Page 110 - DCAP210_INTRODUCTION__TO_MICROPROCESSORS

Introduction to Microprocessors

Notes memory. The microcontroller has fewer memory-move instruct ions and more bit -handling
instructions. The reason for the lack of memory-move instructions is that the microcontroller
typically has only a small amount of RAM, which it uses only as a “scratch pad.” The additional
bit handling instructions were included because they are so useful in control system applications.
For example, in a control system, each separate bit of a parallel output word might control a
different device, such as a motor or indicator light. The bit-handling instructions al low the software
to turn one device easily on or off without affecting the others.
In March 1976, Intel announced a single-board computer product that integrated all the support
components required for their 8080 microprocessor, along with 1 kbytes of RAM, 4 kbytes of
user-programmable ROM, and 48 lines of parallel digital I/O with line drivers. The board also
offered expansion through a bus connector, but could be used without an expansion card cage
where applications didn’t require additional hardware. Software development for this system
was hosted on Intel’s Intellec MDS microcomputer development system; this provided asembler
and PL/M support, and permitted in-circuit emulation for debugging.
Processors of this era required a number of support chips in addition. RAM and EPROM were
separate, often requiring memory management or refresh circuitry for dynamic memory as well.
I/O processing might be carried out by a single chip such as the 8255, but frequently required
several more chips.
A single-board microcontroller differs from a single-board computer in that it lacks the general
purpose user interface and mass storage interfaces that a more general-purpose computer would
have. Compared to a microprocessor development board, a microcontroller board would
emphasize digital and analog control interconnections to some controlled system, where a
development board might by comparison have only a few or no discrete or analog input/output
devices. The development board exists to showcase or to train on some particular processor family
and this internal implementation is more important than the external function.
Single-board microcontrollers appeared in the late 1970s when the first
generations of microprocessors, such as the 6502 and the Z80,made it practical

to build an entire controller on a single board, and affordable to dedicate a
processor chip to such a relatively minor task.

7.4.2 Input and Output
Microcontroller systems provide multiple forms of input and output signals to allow application
software to control an external “real-world” system. Discrete digital I/O provides a single bit of
data (on, or off). Analog signals, representing a continuously variable range such as temperature
or pressure, can also be inputs and outputs for microcontrollers.
Discrete digital inputs and outputs might only be buffered from the microprocessor data bus by
an addressable latch, or might be operated by a specialized input/output integrated circuit such
as an Intel 8155 or Motorola 6821 parallel input/output adapter. Later single-chip micrcontrollers
have input and output pins available. The input/output circuits usually do not provide enough
current to directly operate such devices as lamps or motors, so solid-state relays are operated by
the microcontroller digital outputs, and inputs are isolated by signal conditioning level-shifting
and protection circuits.
One or more analog inputs, with an analog multiplexer and common analog to digital converter,
are found on some microcontroller boards. Analog outputs may use a digital-to-analog converter,
or on some microcontrollers may be controlled by pulse-width modulation. As for discrete inputs,
external signal conditioning may be required to scale inputs, or provide such functions as bridge
excitation or cold-junction compensation.

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