Lecture 7 COMBINATIONAL CIRCUITS 1 DECODERS 2 ENCODERS

Lecture 7: COMBINATIONAL CIRCUITS. 1. DECODERS. 2. ENCODERS. 3. MULTIPLEXERS. 1

DECODER A decoder provides 2 n mainterms of n input variables 3 to 8 decoder 2

TRUTH TABLE OF A 3 -TO-8 LINE DECODER INPUTS OUTPUTS X Y Z D 0 D 1 D 2 D 3 D 4 D 5 D 6 D 7 0 0 0 1 0 0 0 0 1 0 0 0 0 1 0 0 0 1 0 0 0 1 1 0 0 0 0 1 1 1 0 0 0 0 1 3

COMBINATIONAL LOGIC IMPLEMENTATION. Ex: Implement a full adder circuit with a decoder and two or gates. S(X, Y, Z)=∑(1, 2, 4, 7) C(X, Y, Z)=∑(3, 5, 6, 7) since, there is 3 inputs , then we can use 3 th , 8 decoder 4 Implementation of a full-adder with a decoder.

ENCODER - An encoder is a digital circuit that perform the inverse operation of a decoder. - It has 2 n inputs and n outputs INPUTS OUTPUTS D 0 D 1 D 2 D 3 D 4 D 5 D 6 D 7 X Y Z 1 0 0 0 1 0 0 0 0 0 0 1 0 0 0 1 1 0 0 0 0 0 1 0 0 0 1 0 1 1 0 0 0 0 1 1 5

ENCODER Only one input is active at any given time So, we use priority (if both D 6 and D 3 is active we count only for D 6) 6

MULTIPLEXERS: - Multiplexing means transmitting a large number of information mnits over a smaller number of channels or lines - A digital multiplexer is a combinational circuit that selects binary information from one of many input lines and directs it to a single output line. - A multiplexer has 2 n input lines n select lines and 1 output line 7

MULTIPLEXERS: 8