M68HC11
REFERENCE MANUAL
CONFIGURATION AND MODES OF OPERATION
MOTOROLA
3-1
SECTION 3
CONFIGURATION AND MODES OF OPERATION
This section discusses the mechanisms that allow the MC68HC11A8 to conform to a
wide variety of applications. These mechanisms include hardware mode selection cir-
cuitry, a nonvolatile EEPROM-based configuration register, and protected control reg-
ister bits. The majority of the control bits in the MC68HC11A8 are accessible at any
time by software and will be discussed throughout this manual.
The term mode is used in more than one context in discussing the microcontroller unit
(MCU). For example, the serial peripheral interface (SPI) is said to be in either the
master or slave mode, the parallel I/O system is said to be in simple strobed mode,
full-input handshake mode, or full-output handshake mode. In most cases, there is no
confusion about what the term mode refers to; however, the use of the term mode in
conjunction with STOP and WAIT is often misunderstood. STOP and WAIT are actu-
ally modes of operation of the central processing unit (CPU) as opposed to single-chip
and expanded modes, which are modes of operation of the MCU integrated circuit. In
this section, the MCU operating modes and other mechanisms controlling the basic
configuration of the MCU are discussed.
Very few MCU functions are influenced by the mode of operation. For example, the
timers, analog-to-digital converter (A/D), and serial I/O functions all work the same in
expanded modes as they do in single-chip modes. The parallel I/O functions of 18 pins
are lost in the expanded modes but can be regained with a special, external, port-re-
placement chip called the MC68HC24. In the two special modes of MCU operation,
some special testing functions become accessible, including the ability for software to
change the MCU mode.
3.1 Hardware Mode Selection
There are only two fundamental modes of operation for the MC68HC11A8 MCU: sin-
gle chip and expanded. Each mode has a normal variation and a special variation.
These four mode variations are selected by the levels on the mode A (MODA) and
mode B (MODB) pins during reset. The special variation of single-chip mode is called
special bootstrap mode; the special variation of the expanded mode is called special
test mode. The special bootstrap mode allows programs to be downloaded through
the on-chip serial communications interface (SCI) into internal random-access mem-
ory (RAM) to be executed. The bootloaded program is used for a variety of tasks such
as loading calibration values into internal electrically erasable programmable read-
only memory (EEPROM) or performing diagnostics on a finished module. The boot-
strap mode is a special user’s mode, not a factory test mode. The special test mode,
which is intended primarily for factory testing, is seldom used by the user except for
emulation, development, or in other rare circumstances.