Friday, August 03, 2012

Binary decoder

A decoder is a multiple-input, multiple-output logic circuit that converts coded inputs into coded outputs, where the input and output codes are different; e.g. n-to-2n, BCD decoders. Enable inputs must be on for the decoder to function, otherwise its outputs assume a single "disabled" output code word.Decoding is necessary in applications such as data multiplexing, 7 segment display and memory address decoding. Figure below shows the pseudo block of a decoder.


                                             



Basic Binary Decoder

AND gate can be used as the basic decoding element, because its output is HIGH only when all its inputs are HIGH. For example, if the input binary number is 0110, then, to make all the inputs to the AND gate HIGH, the two outer bits must be inverted using two inverters as shown in figure below.




Binary n-to-2n Decoders


A binary decoder has n inputs and 2n outputs. Only one output is active at any one time, corresponding to the input value. Figure below shows a representation of Binary n-to-2n decoder

                                

2-to-4 Binary Decoder

A 2 to 4 decoder consists of two inputs and four outputs, truth table and symbols of which is shown below.

Truth table

X
Y
F0
F1
F2
F3
0
0
1
0
0
0
0
1
0
1
0
0
1
0
0
0
1
0
1
1
0
0
0
1

           Symbol
                                          
                               Using k-map we can do the minimization of truth table.
               Circuit









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