Combinational Logic Design: MSI Building Blocks - Decoders and Encoders, Lecture notes of Digital Systems Design

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COMBINATIONAL LOGIC DESIGN

MSI BUILDING BLOCKS

In this Module

Decoders

- Building decoders

- Applications of Decoders

Encoders

- Building encoders

- Applications of encoders

A 3: 8 Decoder

Decoder with Enable Line

Decoders can also have an enable line. If Enable is active

high then, If enable= 0 , decoder is off. It means all output lines

are zero. If enable= 1 , decoder is on and depending on input,

the corresponding output line is 1 , all other lines are 0

E A B O0 O1 O2 O 0 X X 0 0 0 0 1 0 0 1 0 0 0 1 0 1 0 1 0 0 1 1 0 0 0 1 0 1 1 1 0 0 0 1

2:4 Decoder with active high enable, active high outputs

Building Decoder of a larger size

Processor

Port address Decoder

Port address

Printer

Monitor

Keyboard

Scanner

Decoder as Device Selector

Decoders are sometimes called minterm generators.

 For each of the input combinations, exactly one output is true.  Each output equation contains all of the input variables.

 These properties hold for all sizes of decoders.

This means that you can implement arbitrary functions with

decoders. If you have a sum of minterms equation for a function, you can easily use a decoder (a minterm generator) to implement

that function X Y F 0 F 1 F 2 F 3 0 0 1 0 0 0 0 1 0 1 0 0 1 0 0 0 1 0 1 1 0 0 0 1

Implementing Boolean Function

F0 = X’Y’ = m 0 F1 = X’Y = m 1 F2= XY’ = m 2 F3 = XY = m 3

A Full Adder Implementation

Decoder with active low outputs