logic gates ghader kurdi adapted from the slides prepared by department of preparatory year

Logic Gates

Ghader Kurdi

Adapted from the slides prepared by DEPARTMENT OF PREPARATORY YEAR.

Introduction to Digital Logic Basics

Hardware consists of a few simple building blocks These are called logic gates

AND, OR, NOT, … NAND, NOR, XOR, …

Both the inputs and output to/from logic gates are expressed in terms of 1s and 0s

Logic gatePerforms logical

operationoutputInput/s

Design of Logic Gates The basic logic gates are:

AND OR NAND NOR XOR XNOR Inverter (NOT) Buffer

Design of Logic Gates Logic gates are built using transistors

NOT gate can be implemented by a single transistor AND requires 3 transistors

Transistors are the fundamental devices that any electronic system consists of

Pentium consists of 3 million transistors Compaq Alpha consists of 9 million transistors Now we can build chips with more than 100 million

transistors

AND gate

A B F0 0 00 1 01 0 01 1 1

AND function finds the minimum between two binary digits

AND gate outputs a 1 only if all the inputs are 1.

Truth table

Graphical representation

F = A . B or F = AB

Logical representation

OR gate

A B F0 0 00 1 11 0 11 1 1

Truth table

F = A + B


OR finds the maximum between two binary digits

OR gate outputs a 1 if any one of the inputs is1.


NAND gate

A B F0 0 10 1 11 0 11 1 0

NAND = AND + NOT The output is "false" if both inputs are "true."

Otherwise, the output is "true."

Truth table


F = A . B


NOR gate

A B F0 0 10 1 01 0 01 1 0

NOR = OR + NOT The output is "true" if both inputs are "false."

Otherwise, the output is "false."

Truth table


F = A + B


XOR gate

A B F0 0 00 1 11 0 11 1 0

XOR implements exclusive-OR function The output is "true" if either, but not both, of the

inputs are "true." The output is "false" if both inputs are "false" or if both inputs are "true."

Truth table


A + B = A · B + A · B


XNOR gate The output is true when both inputs A and B are

true and when neither A nor B is true.

A B F0 0 10 1 01 0 01 1 1

Truth table


A + BLogical representation

NOT (Inventer) gate

A F0 11 0

Has only one input Outputs the inverse of the value inputted

Truth table


F = A or A’


Buffer

A Y0 01 1

Has only one input Outputs the same value inputted

Truth table


Y = ALogical representation

Integration levels

SSI (small scale integration) Introduced in late 1960s 1-10 gates (previous examples)

MSI (medium scale integration) Introduced in late 1960s 10-100 gates

LSI (large scale integration) Introduced in early 1970s 100-10,000 gates

VLSI (very large scale integration) Introduced in late 1970s More than 10,000 gates

Logic Functions Logical functions can be expressed in several

ways: Truth table Logical expressions Graphical form

Example: Majority function

Output is 1 whenever majority of inputs is 1 We use 3-input majority function

Truth Table

A B C …. F0 0 0 ….0 0 1 ….0 1 0 ….0 1 1 ….1 0 0 ….1 0 1 ….1 1 0 ….1 1 1 ….

…. …. …. …. ….

In a truth table, the number of rows = (number of inputs)2

The truth table has a column foe each input on the left hand side

The outputs are listed in columns on the right hand side

Logic Functions (cont.)3-input majority function

A B C F

0 0 1 00 1 0 00 1 1 11 0 0 01 0 1 11 1 0 11 1 1 1

Logical expression formF = A B + B C + A C

0 0 00

0

00

0

0

0

0

0

0

0 0 0 0

Logic Functions (cont.) Logical expression: Q= A’B + AB’A B A’ B’ A’B AB’ Q

0

0

1

1

0

1

0

1

1

1

0

0

1

0

1

0

0

1

0

0

0

0

1

0

0

1

1

0

Logic Circuit Design Process A simple logic design process involves

Problem specification Truth table derivation Derivation of logical expression Simplification of logical expression Implementation

Logical Equivalence All three circuits implement F = A B function

Logical Equivalence (cont.) Proving logical equivalence of two circuits

1. Derive the logical expression for the output of each circuit

2. Show that these two expressions are equivalent Two ways:

You can use the truth table method For every combination of inputs, if both expressions

yield the same output, they are equivalent Good for logical expressions with small number of

variables You can also use algebraic manipulation

Need Boolean identities

Logical Equivalence (cont.)

A

B

A+B

Derivation of logical expression from a circuit Trace from the input to output Write down intermediate logical expressions along the path

Logical Equivalence (cont.) Proving logical equivalence: Truth table method

A B A B (A + B) F2 = (A + B)0011

0101

1100

1010

1110

0001

Logical Equivalence (cont.) Derivation of logical expression from a circuit

Logical Equivalence (cont.) Proving logical equivalence: Truth table method

A B F1 = A B F3 = (A + B) (A + B) (A + B)

0 0 0 00 1 0 01 0 0 01 1 1 1

The Universal NAND Gate NAND gate is the main fundamental universal gate

for being easier in manufacturing process.

The Universal NAND Gate (cont.)


A B F0 00 11 01 1

A B F0 0 10 1 11 0 11 1 0

0

01

1

10

0


A B F0 0 00 11 01 1

A B F0 0 10 1 11 0 11 1 0

0

11

1

10

0


A B F0 0 00 1 01 01 1

A B F0 0 10 1 11 0 11 1 0

1

01

1

10

0


A B F0 0 00 1 01 0 01 1

A B F0 0 10 1 11 0 11 1 0

1

10

0

01

1


A F01

A B F0 0 10 1 11 0 11 1 0

0

01

1


A F0 11

A B F0 0 10 1 11 0 11 1 0

1

10

0

The Universal NOR Gate NOR gate is also another main fundamental

universal gate

Deriving Logical Expressions Derivation of logical expressions from truth tables

sum-of-products (SOP) form product-of-sums (POS) form

SOP form Write an AND term for each input combination that

produces a 1 output Write the variable if its value is 1; complement

otherwise OR the AND terms to get the final expression

POS form Dual of the SOP form

Deriving Logical Expressions (cont.)

3-input majority function

A B C F0 0 0 00 0 1 00 1 0 00 1 1 11 0 0 01 0 1 11 1 0 11 1 1 1

SOP logical expression Four product terms

Because there are 4 rows with a 1 output

F = A B C + A B C + A B C + A B C

Deriving Logical Expressions (cont.)

3-input majority function

A B C F0 0 0 00 0 1 00 1 0 00 1 1 11 0 0 01 0 1 11 1 0 11 1 1 1

POS logical expression Four sum terms

Because there are 4 rows with a 0 output

F = (A + B + C) (A + B + C) (A + B + C) (A + B + C)

Boolean Algebra Boolean algebra can be used to formalize

the combinations of binary logic states. In these relations A and B are binary

quantities, they can be either logical true (T or 1) or

logical false (F or 0).

Boolean Algebra

Boolean Algebra (cont.)

Logical Expression SimplificationProve that: A B = (A + B) (A + B) (A + B)

Used rules

(A + B) (A + B) (A + B) Distribution

(A + BB) (A + B) Complement

(A + 0) (A + B) Identity

A (A + B) Distribution

AA + AB Complement

0 + AB Identity

AB

Logical Expression SimplificationProve that: A B = (A + B) (A + B) (A + B)

Used

rules

(A + B) (A + B) (A + B)

Idempotent

(A + B) (A + B) (A + B) (A + B)

((A + B) (A + B)) ((A + B) (A + B)) Distribution

(A + BB) (B + AA) Complement

(A + 0) (B + 0) Identity

(A)(B)

AB

Logical Expression Simplification (cont.)

Prove that: X+XY = XUsed rules

X+XY X(1+Y) NullX(1) IdentityX


Prove that: X(X+Y) = XUsed rules

X(X+Y) Distribution

XX+XY Idempotent

X+XY

X(1+Y) Null

X(1) Identity

X


Prove that: X.(Y+Z) = X.Y + X.ZUsed rules

X.Y + X.Z Distribution

(X.Y + X) (X.Y + Z) Absorption

X . (X.Y + Z) Distribution

X . (Z+X . Z+Y) Associative

(X . X+Z) . (Y+Z) Absorption

X . (Y+Z)


Prove that: X(X+X') = XUsed rules

X(X+X') Distribution XX + XX’ IdempotentX + XX’ ComplementX + 0 Identity X


Majority function exampleA B C + A B C + A B C + A B C =

A B C + A B C + A B C + A B C + A B C + A B C

We can now simplify this expression as

B C + A C + A B

A difficult method to use for complex expressions

Added extra


Simplify the next:WY + XY + WZ + XZ = (W + X) (Y + Z)

Used rules

WY + XY + WZ + XZ Distribution

Y(W + X) + Z(W + X)

(W + X) (Y + Z)


Simplify the next: (X + Y) + (X + Y) = 0Used Rules

(X + Y) + (X + Y) Associative

(X + X) + ( Y + Y) Complement,

Idempotent

1 + Y Null10


Simplify the next: (X + Y) + (X + Y) = 0Used Rules

(X + Y) . (X + Y) De Morgan(X . Y) . ( X . Y) De Morgan (X . Y) . ( X . Y) Associative

(X . X) . (Y . Y) Idempotent

(X . X) . (Y) Complement

0 . Y Null0

Simplification Using Boolean Algebra

AB+A(B+C)+B(B+C) (distributive law)

AB+AB+AC+BB+BC (Idempotent; BB=B)

AB+AB+AC+B+BC (Idempotent; AB+AB=AB)

AB+AC+B+BC (Absorption; B+BC=B)

AB+AC+B (Absorption; AB+B=B)

B+AC

A

B

C

B+AC

A

BC AB+A(B+C)+B(B+C)


Two basic methods Algebraic manipulation

Use Boolean laws to simplify the expression Difficult to use Don’t know if you have the simplified form

Karnaugh map (K-map) method Graphical method Easy to use

limited to problems with up to 4 binary inputs. Let's start with a simple example

Karnaugh Map Method

Karnaugh Map Of Two Variables

A = 0B = 0

A = 0B = 1

A = 1B = 0

A = 1B = 1

Karnaugh Map Method (cont.)

Step 1: Find a truth table Step 2: Draw a K-map

A B A’

B’ A’B’

A’B

F

0 0 1 1 1 0 10 1 1 0 0 1 11 0 0 1 0 0 01 1 0 0 0 0 0

1 0 1 0

0 10 1

AB

F = A’B’ + A’B


Step 3: Group adjacent 1’sStep 4: Read off a reduced expression

1 0 1 0

0 1

0 1

AB

1 0 1 0

0 10 1

AB

A’B’

A’B

A’B’ + A’B = A’


1 0 1 0

0 1

0 1

AB

0 1 0 1

0 1

0 1

AB

1 1 0 0

0 10 1

AB

0 0 1 1

0 1

0 1

AB

1 1 1 0

0 1

0 1

AB

1 1 1 1

0 10 1

AB

How to group adjacent 1’s ?

logic gates ghader kurdi adapted from the slides prepared by department of preparatory year

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