Chapter 3Chapter 3

Basic Logic GatesBasic Logic Gates

1

ObjectivesObjectives

You should be able to:You should be able to:– Describe the operation and use of AND gates Describe the operation and use of AND gates

and OR gates.and OR gates.– Construct truth tables for AND and OR gates.Construct truth tables for AND and OR gates.– Draw timing diagrams for AND and OR gates.Draw timing diagrams for AND and OR gates.– Use timing analysis to describe operation of an Use timing analysis to describe operation of an

ENABLE function.ENABLE function.

2

ObjectivesObjectives

You should be able to:You should be able to:– Sketch external connections to IC chips to Sketch external connections to IC chips to

implement AND and OR logic circuits.implement AND and OR logic circuits.– Explain how to use a logic pulser and a logic Explain how to use a logic pulser and a logic

probe to troubleshoot digital ICs.probe to troubleshoot digital ICs.– Describe the operation and use of NAND and Describe the operation and use of NAND and

NOR gates.NOR gates.– Construct truth tables for NAND and NOR Construct truth tables for NAND and NOR

gates.gates.

3

ObjectivesObjectives

You should be able to:You should be able to:– Draw timing diagrams for NAND and NOR Draw timing diagrams for NAND and NOR

gates.gates.– Use the outputs of a Johnson shift counter to Use the outputs of a Johnson shift counter to

generate specialized waveforms using generate specialized waveforms using combinations of the five basic gates.combinations of the five basic gates.

– Develop a comparison of Boolean equations Develop a comparison of Boolean equations and truth tables for the five basic gates.and truth tables for the five basic gates.

4

The AND GateThe AND Gate

The output, X, is HIGH if input A AND input The output, X, is HIGH if input A AND input B are both HIGHB are both HIGH

Figure 3-1Figure 3-1

5

The AND GateThe AND Gate

Truth Table – Truth Table –

6

The AND GateThe AND Gate

Boolean Equation X = A AND B orBoolean Equation X = A AND B or X = AB X = AB

Can have more than two inputsCan have more than two inputs Number of combinations = 2Number of combinations = 2NN

7

The OR GateThe OR Gate

The output at X will be HIGH whenever input The output at X will be HIGH whenever input A OR input B is HIGH or both are HIGHA OR input B is HIGH or both are HIGH

Figure 3-5Figure 3-5

8

The OR GateThe OR Gate

Truth Table -Truth Table -

9

The OR GateThe OR Gate

Boolean Equation X = A OR B or Boolean Equation X = A OR B or X = A+BX = A+B

Can have more than two inputsCan have more than two inputs Number of combinations = 2Number of combinations = 2NN

10

Timing AnalysisTiming Analysis

Timing diagramTiming diagram OscilloscopeOscilloscope

– Voltage versus timeVoltage versus time– Up to twoUp to two

Logic analyzerLogic analyzer– State tableState table– Up to 16Up to 16

11

Timing AnalysisTiming Analysis

MultiSIM AND gate simulation.MultiSIM AND gate simulation.

12

Timing AnalysisTiming Analysis Example of timing analysis –Example of timing analysis –

– Determine the output.Determine the output.

13

Enable and Disable FunctionsEnable and Disable Functions Enable function using AND gateEnable function using AND gate

Figure 3-17

14

Enable and Disable FunctionsEnable and Disable Functions Disable function using OR gateDisable function using OR gate

Figure 3-18

15

Using Integrated Circuit Logic Using Integrated Circuit Logic GatesGates

Enable and DisableEnable and Disable Pin ConnectionsPin Connections VVCCCC and GND and GND Figure 3-20

16

Introduction to Introduction to Troubleshooting TechniquesTroubleshooting Techniques

The procedure used to find the fault or trouble The procedure used to find the fault or trouble in a circuit.in a circuit.

Logic ProbeLogic Probe– Metal tipMetal tip– Indicator lamp(s)Indicator lamp(s)

Floating - open circuit, neither HIGH nor LOWFloating - open circuit, neither HIGH nor LOW Logic Pulser - provide known digital signal to Logic Pulser - provide known digital signal to

a circuita circuit

17

Discussion PointsDiscussion Points

Describe how a logic probe and pulser Describe how a logic probe and pulser could be used to troubleshoot an AND could be used to troubleshoot an AND gate.gate.

Describe how a logic probe and pulser Describe how a logic probe and pulser could be used to troubleshoot an OR could be used to troubleshoot an OR gate.gate.

18

The InverterThe Inverter

Used to complement or invert a digital signalUsed to complement or invert a digital signal Figure 3-27Figure 3-27

19

The InverterThe Inverter

Truth Table Truth Table Boolean Equation X = ABoolean Equation X = A Inversion barInversion bar NOT gateNOT gate

20

The NAND GateThe NAND Gate

Same as the AND gate except that its Same as the AND gate except that its output is invertedoutput is inverted

Figure 3-29Figure 3-29

21

The NAND GateThe NAND Gate Truth TableTruth Table Boolean Equation X = ABBoolean Equation X = AB Multiple inputs - the output is always Multiple inputs - the output is always

HIGH unless all inputs go HIGHHIGH unless all inputs go HIGH

22

The NOR GateThe NOR Gate

Same as the OR gate except that its Same as the OR gate except that its output is invertedoutput is inverted

Figure 3-36Figure 3-36

23

The NOR GateThe NOR Gate

Truth TableTruth Table Boolean Equation X = A + BBoolean Equation X = A + B

See Table 3-8 in your textSee Table 3-8 in your text

24

Logic Gate Waveform GenerationLogic Gate Waveform Generation

Repetitive waveformRepetitive waveform Waveform generatorWaveform generator

– Johnson shift counterJohnson shift counter– Figure 3-43Figure 3-43

25

Discussion PointDiscussion Point Which Johnson counter outputs must be Which Johnson counter outputs must be

connected to a 3 input AND gate to connected to a 3 input AND gate to enable only Cenable only CPP #4? #4?

26

Discussion PointDiscussion Point Sketch the output waveform resulting Sketch the output waveform resulting

from inputting the Johnson counter from inputting the Johnson counter outputs shown:outputs shown:

27

Using IC Logic GatesUsing IC Logic Gates

Hex - six gatesHex - six gates Figure 3-60Figure 3-60

28

Using IC Logic GatesUsing IC Logic Gates Quad - four gatesQuad - four gates three-, four-, and eight-input three-, four-, and eight-input

configurationsconfigurations Figure 3-61Figure 3-61

29

Summary of Basic Logic Gates Summary of Basic Logic Gates and IEEE/IEC Standard Logic and IEEE/IEC Standard Logic

SymbolsSymbols

30

Summary of Basic Logic Gates Summary of Basic Logic Gates and IEEE/IEC Standard Logic and IEEE/IEC Standard Logic

SymbolsSymbolsFigure 3-65

31

Discussion PointDiscussion Point

Briefly describe the operation of each of the Briefly describe the operation of each of the basic logic gates:basic logic gates:– ANDAND– OROR– NOT (inverter)NOT (inverter)– NANDNAND– NORNOR

32

Discussion PointDiscussion Point

Create a truth table for a three input NAND Create a truth table for a three input NAND gate.gate.

Write the Boolean equation for a 3 input OR Write the Boolean equation for a 3 input OR gate.gate.

33

Discussion PointDiscussion Point

Sketch the output waveform X for the 2 input Sketch the output waveform X for the 2 input AND gate shown.AND gate shown.

34

Discussion PointDiscussion Point Sketch the output waveforms for the Johnson Sketch the output waveforms for the Johnson

shift counter outputs shown:shift counter outputs shown:

35

Discussion PointDiscussion Point Determine the problem (if any) with the 7427 Determine the problem (if any) with the 7427

NOR IC using the logic probe results shown:NOR IC using the logic probe results shown:

36

SummarySummary

The AND gate requires that all inputs are The AND gate requires that all inputs are HIGH in order to get a HIGH outputHIGH in order to get a HIGH output

The OR gate outputs a HIGH if any of its The OR gate outputs a HIGH if any of its inputs are HIGHinputs are HIGH

An effective way to measure the precise An effective way to measure the precise timing relationships of digital waveforms is timing relationships of digital waveforms is with an oscilloscope or a logic analyzerwith an oscilloscope or a logic analyzer

37

SummarySummary

Beside providing the basic logic functions, Beside providing the basic logic functions, AND and OR gates can also be used to AND and OR gates can also be used to enable or disable a signal to pass from one enable or disable a signal to pass from one point to anotherpoint to another

There are several integrated circuits There are several integrated circuits available in both TTL and CMOS that available in both TTL and CMOS that provide the basic logic functionsprovide the basic logic functions

38

SummarySummary

Two important troubleshooting tools are the Two important troubleshooting tools are the logic pulser and the logic probe. The pulser logic pulser and the logic probe. The pulser is used to inject pulses into a circuit under is used to inject pulses into a circuit under test. The probe reads the level at a point in test. The probe reads the level at a point in a circuit to determine is it is HIGH, LOW, or a circuit to determine is it is HIGH, LOW, or floatingfloating

An inverter provides an output that is the An inverter provides an output that is the complement of its inputcomplement of its input

39

SummarySummary

A NAND gate outputs a LOW when all of its A NAND gate outputs a LOW when all of its inputs are HIGHinputs are HIGH

A NOR gate outputs a HIGH when all of its A NOR gate outputs a HIGH when all of its inputs are LOWinputs are LOW

Specialized waveforms can be created by Specialized waveforms can be created by using a repetitive waveform generator and using a repetitive waveform generator and the basic gatesthe basic gates

40

SummarySummary

Manufacturers’ data manuals are used by Manufacturers’ data manuals are used by the technician to find the pin configuration the technician to find the pin configuration and operating characteristics for the ICs and operating characteristics for the ICs used in modern circuitry.used in modern circuitry.

41