Digital Electronics

Introduction to Number Systems & Codes

Digital & Analog systems, Numerical representation, Digital number systems, Binary to Decimal conversions, Decimal to Binary conversions, Hexa-decimal

number systems, BCD code, The Gray code, Alphanumeric codes, Parity method for error detection, Octal & Hexa-decimal numbers, Complements.

1St Cycle

Data can be analog or digital

Analog data refers to information that is continuous

Analog data take on continuous values

Analog signals can have an infinite number of values in a range

Digital data refers to information that has discrete states

Digital data take on discrete values

Digital signals can have only a limited number of values

In data communications, we commonly useperiodic analog signals and non-periodic digital signals.

ANALOG AND DIGITAL

ANALOG & DIGITAL SYSTEMS

An Analog system contains devices that manipulate physical quantities that are represented in analog form.

In an analog system, the quantities can vary over a continuous range of values.

For example, the amplitude of the output signal to the speaker in a radio receiver can have any value between zero and its maximum limit.

Other common analog systems are audio amplifiers, magnetic tape recording and playback equipment and a simple light dimmer switch.

More over, a 1 can be encoded as a positive voltage and a 0 as zero voltage.

A digital signal can have more than two levels.

In this case, we can send more than 1 bit for each level.

A Digital system is a combination of devices designed to manipulate logical information or physical quantities that are represented in digital form; that is, the quantities can take on only discrete values.

These devices are most often electronic, but they can also be mechanical, magnetic or pneumatic.

Some of the more familiar digital systems include digital computers, calculator, Digital audio and video equipments and telephone system etc.

Continue………………

Digital Signal

Analog Signal

Advantages of Digital Techniques

Very much easier to design.

Information storage is easy.

Accuracy and precision are easier to maintain throughout the system.

Operation can be programmed.

Less affected by noise.

More circuits can be fabricated on IC chips.

Limitations of Digital Techniques:

The real world is analog

Processing digitized signals takes time.

NUMBER SYSTEMS

Many Number systems are in use in digital technology.

The most common are-DecimalBinary Octal andHexadecimal.

Second Cycle

Analysis & synthesis of digital logic circuitsBasic logic functions

OR operation with OR gates, AND operation with AND gates, NOR operation with NOR gates, Describing logic circuits algebraically, Evaluating logic circuit outputs, Implementing circuits from Boolean expressions, NOR gates & NAND

gates.

Boolean Algebra

Boolean theorems, Demorgan’s Theorems, Universality of NAND gates & NOR gates

DIGITAL LOGIC GATES

Boolean functions are implemented in digital

computer circuits called gates.

A gate is an electronic device that produces a result

based on two or more input values.

– In reality, gates consist of one to six transistors,

but digital designers think of them as a single unit.

– Integrated circuits contain collections of gates

suited to a particular purpose.

Logic gates

Electronic gates require a power supply.

Gate Input’s are driven by voltage having two nominal values.

The Output of a gate provides two nominal values of voltage.

There is always a Time Delay between an input being applied and the output responding.

Point’s Need To Understand

Different Types of Logic gates

Digital systems are said to be constructed by using gates. These gates are --

AND GATE

OR GATE

NOT GATE

NAND GATE

Ex-OR GATE

Ex-NOR GATE

Truth Tables

Used to describe the functional behavior of a Boolean

expression and/or Logic circuit.

Each row in the truth table represents a unique

combination of the input variables.

For n input variables, there are 2n rows.

The output of the logic function is defined for each row.

Each row is assigned a numerical value, with the rows

listed in ascending order.

The order of the input variables defined in the logic

function is important.

AND Gate

The AND gate is an electronic circuit that gives a high output only if all its inputs are high.

A dot(.) is used to show the AND operation.

Truth table

2 Input AND gate

A B Z=A.B

0 0 0

0 1 0

1 0 0

1 1 1

Circuit Symbol

A

B

AND

Z=A.B

Summary of the AND gates

The AND operation is performed the same as ordinary multiplication of 1s and 0s.

An AND gate is a logic circuit that performs the AND operation on the circuit’s input.

An AND gate output will be 1 only for the case when all inputs are 1; for all other cases, the output will be 0.

The expression Z=A.B is read as “Z equals A AND B.”

OR Gate

The OR gate is an electronic circuit that gives a high output. One or more of its inputs are high.

A plus (+) is used to show the OR operation.

Truth table

2 Input OR gate

A B Z=A+B

0 0 0

0 1 1

1 0 1

1 1 1

A

B

OR

Z=A+B

Circuit Symbol

Summary of the OR gates

The OR operation produces a result (output) of 1

whenever any input is a1. Otherwise the output is 0.

An OR gate is a logic circuit that performs an OR

operation on the circuit’s input.

The expression Z=A+B is read as “Z equals A OR

B.”

NOT Gate

The NOT gate is an electronic circuit that produces an inverted version of the input at its output.

More commonly called an Inverter.

Truth tableCircuit Symbol

NOT GATE

A A

0 1

1 0

A A

NAND Gate

The NAND gate which is to NOT - AND gate.

Truth table

Circuit Symbol

A.BAB

NAND

2 Input NAND gate

A B A.B

0 0 1

0 1 1

1 0 1

1 1 0

NOR Gate

The NOR gate which is to NOT - OR gate.

Truth table

2 Input NOR gate

A B A.B

0 0 1

0 1 0

1 0 0

1 1 0

Circuit Symbol

A+BA

B

NOR

EX-OR Gate

The Exclusive –OR gate is a circuit which will give a

high output.An encircled plus sign() is used to show

the EX-OR operation.

Truth table

Circuit Symbol

A BA

B

EX-OR

2 Input EXOR gate

A B A B

0 0 0

0 1 1

1 0 1

1 1 0

EX-NOR Gate

The Exclusive –NOR gate is circuit does the opposite to

the EX-NOR gate. The symbol is an EX-NOR gate with a

small circle on the output. The small circle represents

inversion. Truth table

2 Input EXNOR gate

A B A B

0 0 1

0 1 0

1 0 0

1 1 1

Circuit Symbol

A BA

B

EX-NOR

NAND and NOR are known as universal gates

because they are inexpensive to manufacture and

any Boolean function can be constructed using only

NAND or only NOR gates.

Universal Gates

Boolean Expressions

Boolean expressions are composed of Literals – variables and their complements Logical operations

Examples F = A.B'.C + A'.B.C' + A.B.C + A'.B'.C'

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

literals logic operations

Boolean Expressions

Boolean expressions are realized using a

network (or combination) of logic gates. Each logic gate implements one of the logic

operations in the Boolean expression

Each input to a logic gate represents one of

the literals in the Boolean expression

f

A B

logic operationsliterals

Boolean Expressions

Boolean expressions are evaluated by Substituting a 0 or 1 for each literal. Calculating the logical value of the

expression.

A Truth Table specifies the value of the Boolean expression for every combination of the variables in the Boolean expression.

For an n-variable Boolean expression, the truth table has 2n rows (one for each combination).

Boolean Algebra

George Boole developed an algebraic description for processes involving logical thought and reasoning.

Became known as Boolean Algebra Claude Shannon later demonstrated that Boolean

Algebra could be used to describe switching circuits. Switching circuits are circuits built from devices that

switch between two states (e.g. 0 and 1). Switching Algebra is a special case of Boolean

Algebra in which all variables take on just two distinct values

Boolean Algebra is a powerful tool for analyzing and designing logic circuits.

Basic Laws and Theorems

Commutative Law A + B = B + A A.B = B.AAssociative Law A + (B + C) = (A + B) + C A . (B . C) = (A . B) . CDistributive Law A.(B + C) = AB + AC A + (B . C) = (A + B) . (A + C)Null Elements A + 1 = 1 A . 0 = 0Identity A + 0 = A A . 1 = A

A + A = A A . A = AComplement A + A' = 1 A . A' = 0Involution A'' = AAbsorption (Covering) A + AB = A A . (A + B) = ASimplification A + A'B = A + B A . (A' + B) = A . BDeMorgan's Rule (A + B)' = A'.B' (A . B)' = A' + B'Logic Adjacency (Combining) AB + AB' = A (A + B) . (A + B') = AConsensus AB + BC + A'C = AB + A'C (A + B) . (B + C) . (A' + C) = (A + B) . (A' + C)

Idempotence

DeMorgan's Laws

Can be stated as follows: The complement of the product (AND) is the

sum (OR) of the complements. (X.Y)' = X' + Y'

The complement of the sum (OR) is the product (AND) of the complements.

(X + Y)' = X' . Y'

Easily generalized to n variables. Can be proven using a Truth table

Proving DeMorgan's Law

(X . Y)' = X' + Y'

DeMorgan's Theorems

x 1

x 2

x 1

x 2

x 1 x 2 x 1 x 2 + = (a)

x 1 x 2 + x 1 x 2 = (b)

x 1

x 2

x 1

x 2

x 1

x 2

x 1

x 2

Importance of Boolean Algebra

Boolean Algebra is used to simplify Boolean

expressions.

– Through application of the Laws and

Theorems discussed

Simpler expressions lead to simpler circuit

realization, which, generally, reduces cost, area

requirements, and power consumption.

The objective of the digital circuit designer is to

design and realize optimal digital circuits.

Algebraic Simplification

Justification for simplifying Boolean expressions:

– Reduces the cost associated with realizing

the expression using logic gates.

– Reduces the area (i.e. silicon) required to

fabricate the switching function.

– Reduces the power consumption of the

circuit.

In general, there is no easy way to determine when

a Boolean expression has been simplified to a

minimum number of terms or minimum number of

literals.

– No unique solution

Algebraic Simplification

Boolean (or Switching) expressions can be

simplified using the following methods:

1. Multiplying out the expression.

2. Factoring the expression.

3. Combining terms of the expression.

4. Eliminating terms in the expression.

5. Eliminating literals in the expression

6. Adding redundant terms to the expression.

Examples