Datasheet

M68HC12B Family Data Sheet, Rev. 9.1
Freescale Semiconductor 75
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 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. During reset an active pullup is connected to the
BKGD pin (as input) and active pulldowns are connected to the MODB and MODA pins. If an open occurs
on any of these pins, the device will operate in normal single-chip mode.
The two basic types of operating modes are:
1. Normal modes — Some registers and bits are protected against accidental changes.
2. 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.
Table 5-1. Mode Selection
BKGD MODB MODA Mode Port A Port B
0 0 0 Special single chip General-purpose I/O General-purpose I/O
0 0 1 Special expanded narrow
ADDR[15:8]
DATA[7:0]
ADDR[7:0]
0 1 0 Special peripheral
ADDR
DATA
ADDR
DATA
0 1 1 Special expanded wide
ADDR
DATA
ADDR
DATA
1 0 0 Normal single chip General-purpose I/O General-purpose I/O
1 0 1 Normal expanded narrow
ADDR[15:8]
DATA[7:0]
ADDR[7:0]
110
Reserved
(forced to peripheral)
——
1 1 1 Normal expanded wide
ADDR
DATA
ADDR
DATA