CHAPTER 2

LOGIC GATES

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BASIC LOGIC GATES Logic gates are the basic

building blocks to form digital circuit.

It has 1 o/p terminal. O/p either HIGH

(logic 1) or LOW (logic 0) depend on digital level at i/p.

7 types of Logic Gates: INVERTER AND OR NAND NOR Exclusive – OR Exclusive - NOR

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2.1.1 NOT / INVERTER Used to complement a digital signal. It has only 1 output.

INPUT OUTPUT

A X

0 1

1 0AX

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2.1.2 AND GATE It has 2 or more input. 2n possible input combinations, where n = no. of

bit. Operation for AND gates is multiplication (x ). O/p is HIGH when both input (A and B) are HIGH.

INPUT OUTPUT

A B X

0 0 0

0 1 0

1 0 0

1 1 1

A

BOUTPUT, X

BAX

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TIMING DIAGRAM

A

B

X

Draw the waveform of X where X = A.B

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2.1.3 OR GATE It has 2 or more input. 2n possible input combinations, where n is no. of bit. Operation for OR gates is adding (+). O/p is HIGH whenever input A and B are HIGH or

both i/p are HIGH.

INPUT OUTPUT

A B X

0 0 0

0 1 1

1 0 1

1 1 1BAX

A

BX

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2.1.4 NAND GATE

The operation is SAME as AND gate. The o/p of NAND gate is inverted from o/p of AND gate.

INPUT Output AND

OutputNAND

A B

X

0 0

0 1

0 1

0 1

1 0

0 1

1 1

1 0

Denotes inversion

BAX

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2.1.5 NOR GATE

The operation is the SAME as OR gate. The o/p NOR gate is inverted from OR gate.

INPUT OUTPUT

A B

X

0 0 1

0 1 0

1 0 0

1 1 0

BAX

BABAX

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2.1.6 EXCLUSIVE – OR GATE

INPUT OUTPUT

A B

X

0 0 0

0 1 1

1 0 1

1 1 0

BABAX

BAX

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2.1.7 EXCLUSIVE-NOR GATE

INPUT OUTPUT

A B

X

0 0 1

0 1 0

1 0 0

1 1 1B.AB.AX

BAX

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SUMMARY of LOGIC GATES

2.2 IC GATES Digital logic gate circuits are manufactured as integrated circuits. A number of gates is enclosed in a single package called Dual Inline

Package (DIP) housing for plug in or feedthrough mounting, or Small Outline Integrated Circuit (SOIC) for surface mounting.

Part numbers specify what type of gates are enclosed. These part numbers are industry standards, meaning that a "74LS02"

manufactured by Motorola will be identical in function to a "74LS02" manufactured by Fairchild or by any other manufacturer.

Logic circuit part numbers beginning with "74" are commercial-grade TTL. If the part number begins with the number "54", the chip is a military-grade unit: having a greater operating temperature range, and typically more robust in regard to allowable power supply and signal voltage levels.

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Packaging - SOIC

Packaging - DIP

Common TTL "DIP" circuit packages

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Universality of NAND & NOR Gates

NAND & NOR gates can be used as: Inverter gates AND gates OR gates

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Universality of NANDA X

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Universality of NOR A X

Exercise

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1.Draw the output waveform C and D, given the input waveforms A and B as shown below.

Exercise (cont’d)

A B C Z

0 0 0

0 0 1

0 1 0

0 1 1

1 0 0

1 0 1

1 1 0

1 1 1

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2. Refer to the circuit and complete the truth table given.