Chapter 20 Digital Circuits © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Objectives Explain the difference

Chapter 20

Digital Circuits

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Objectives

• Explain the difference between analog and digital systems.

• Convert decimal numbers to their binary equivalents and binary numbers to their decimal equivalents.

• Name seven types of logic gates.


Objectives

• Explain the operation of various types of logic gates.

• Use truth tables to determine the output of a logic gate.

• Discuss two types of logic families.


Objectives

• Explain the digital encoders and decoders.

• Explain analog-to-digital and digital-to-analog devices.

• List three types of flip-flops and explain their truth tables.


ICs

• Include transistors, diodes, resistors, and capacitors

• Major advantage is size• Linear circuits

– Used as amplifiers and have variable outputs

• Digital circuits– Used as switches and work in the on or off

state


Digital Fundamentals

• Binary numbering system

• Voltage logic levels

• Bits, nibbles, and bytes


Digital Advantages

• Inexpensive, compared to analog systems

• Easier information storage

• Speed is greater than in analog systems

• Compatibility with computers

• Less effect from temperature changes


Digital Characteristics

• Capable of replacing analog systems

• Cannot be distinguished from analog systems in most cases

• Information is handled using switching circuits

• Combination of logic gate and flip-flop circuits


Binary Numbering System

• Digital circuits act in on or off states

• Comparable to a single-pole single-throw (SPST) switch– Switch in “on” position

represents a 1


Basic Counting Rules

• Digits must be recorded one after the other for each counting unit

• When count exceeds total number of available digits, a second column begins

• Decimal system’s first column ends at 9, and second column begins at 10


Binary System

• 2 is comparable to decimal system’s 10

• 2 moves to second column, becomes a 0 or 1


Decimal to Binary Conversion Table


Binary Numbers

• Count binary numbers on your fingers

• Number the fingers on your left hand as shown

• Fingers pointing up are 1s, fingers folded down are 0s


Large Binary Numbers and Addition


Voltage Logic Levels in Digital Circuits

• Type of logic circuitry or family affects operating voltages needed in a circuit

• Valid logic high range

• Valid logic low range

• Invalid value range, or intermediate range


Bits, Nibbles, and Bytes

• Bit comes from joining binary and digit

• Bit is smallest unit of information

• Computer memory– Kilobytes– Megabytes– Gigabytes

• Computer storage abilities are growing


Review

What is the primary difference between the decimal numbering system and the binary numbering system?

The decimal system has numbers 0–9, and the binary system has 0 and 1


Review

What is the smallest unit of information in binary code?

A bit


Logic Gates

• AND gates

• OR gates

• NOT gates

• NAND gates

• NOR gates

• XOR gates

• XNOR gates


AND Gates

• Accepts high and low inputs (1 and 0)

• High and low outputs• Output of 1 if all

inputs are 1• If only one gate is on,

circuit will not work• Truth tables


AND Gates (Cont.)


AND Gates (Cont.)

• Valid logic highs and lows


OR Gates

• Output signal of 1 if either or both inputs is 1

• When all inputs are 0, output is 0

• Acts similar to two switches in parallel


OR Gates (Cont.)



NOT Gates

• Used to invert polarity of input signal– Also called inverters

• If input is 1, output is 0• If input is 0, output is 1


NOT Gates (Cont.)



NAND Gates

• Made of an AND gate and a NOT gate

• Reverse of AND application

• Also called NOT AND gates


NAND Gates (Cont.)



NOR Gates

• Opposite of OR gate• Made of OR and NOT

gates• Used to test for any

kind of input• No input will have

output of 1


NOR Gates (Cont.)



XOR Gates

• Also called exclusive OR gates

• Provide high output if any, but not all, inputs are logic high

• Provide low output if all inputs are logic high


XOR Gates (Cont.)



XNOR Gates

• Also called exclusive NOR gates

• XOR gate with inverted output

• High output only when all inputs are logic high or logic low


Review

Which gate has an output signal of 1 if either or both inputs is 1?

OR gate


Review

Which gate provides high output if any, but not all, inputs are logic high, but provides a low output if all inputs are logic high?

XOR gate


Review

Which gate is made of an AND and a NOT gate?

NAND gate


Review

Which gate has an inverted output so it has a high output only when all inputs are logic high or logic low?

XNOR


Logic Families

• Traits of one logic family must match traits of another family when many digital ICs are used in one device

• Complementary metal-oxide semiconductor logic (CMOS)

• Transistor-transistor logic (TTL)


CMOS Logic

• Circuits have good resistance to noise

• Use FETs

• Require small amounts of power

• Can be damaged by static electricity

• Worker and work surface must be grounded through high-resistance resistor


TTL

• Circuits work quickly

• Function from 20 megahertz to 60 megahertz

• Faster than CMOS ICs

• Require high power dissipation and high current

• Many gates can be placed in one TTL IC


Review

If many digital circuits are used in one device, how should logic families be treated?

Traits of one logic family must match traits of another family


Review

Which of the two types of logic families can be easily damaged by static electricity?

CMOS


Digital Applications

• Logic probes

• Digital encoders and decoders

• Digitized analog signals

• Flip-flops

• Counters


Logic Probes

• Indicate a high or low signal using LEDs

• Connect to power supply of circuit being tested

• Equipped with a slide switch to select the logic family

• If both LEDs light, there is an invalid logic level voltage


Digital Encoders and Decoders

• Electronic systems translate binary system into decimal system

• Analog-to-digital converters can change analog numbers to a digital number equivalent– Digital multimeter as example

• Can convert electrical pulses into binary numbers


Digitized Analog Signals

• Digital signals representing sound waves or linear voltage

• Analog-to-digital encoders are used


Flip-Flops

• Set-reset (R-S) flip-flops

• J-K flip-flops

• D flip-flops


R-S Flip-Flops

• When S is high, Q is high• Output represents last input setting if both inputs are low• Outputs are complementary• Clocked R-S flip-flops


J-K Flip-Flops

• Clock driven• Retain output status

when two inputs are low

• When both inputs are high, outputs toggle on and off


D Flip-Flops

• Do not require two inputs

• Q outputs toggle when input signal is received if clock signal is applied

• Output state of Q will not change without clock signal

• Output Qs are always complementary


Counters

• Can carry and borrow• Fabricated from individual flip-flops• Available as integrated chips


Decade Counters

• Can count based on ten• Can be used to divide by ten• Used as frequency dividers for oscilloscopes or

in digital clocks


Computerized Tomography (CT)

• Uses photomultiplier tubes, computer scans, and X-rays to produce digitized images of the body

• Photocathodes

• Dynodes

• Anode grid

• Gantry

• Contrast agent


Review

How can binary numbers be translated into the decimal system?

Digital decoders


Review

What is a flip-flop?

A semiconductor device that can assume one of two stable states


Review

What kind of flip-flop does not require two inputs?

D flip-flop


Review

What does it mean if two outputs are complementary?

When one output is high, the other is low


For Discussion

• Give examples of advantages integrated circuits have over transistors.

• What are the differences among the logic gates?

• Give examples of possible advances that will be made in the computer field in the next 25 years, based on what you have learned in this chapter.

Documents

Chapter 20 Digital Circuits © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Objectives Explain the difference