Microprocessors from A to Z---Doing it the easy way (ET/D, Nov. 1979)

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By Bernard B. Daien

ET/D begins a series for people who want to know what a microprocessor is, how it works, and how to use it. It is a "stand alone" course with all terms defined and no previous computer technology knowledge required.

This series provides an easy entry to the world of microprocessors by stressing concepts, instead of details. These articles can be quickly read by those who understand basic electronics, but not digital electronics.

Upon completion, the reader should be capable of understanding the trade literature covering the installation, use, troubleshooting, and programming of microprocessors.

Fig. 1--A sophisticated, programmable microprocessor is the heart of this advanced heat pump control system from Honeywell's Residential Controls Center. Transducers encode indoor, outdoor and wall temperatures in home or office buildings for computerized control of indoor air flows.

Microprocessors' growth

Although microprocessors (MPUs) are usually viewed as "computers," the computing function is only a portion of the MPUs applications, since it is well suited for data processing and controller uses. "Data processing" includes, but is not limited to, the gathering, analyzing, sorting, organizing, arranging, and distributing of data. To do this, data may have to be stored, retrieved from storage, displayed, printed out, or altered, as needed. The data may be used in computation, but as you can now appreciate, the computation would only be a small part of the data processing.

For example: customer lists are first sorted alphabetically, then placed on punch cards, which are used later for billing, mailing lists, or other uses. The customer list might also be placed on magnetic tape for storage, encoded in digital form. 'The term "digital" will be explained later.) The MPU would be used to encode and decode the information, and use it to perform desired tasks in accordance with the instructions given to the MPU in the form of a "program" written by a programmer.

This handling of digital data is a very large and important use of MPUs. Data transmission over telephone and other communication systems is now a huge business, and growing faster than any other type of communication. Banks communicate about credit cards and accounts. Stock markets reports and other business transactions are included in data transmissions.

In some cases, large computers are required because of the amount of data to be handled, and the speed required, but there are many situations in which a smaller, slower, and less expensive machine (the MPU) is adequate.

Because of the tremendous amount of data being handled every day, manual handling is far too slow and error prone to suit our modern society. The MPU can be utilized to perform thousands of operations per second, involving different types of data, performing many different operations, and is faster than all other methods of data processing, excepting only a larger, more expensive, general purpose computer.

MPUs also are used extensively in "process control." In this case the MPU acts as a "controller," achieving desired conditions and results, by controlling machines in accordance with programmed instructions. Some automobiles now have MPUs which control the mixture of gasoline and air so as to achieve complete combustion.

These MPUs take into account such variables as altitude, air temperature, barometric pressure variations due to weather, and engine loading. In the past, controllers were devised using combinations of mechanical, electrical, hydraulic, and pneumatic systems. But the MPU is faster, more accurate, smaller, lighter, cheaper, and it is programmable. It is therefore very flexible, and can be used in a wide Fig. 2--An example of types of test gear field service technicians of the future will be working with is this portable, microprocessor controlled diagnostic tool. For use in troubleshooting Honeywell's heat pump control system, the field technician simply connects to the unit under test and automatic "error" messages display location and type of trouble.

variety of applications, which results in large sales, which further reduces the price due to mass production. This is a main reason for the MPUs' success ... wide applicability, mass production, low price, due directly to its programmability.

The MPU invasion

In many cases the MPU is a built-in unrecognizable part of the equipment it controls. Using data from the equipment itself, and instructions from humans, and drawing upon its memory banks, the MPU makes fast, accurate, complex decisions, which control the host equipment in a very versatile manner.

The MPU is being used more and more to control machinery, and again the word "speed" crops up.

Of course it isn't practical to use a $500,000 computer to control a $50,000 machine in most cases. The MPU with its lower cost makes practical the use of digital control for medium, and even small sized, business.

The MPU can be programmed, and readily reprogrammed, to do many different tasks. Thus, one MPU can be many things for many people, like a magic circuit board, which changes its functions in accordance with the wishes of the operator! An MPU can be purchased, secure in the knowledge that it will not be rendered useless by some change required in the manufactured product.

So ... if you think about it, we are saying that MPUs use programming to replace hardware. By changing the programmed instructions, the MPU can be made to perform a new and different task, without the necessity of purchasing new equipment. And that is the secret of success of the MPU.

What a microprocessor is

MPUs have been defined many different ways, which often confuses readers of most microprocessor texts. Usually the discussion concerns computers, and then the MPU is defined as "part of a computer." We will attempt to define the MPU in simpler terms.

Modern MPUs are large scale integrated circuits (LSIC), containing 8,000-to-10,000 transistors which can be used to replace many different types of integrated circuits, by imitating their functions. The MPU is fed an input, called "data," and instructions telling it what to do with the data. It is the instruction that tells the MPU what type of circuit it must emulate.

The term "large scale integrated circuit" indicates the size and complexity of the integrated circuit. An IC which replaces a single stage of an amplifier, is considered "small scale." If it replaces an entire sub-section, such as the entire sound amplifier, or color section, in a TV set, it would be regarded as "medium scale." Larger ICs with thousands of transistors are "large scale." Today we have "very large scale" ICs, with hundreds of thousands of transistors, and the end is not in sight! Let's be specific. Suppose we want an MPU to add some numbers. The numbers to be added are the "data." An instruction, "add" indicates what we want to do with the data. In this case, the MPU is performing the function of an adder.

The MPU can perform a very wide variety of functions, some requiring an entire series of steps to complete. The MPU can perform several functions, in sequence, in order to accomplish a given task. This requires a series of instructions, called a "program." Since we wish to enter the program into the MPU all at once, we need some "memory" circuits, which can accept the entire program of instructions, and then implement it, step by step, in the necessary sequence. This establishes the fact that the MPU IC, by itself, is very limited in usefulness. Some memory circuitry is required to "support" the MPU. Other "supporting" circuitry also includes input and output "interface devices. Interface devices are not new to you. A hi-fi amplifier often uses a matching transformer to interface between the amplifier and the speakers, for example. MPUs often communicate with the user over the telephone, in which case the interface device would be a telephone coupler.

Interfacing can be defined as the process of connecting different circuits together in such a way that they operate in a coordinated manner. When we interface "input" or "output" devices, we abbreviate them to "I/O devices."

Let's get some feeling for how complex the MPU IC is. Relating to what you are familiar with, the MPU is more complex than the student type electronic pocket calculators, which can perform only the functions indicated on the keyboard. The MPU remember, can be programmed for a wide variety of tasks.

On the other hand the MPU is less complex than the larger general purpose computers, because the MPU by itself cannot do very much. It requires support circuitry consisting of a power supply, interface devices, memory circuitry, a control panel, some kind of readout, etc.

The general purpose computer has all of these things within its cabinet.

Before the advent of the MPU, logic circuits were constructed on printed circuit boards to perform specific functions. If the function had to be changed, the circuitry had to be rebuilt, with loss of time, labor, and money. With the MPU, a library of programs permits frequent, quick, task changes.

Hardware vs. software

It is important to understand that programs do not materialize out of the air! They are costly. A programmer needs skill, time, and equipment to write a program of instructions. The program must be run on equipment similar to the MPU it is to be used with, in order to detect any errors in the program.

Eliminating all of the errors requires running, and rerunning, and correcting, and re-correcting the program. This is called "debugging," and may take weeks, depending on the length and complexity of the program. We are stating that we have exchanged one set of problems for another set of problems.

In the language of computers, we have traded "hardware" for "software." Hardware is defined as actual physical apparatus including electrical, electronic and mechanical parts. "Software" is defined as programs and instructions, on paper. Sometime the MPU and software can be used to replace hardware with economic advantages. At times the reverse is true. Each application must be examined to determine the most cost effective approach.

Microprocessors vs microcomputers

The MPU alone is not a self sufficient device. It needs much support circuitry to perform most tasks. If we add the required circuitry ... power supplies, memory, I/O to interface devices, etc., we can perform computing tasks, and at that point we have expanded our MPU into a minimal computer ... a "microcomputer." This small computer is not as fast, and not as flexible as a large computer. It can only do one operation at a time, step by step, in accordance with the program. Most tasks take the MPU through many steps, requiring milliseconds to accomplish. A large computer would do the same task in a few microseconds, thus accomplishing more work per second.

This ability to do work fast is called "computing power," therefore MPUs are not considered to be very powerful computers. They are adequate where great speed and flexibility are not paramount.

MPUs are also limited in the amount o memory capacity that can be controlled Great strides are currently being made in MPU design, and the new MPUs are as powerful as some of the larger computers of a few years ago ... but ... in the meantime, the larger computers have also grown more powerful, so that the gap still remains when comparing the performance of an MPU with a large computer.

A large bank needs a large computer to handle the thousands of transactions occurring daily, and recording and storing enormous amounts of data in memory. A small factory, on the other hand, might only require an MPU with appropriate supporting hardware to control a production machine. The difference in cost between the two systems would be large. As a matter of fact, the MPU would cost so little, that the cost of the software programs required would probably be greater than the hardware costs!

Fig. 3--Leader Instruments Corporation's "computerized" 5800 production test oscilloscope system is perhaps typical of the types of sophisticated production control units soon to be common. This system permits anyone who can use an oscilloscope to select up to 32 preset configurations for the viewing of waveforms at up to eight preselected test points.

The MPU and you

By now you should be starting to understand the implications of the MPU and the new industry it has created.

People are needed to build MPUs, sell MPUs, install MPUs, maintain MPUs, and program MPUs. Other people are needed to build the required support hardware ... power supplies, keyboards, cabinets, semiconductors, etc.

Technicians who understand the microprocessor (MPU), can take their pick of jobs offered in all the trade periodicals. There is an optimum time to enter each new technology. For example, five years ago a TV technician who understood vacuum tubes could get along nicely, but today a TV technician needs to know both vacuum tube and solid state to service the existing sets. In another five years a tech will only need to know solid state. Most of the tube type sets will have worn out. Another example: A tech working in a factory making auto radios would have had to learn about solid state when technology mandated a fast changeover to transistor auto radios. If he did study solid state in the 1950's, he had only to learn about the bipolar transistor, and then, when JFETs, MOSFETs, Darlingtons, integrated circuits, etc., came along, they were learned easily, one at a time. The tech who delayed getting into solid state, had to learn all the devices at once, a much more difficult task. So there is an optimum time to "get into" a new technology, and it is now.

It has been said that a programmer really does not need to know what is inside the MPU in order to write a program. This is only partially true. The programmer who understands how the MPU works, however, can generally generate shorter, more concise programs than the programmer who merely follows certain rules, without knowing why. You will certainly be working with apparatus that uses, or interfaces with, MPUs. You will benefit from the knowledge about how the MPU works. This fact has not escaped the notice of most employers, and there is now arising a good demand for employees who understand the MPU.

There are several well known companies now manufacturing MPUs, and others are still entering the race, as the market grows. Each brand has its advantages, and disadvantages. Some have more internal memory, others more speed, others more flexibility of application. There is still no real standardization in many areas ... therefore you cannot replace one MPU with another in most cases. Certain brands have arranged for other manufacturers to build the same device so as to have a "second source," which most customers require.

Some brands have a wide variety of support hardware available, while others do not. There is no hard and fast rule. This series of articles has been written to enable you to deal with most of the popular MPUs now on the market.


You already know the terms hardware and software ... there is a third term coming into popular use, "firmware." Firmware, as you might guess, is somewhere between hardware and software. Hardware is by its very nature, quite permanent. Software can be easily changed. Firmware consists of memory that can not be readily changed by the user, such as a permanent memory IC.

Such devices are called "Read Only Memories" (ROMs), because, like a book, once written they can be read over and over again without changing the contents of the book. It is not possible to add, delete, or change information stored in a ROM. The information is unchanged by use.

ROMs can be purchased already programmed with the desired information, ready to plug into an MPU system. Other ROMs can be programmed by the user, or by the local distributor, and these are called "Programmable Read Only Memories" (PROMS). We will discuss memories further, later on, but for now it is only necessary to know that it is possible to buy memories large enough to store more information than most MPU users will need. These add-on memories have already enabled the MPU to compete with larger machines in some areas which were formerly the sole domain of the large computer. This accounts, in part, for the growth of the MPU market.

As MPU costs drop it becomes feasible to use them in more applications. Today they are being used in food mixers, microwave ovens, and washing machines. They are used in automobiles to control air gas mix in order to prevent pollution, which is a lot easier than trying to clean up pollution.

(An advantage is better performance and fuel economy.) Perhaps one of the largest factors limiting faster MPU growth is the lack of personnel trained to use the MPU. The MPU is a high technology device, and skilled personnel are required for this technology. By reading the following articles in this series, you will gradually, and easily, move into this technology.


In this article you learned what an MPU is, and what it does. You also acquired some "buzz words," the working language of the MPU technician, trade definitions. More than this, you have now an insight into the implications of the MPU, the factors responsible for its rapid growth, and how the MPU is affecting you and your occupation. This lays the groundwork for the more specific articles to follow, covering the different parts of the MPU, and how they coordinate with each other.

(source: Electronic Technician/Dealer, Nov. 1979)

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