LEARNING BY EXPERIMENTS

DIGITALINTEGRATED CIRCUITS

B y :

M. C. SHARMA, M. Sc.

Publishers :

B.P.B. PublicationsDELHI MADRAS HYDERABAD

By the same author

1. Practical Transistor Novelties2. Simple Audio Projects

3. Practical SCR/Triac Projects

4. 4 Easy to Build Alarms

5. Using Field Effect Transistors6. 555 Timer-Use and Applications

7. Build Your Own Test Instruments8. Understanding and Using Multimeters

9. Using Semi-conductor Diodes

10. Power Supplies for all occasions11. Care & Repair of Electronic Flash Guns12. Learning by Experiments-CMOS I. Cs.

First Printing 1978

Reprinted 1980

Published by G. C. Jain for Business Promotion Bureau, Delhi-110007 and printed by him at Goyal Offset

printers, Shahzada Bagh, Delhi - 35

Oc Business Promotion Bureau

CONTENTS

Page

Preface. 5

1. Introduction

The b inary sy stem, binary coded dec imal (BCD),data representation, logic levels and logic circuits. 7logi

2. Logic Circuits

The inverter, AND gate, NAND gate, OR gate, NOR gate, Wire AND gate, Schmitt trigger NAND gate, Exclusive OR

and Exclusive-NOR gates. 14

3. Flip-Flops

The RS flip-flop, D flip-flop and JK flip-flop. 30

4. Sequential Circuits

Ripple counte r s , down counte r , up-down counte r , special counters, BCD counters, shift registers, Left-Right Shifting, Ring counters and Johnson counters. 41

g ,

Combinational Circuits

BCD to decimal decoders, seven segment decoders, multiplexers and de-multiplexers.

10. Clocks and One ShotsClock oscillators, one shot multivibrator and re-trig-gerable one shots.

5. Applications

Simple alarm, coin-toss, RF phase shifter, three phase square wave generator, sequential switching,-electro-n i c d i ce . t ime ba se to ta l i z e r , f r equency counte r , t ime in t e r va l me te r , d i g i t a l c l ock and numbe r of pulses generator.

6. Appendix

List of parts, power supply and logic board. 102

8. Answers

91

105

PREFACE

Digital techniques are now used virtually in every area of electronics. The greatest use is in computers; but apart from com-puters, digital methods are now employed in communications, telemetry, industrial controls and consumer equipment like radio, TV, watch, clocks etc. As an electronics engineer, technician or a hobbyist, you can benefit by knowing digital techniques.

The classical approach to electronics education had been to stress on circuit theory and to use simple block diagrams to show interconnections. As a result, most of us are oriented in such a manner that we allot a disproportionate importance to circuitry and consider organization as being of nominal importance. How-ever, more elaborate systems cannot be understood solely in terms of circuit operations. In fact, as systems become more complex, understanding must be based more and more on the way in which circuits are interconnected, with much less attention to the fine details of circuit operation, so long as it performs its function (like amplification, oscillation etc.). Digital systems are a represent-ative of this class. The basic circuits are very simple and of very few types, but it is possible to make a wide variety of devices by interconnecting them in different ways.

Digital techniques are not new. They have been known for years, The key and dc relay telegraphy is the ancient digital system. It is only the availability of modern, low cost integrated circuits that has made their widespread use practical.

The aim of this book is to present, in a simple and practical manner. the principles of digital electronics. Along with explana-tions of circuit functions, practical experiments, which can be per-formed without the necessity of elaborate equipment or external gui-dance, have been described. A knowledge of transistors on the part of reader is presumed. To help in providing a deeper understanding of the subject, exercises with answers have also been included. A

LEARNING BY EXPERIMENTS

DIGITALINTEGRATED CIRCUITS

B y :

M. C. SHARMA, M. Sc.

Publishers :

B.P.B. PublicationsDELHI MADRAS HYDERABAD

By the same author

I. Practical Transistor Novelties2. Simple Audio Projects

3. Practical SCR/Triac Projects

4: Easy to Build Alarms

5. Using Field Effect Transistors

6. 555 Timer-Use and Applications

7. Build Your Own Test Instruments8. Understanding and Using Multimeters

9. Using Semi-conductor Diodes

10 Poaer Supplies on all occasions11 Care and Repair of Electronic FlashI Learning by Experiments-CMOS I. Cs.

First Printing 1978

Reprinted 1980

Published by G. C. Jain for Business Promotion Bureau, Delhi-110007 and printed by him at Goyal Offset

printers, Shahzada Bagh, Delhi - 35

Oc Business Promotion Bureau

CONTENTS

Page

Preface 5

1. Introduction

The b inary sy stem, binary coded dec imal (BCD),data representation, logic levels and logic circuits. 7logi

Logic Circuits

The inverter, AND gate, NAND gate, OR gate, NOR gatee xc lusive-OR and exclusive-NOR gates. 14

3. Flip-Flops

The RS flip-flop, L ' flip-Plop and JK flip-flop. 30

4, Sequential Circuits

special counters, BCD counters, shift registers, left-rightR ipple counte r s , down counter , up-down counte r , shifting, ring counters and Johnson counters. 41

g ,

5. Combinational Circuits

BCD to decimal decoders, seven segment decoders, multiplexers and de-multiplexers.

6. Clocks and One ShotsClock oscillators, one shot multivibrator and re-trig-gerable one shots.

7. Applications

Simple alarm, coin-toss, RF phase shifter, three phase square wave generator, sequential switching,-electro-n i c d i ce . t ime ba se to ta l i z e r , f r equency counte r , t ime in t e r va l me te r , d i g i t a l c l ock and numbe r of pulses generator.

8. Appendix

List of parts, power supply and logic board. 10210. Answers

91

105

PREFACE

Digital techniques are now used virtually in every area of electronics. The greatest use is in computers; but apart from com-puters, digital methods are now employed in communications, telemetry, industrial controls and consumer equipment like radio, TV, watch, clocks etc. As an electronics engineer, technician or a hobbyist, you can benefit by knowing digital techniques.

The classical approach to electronics education had been to stress on circuit theory and to use simple block diagrams to show interconnections. As a result, most of us are oriented in such a manner that we allot a disproportionate importance to circuitry and consider organization as being of nominal importance. How-ever, more elaborate systems cannot be understood solely in terms of circuit operations. In fact, as systems become more complex, understanding must be based more and more on the way in which circuits are interconnected, with much less attention to the fine details of circuit operation, so long as it performs its function (like amplification, oscillation etc.). Digital systems are a represent-ative of this class. The basic circuits are very simple and of very few types, but it is possible to make a wide variety of devices by interconnecting them in different ways.

Digital techniques are not new. They have been known for years, The key and dc relay telegraphy is the ancient digital system. It is only the availability of modern, low cost integrated circuits that has made their widespread use practical.

The aim of this book is to present, in a simple and practical manner. the principles of digital electronics. Along with explana-tions of circuit functions, practical experiments, which can be per-formed without the necessity of elaborate equipment or external gui-dance, have been described. A knowledge of transistors on the part of reader is presumed. To help in providing a deeper understanding of the subject, exercises with answers have also been included. A

few practical applications have also been given to enable the person to assemble and enjoy interesting projects. All circuits have been limited to TTL logic only which is the most popular one at present. But, once the basic principles are understood, it is not difficult to handle other types. I hope readers will find it useful. Suggestions for improvement shall be gratefully accepted.

M. C. Sharma

1 Introduction

In electronics there are two basic types of signals, Analog and Digital. Of these the analog signals are more famil iar. An analog is an ac or dc voltage (or current) that varies smoothly and continuously and which does not change in steps. Digital signals are essentially a series of pulses or rapidly changing voltage (or current) levels that vary in discrete steps or increments. Examples of analog and digital signals are shown in Fig. 1. Electronic circuits that process these step signals are cal led digi tal , logic or pulse circuits.

ANALOG SIGNALS DIGITAL SIGNALS

Fig. 1. Examples of Analog and Digital Signals.

In practical life there are many examples of both. The speedometer of a car is an analog device while the odometer, which changes in steps of 1 km, is digital. The tuning control in a radio receiver is analog while the channel selector in a TV is digital. In electronics, digital methods permit new approaches in equipment design. They also make it possible to do things that have no analog equivalent.

The same device can behave as an analog or as a digital device depending on the way it isused. Consider an electric bulb. We

can vary the current through it continuously and its brightness vary. Used in this way, the bulb is an analog device. There are virtually an infinite number of brightness levels.

The most common way of us ing a l ight bulb as a digi talg

device , is to give i t s br i l l iance two levels ; 'o ff ' and on ' . The lamp thus has only two states and we can say that lamp is binary in nature.

The term 'binary' designates any two state device or signal. The two states can be represented in many ways, e.g. off and on; no and yes; open and closed etc. The most common way of represe-nting them is by '0' and '1'. The meanings to 0 and 1 can be given arbitrarily.

In digital electronics, the binary system has become very popu-lar because of ease of implementation. An electronic unit that has only two states, on and off, is very much simpler in design, less expensive, faster and reliable. In the above example of bulb, when used as an analog device, any variation in line voltage will show up as variation i n brilliance. But in the digital mode, it is very easy to say definitely whether the lamp is off or on inspire of wide var iat ions in the l ine vo ltage. This i l lustrates that the digi tal systems have high noise rejection capability.

The Binary System

In the binary system, we use only two digits, 0 and 1. These binary digits, or bits, when, appropriately arranged can also rep-resent any decimal number. For example, the binary number 1101 represents the decimal number 13. The basic distinguishing feature of any number system is its base or 'radix'. The decimal system has a radix of ten. i.e. we go to the next column on the tenth count. The binary system has a radix of two only. Both systems are positional or weighted number systems. This means that each digit or bit posi t ion in a number carr ies a part icular weight in determining the magnitude of that number. To determine the number, you multiply each digit by the weight of the position and add your results. This is illustrated for both systems in Fig. 2.

8

Table in Fig-3 shows the binary equivalents of some decimal numbers.

Fig-2. Weighted Decimal and Binary Systems

Decimal No

BinaryEquivalent

Decimal No

BinaryEquivalent

0 0000 10 10101 0001 11 10112 0010 12 11003 0011 13 11014 0100 14 11105 0101 15 11116 0110 16 100007 0111 32 1000008 1000 64 10000009 1001 128 10000000

Fig. 3. Binary Equivalents of Decimal Numbers,

9

Binary numbers are also referred to as binary words; e.g. an 8 bit binary number is also an 8 bit binary word. Fig. 4 illustrates as how a binary word can be represented in electronic circuits by switches or by lamps. In the illustration a switch in on position or a glowing lamp is taken to represent a binary 1 while an off switch or a dark lamp represents a binary 0. The weights of the positions is also shown in the figure.

Fig. 4. Representation of Binary Words by Switches and by Lamps

The number of bits in a word determine the number of discrete states that can exist; or in other words, the maximum decimal number value that can be represented. The total number of states is given by the formula N=211, where N is the total number of states and n is the number of bits in the word used. For example, with a four bit word we can represent a maximum of 2^4 or 16 states. These can be seen as binary numbers from binary 0000 (decimal 0) to binary 1111 (decimal 15) in fig. 3.

Binary Coded Decimal (BCD)

Because the decimal number is so familiar, it is easy to use. The binary system is less convenient. Digital engineers recognized this problem and developed a special form of binary code that was more compatible with the decimal system. This special code is knowna Binary Coded Decimal or BCD. This code uses the standard 8421 binary weighted code for decimal numbers from 0 through 9. A four bit word can actually represent decimal numbers upto 15, but in this case the decimal equivalents from 10 through 15 are considered invalid and therefore ignored. To carry to next set of 4 bit binary word is given on the tenth count of the previous number, as done in the decimal system.

Data Representation

There are two basic ways in which digital numbers are tran-smitted; processed or manipulated. These are known as series' and 'Parallel'. In the series method, each bit of binary word is processed or transmitted serially one after another, one at a time The primary advantage is that this system requires only a single line or channel for data transmission from one place to another. It is, therefore, simplest and economical, but it takes a longer processing time.

In the parallel method, all bits of binary words are processed simultaneously. This system requires as many separate channels or wires as the number of bits in the word. The transmission is, therefore, more complex but fast.

Logic Levels

The basic element for representing a single bit of data is a switch. Its on-off nature makes it perfect for binary data repres-entation. A mechanical switch however, is too sluggish and cannot be manipulated fast. A transistor on the other hand, can readily assume two distinct states, conductive and cut-off. It can also change states at a very fast rate. The transistor is, therefore, very much used as a switch in digital electronic circuits.

The bit assignments are represented by voltage levels. For example, a binary 0 may be represented by 0 volts. And a binary I by +5 volts. This is called a positive logic system and used in the TTL circuits. In a negative logic system, 0 volts will represent binary 1 and +5 volts will represent binary 0. The switching element merely controls the voltage levels.

Logic Circuits

Our ability to think logically requires both, a capability for making decisions based on the available data or facts and our ability to remember. This holds true for d igital circuits and equipment as well. The decision making circuits are called `gates' while the memory circuits are made up of circuits called `flip-flops'.

11

I I