ch2 boolean algebra

8/3/2019 Ch2 Boolean Algebra

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Boolean Algebra &

Logic Gates

ECE 223Digital Circuits and Systems


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Binary (Boolean) Logic

Deals with binary variables and binary logicfunctions

Has two discrete values

0 False, Open

1 True, Close

Three basic logical operations

AND (.); OR (+); NOT ()


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Logic Gates & Truth Tables

AND ORNOT

101001

1

0

0

A

OR

1

1

0

B

1

1

0

A+B

1

0

A

1

0

0

A

AND

1

1

0

B

1

0

0

A.B

0

1

A

NOT

AND; OR gates may have any # of inputs

AND 1 if all inputs are 1; 0 other wise

OR 1 if any input is 1; 0 other wise


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Boolean Algebra

Branch of Algebra used for describing and designingtwo valued state variables Introduced by George Boole in 19th centaury Shannon used it to design switching circuits (1938)

Boolean Algebra Postulates

An algebraic structure defined by a set of elements, B,together with two binary operators + and . that satisfy thefollowing postulates:

1. Postulate 1:Closure with respect to both (.) and ( +)

2. Postulate 2:An identity element with respect to +, designated by 0. An identity

element with respect to . designated by 1


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Boolean Algebra - Postulates

3. Postulate 3:Commutative with respect to + and .

4. Postulate 4:

Distributive over . and +

5. Postulate 5:

For each element a of B, there exist an element a such that(a) a + a = 1 and (b) a.a = 0

6. Postulate 6:

There exists at least two elements a, b in B, such that a b


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Boolean Algebra - Postulates

Postulates are facts that can be taken as true; they donot require proof

We can show logic gates satisfy all the postulates

101001

1

0

0

A

OR

1

1

0

B

1

1

0

A+B

1

0

A

1

0

0

A

AND

1

1

0

B

1

0

0

A.B

0

1

A

NOT


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Boolean Algebra - Theorems

Theorems help us out in manipulating Booleanexpressions They must be proven from the postulates and/or other already

proven theorems

Exercise Prove theorems from postulates/other proventheorems


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Boolean Functions

Are represented as

Algebraic expressions;

F1 = x + yz Truth Table

Synthesis

Realization of schematic from theexpression/truth table

Analysis Vice-versa

10011101

0110

1011

1

0

0

0

x

001

1

0

0

y

1

1

0

z

1

1

0

F1

x

y

z

F1


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Synthesis F1

Assume true as well as complement inputsare available

Cost

A 2-input AND gate

A 2-input OR gate

4 inputs1001

1101

0110

1011

1

0

0

0x

001

1

0

0y

1

1

0z

1

1

0F1

x

y

z

F1


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Canonical and Standard Forms

Minterms A minterm is an AND term in which every literal

(variable) of its complement in a function occurs once

For n variable 2n minterms

Each minterm has a value of 1 for exactly one

combination of values of n variables(e.g., n = 3)


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Minterms

One method of Writing Boolean function is thecanonical minterm (sum of products or SOP) form F = xyz +xyz + xyz = m1 + m5 + m6 = (1,5,6)

xyz

xyz

xyz

xyz

xyz

xyz

xyz

xyz

Corresponding

minterm

m4001

m5101

m3110

m6011

1

0

0

0

x

m201

1

0

0

y

1

1

0

z

m7

m1

m0

Designation


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Minterms examples

F2 = (0,1,2,3,5)

= xyz + xyz + xyz + xyz + xyz

0

0

1

0

1

1

1

1

F2 (Given)

001

m5101

m3110

011

1

0

0

0

x

m201

1

0

0

y

1

1

0

z

m1

m0

Designation


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Minterms examples

(F2) = (all minterms not in F2) = (4,6,7)

= xyz + xyz + xyz

0

0

1

0

1

1

1

1

F2 (Given)

001

m5101

m3110

011

1

0

0

0

x

m201

1

0

0

y

1

1

0

z

m1

m0

Designation


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Maxterms

A maxterm is an OR term in which every literal(variable) or its complement in a function occurs once Each maxterm has a value 0 for one combination of values of

n variables

x +y +z

x +y +z

x +y +z

x +y +z

x +y +z

x +y +z

x +y +z

x +y +z

Corresponding

maxterm

M4001

M5101

M3110

M6011

1

0

0

0

x

M201

1

0

0

y

1

1

0

z

M7

M1

M0

Designation


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Minterms & Maxterms

Conversion between minterms & maxtermsm0 = xyz = (x+y+z) = (M0)

In general, mi = (Mi)

An alternative method of writing a Boolean function isthe canonical maxterm (product of sums or POS) form

The canonical product of sums can be written directlyfrom the truth table


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Maxterms

F3 = (x+y+z)(x+y+z)(x+y+z)(x+y+z)(x+y+z)= (0,2,3,4,7)

(F3) = (all maxterm not in F3)

0

1

1

0

0

0

1

0

F3 (Given)

M4001

101

M3110

011

1

0

0

0

x

M201

1

0

0

y

1

1

0

z

M7

M0

Designation


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Standard Forms

In canonical forms, each minterm (or maxterm)must contain all variables (or its complements)The algebraic expressions can further be simplified

ExampleF4 (x,y,z) = xy +yz (sum of products, standard form)

F5 (x,y,z) = (x+y)(y+z) (product of sums, standard form) Conversion

Standard form can be converted into canonical form using identity elements

F4 = xy + yz = xy.1 +1.yz = xy(z+z) + (x+x)yz

= xyz + xyz + xyz + xyz = m7 +m6 +m5 +m1

How about the conversion from canonical forms tostandard forms?

Exercise convert F5 into maxterms


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Non-Standard Forms

A Boolean function may be written in non-standard formF6 (x,y,z) = (xy + z)(xz + yz)

= xy(xz + yz) + z(xz + yz)

= xyz + xyyz + xz +yz

= xyz + xz + yz

= xz + yz (standard form)


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Other Logic Gates NAND Gate So far, we discussed AND, OR, NOT gates

2-input NAND (NOT-AND operation)

Can have any # of inputs

NAND gate is not associative

Associative property to be discussed later

x

y

z

011

1

0

0

x

10

1

0

y

1

1

z


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Other Logic Gates NOR Gate 2-input NOR (NOT-OR operation)


NOR gate is not associative

Associative property to be discussed later

x

y

z

011

1

0

0

x

00

1

0

y

0

1

z


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Other Logic Gates XOR Gate 2-input XOR

Output is 1 if any input is one and the other input is 0


x

y

z

011

1

0

0

x

10

1

0

y

1

0

z


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Other Logic Gates XNOR Gate 2-input XNOR

Performs the NOT-XOR operation

Output is 1 if both inputs are 1; or both inputs are 0


x

y

z

111

1

0

0

x

00

1

0

y

0

1

z


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Extension to Multiple Inputs So far, we restricted ourselves to 1 or 2-input gates

A logic gate (except inverter) can have any number of inputs

AND, OR logic operations have two properties

x +y = y +x (commutative)

(x +y)+ z = x + (y +z) = x +y +z (associative)

NAND and NOR operations are commutative, but notassociative

(xy)z x(yz) = NOR operation

(xy)z x(yz) = NAND operation

How about XOR


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Positive & Negative Logic

Positive Logic 0 = False (Low Voltage)

1 = True (High Voltage)

Negative Logic

0 = True (High Voltage)

1 = False (Low Voltage) Implement truth table with

positive & negative logic

Positive logic AND gate

Negative logic ?

HHH

H

L

L

x

LL

H

L

y

L

L

z


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Integrated Circuit - Evolution

Transistor was invented in 1947/48

Integrated Circuits were invented in 1959/60

Since then, larger # of transistors/chip are integrated

101 Small Scale Integration

102-3 Medium Scale Integration

103-6 Large Scale Integration

106-9 Very Large Scale Integration

Digital Logic Families (technologies)

TTL Transistor-Transistor Logic

ECL Emitter Coupled Logic

MOS Metal Oxide Semiconductor

CMOS Complementary Metal Oxide Semiconductor


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Book Sections Boolean Algebra & Logic

Gates Material is covered in Sections 2.1 2.8

ch2 boolean algebra

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