operating mode and provide
limited mode switching during operation. The states of the BKGD, MODB, and MODA pins are latched
into these bits on the rising edge of the reset signal.
The two basic types of operating modes are:
• Normal modes — Some registers and bits are protected against accidental changes.
• Special modes — Protected control registers and bits are allowed greater access for special
purposes such as testing and emulation.
A system development and debug feature, background debug mode (BDM), is available in all modes. In
special single-chip mode, BDM is active immediately after reset.
5.2.1 Normal Operating Modes
These modes provide three operating configurations. Background debugging is available in all three
modes, but must first be enabled for some operations by means of a BDM command. BDM can then be
made active by another BDM command.
Table 5-1. Mode Selection
BKGD MODB MODA Mode
Port A
Port B
Port C Port D
0 0 0 Special single-chip
G.P.
(1)
I/O
1. G.P. = General purpose
G.P. I/O G.P. I/O
0 0 1 Special expanded narrow ADDR DATA G.P. I/O
0 1 0 Special peripheral ADDR DATA DATA
0 1 1 Special expanded wide ADDR DATA DATA
1 0 0 Normal single chip G.P. I/O G.P. I/O G.P. I/O
1 0 1 Normal expanded narrow ADDR DATA G.P. I/O
1 1 0 Reserved (forced to peripheral) — — —
1 1 1 Normal expanded wide ADDR DATA DATA