DIGITAL ELECTRONICS

RC1225_013Nandhini V. L (Team Leader)Kusuma M.S.

Digital Electronics is a branch of Electronics deals with the digital circuits and digital signals.

BasicsCombinational CircuitsSequentional Circuits

BASIC GATES

Digital Computers store the data in Binary format that is in terms of logic ‘0’ and logic ‘1’.

Basic logic block –GATE

BinaryDigitalInputSignal

BinaryDigitalOutputSignal

Gate

Types of Basic Logic Blocks

- Combinational Logic Block Logic Blocks whose output logic value depends only on the input logic

values

- Sequential Logic Block Logic Blocks whose output logic value depends on the input values

and the previous state of the blocks

Functions of Gates can be described by

- Truth Table - Boolean Function - Karnaugh Map

BASIC CONCEPTS Simple gates

AND OR NOT

Functionality can be expressed by a truth table A truth table lists output

for each possible input combination

Precedence NOT > AND > OR F = A B’ + A’ B = (A (B’)) + ((A’) B)

Additional useful gates

Universal gates NAND NOR

NAND = AND + NOT NOR = OR + NOT Additional gate: XOR gate XOR implements

exclusive-OR function NAND and NOR gates

require only 2 transistors AND and OR need 3 transistors

COMBINATIONAL CIRCUITS

Output depends only on the present inputs. Combinational circuits provide a higher level of

abstraction.Help in reducing design complexity.Reduce chip count.

We look at some useful combinational circuits

COMBINATIONAL LOGIC CIRCUITS

Half AdderFull Adder Multiplexer

Encoder Decoder Parity Checker Parity Generator etc

ADDERS

Half-adderAdds two bits

Produces a sum and carry

Full-adderAdds three 1-bit values

Like half-adder, produces a sum and carryAllows building N-bit adders

Simple techniqueConnect Cout of one adder to Cin of the next

These are called ripple-carry adders

a sumb carr

y

Coutsum

Cin

ab

HA

FA

MULTIPLEXER

2n data inputs n selection inputs a single output

Selection input determines the input that should be connected to the output

Multiplexers(Continued…)

4-data input MUX implementation

Multiplexers(Continued…)

4-data input MUX

Multiplexers(Continued…)

Example chip: 8-to-1 MUX

Demultiplexer (DeMUX)

DEMULTIPLEXERS

Decoders

Decoder selects one-out-of-N inputs

Comparator

Used to implement comparison operators (= , > , < , , )

Comparator (cont.) A=B: OX = IX (X=A<B, A=B, & A>B)

4-bit magnitude comparator chip

Thank you