Unit5 Encoder Decoder

Design Procedure
Prepared and Delivered By: Pawandeep Kaur

Design Procedure:
BCD to Excess-3 Code Conversion
How many
Design of
inputs and
Truth Table
outputs
Simplification Design the

with K-map circuit

Encoder
 An encoder is a combinational circuit that performs the inverse operation of a decoder.
 An encoder has (or fewer) input lines and n output lines.
 The output lines, as an aggregate, generate the binary code corresponding to the input value.
1) 4:2 Encoder
2) 8:3 Encoder (Octal to binary)
3) Decimal to BCD Encoder
4) Hexadecimal to Binary Encoder
5) Special :Priority Encoder (not in curriculum)

Encoder: Example
Qsn: Design an octal to binary encoder?

Encoder: Example

Encoder: Example

QUICK QUIZ (POLL)
How is an encoder different from a decoder?
a) The output of an encoder is a binary code for 1-of-N input
b) The output of a decoder is a binary code for 1-of-N input
c) The output of an encoder is a binary code for N-of-1 output
d) The output of a decoder is a binary code for N-of-1 output

QUICK QUIZ (POLL)
How is an encoder different from a decoder?
a) The output of an encoder is a binary code for 1-of-N input
b) The output of a decoder is a binary code for 1-of-N input
c) The output of an encoder is a binary code for N-of-1 output
d) The output of a decoder is a binary code for N-of-1 output
An encoder is a combinational circuit encoding the information of 2n input lines to n output lines, thus producing the
binary equivalent of the input. It performs the opposite operation of a decoder which results in 2 n outputs from n
inputs. Thus, an encoder different from a decoder because of the output of an encoder is a binary code for 1-of-N
input.

QUICK QUIZ (POLL)
1.How many outputs will a decimal-to-BCD encoder have?
a) 4 3.Can an encoder be a transducer?
b) 8 a) Yes
c) 12 b) No
d) 16 c) May or may not be
2.If we record any music in any recorder, such d) Both are not even related slightly
types of process is called ___________
a) Multiplexing
b) Encoding
c) Decoding
d) Demultiplexing

QUICK QUIZ (POLL)
1.How many outputs will a decimal-to-BCD encoder have?
a) 4 3.Can an encoder be a transducer?
b) 8 a) Yes
c) 12 b) No
d) 16 c) May or may not be
2.If we record any music in any recorder, such d) Both are not even related slightly
types of process is called ___________
a) Multiplexing
b) Encoding
c) Decoding
d) Demultiplexing
If we record any music in any recorder, it means that we are
giving data to a recorder. So, such process is called encoding.
Getting back the music from the recorded data is known as
decoding.
Limitations of Encoder
 There is an ambiguity, when all outputs of encoder are equal to zero. Because, it could be the
code corresponding to the inputs, when only least significant input is one or when all inputs
are zero.
 If more than one input is active High, then the encoder produces an output, which may not be
the correct code. For example, if both Y3 and Y6 are ‘1’, then the encoder produces 111 at the
output. This is neither equivalent code corresponding to Y3, when it is ‘1’ nor the equivalent
code corresponding to Y6, when it is ‘1’.
 So, to overcome these difficulties, we should assign priorities to each input of encoder. Then,
the output of encoder will be the binary code corresponding to the active High inputs, which
has higher priority. This encoder is called as priority encoder.

Priority Encoder
 A priority encoder is an encoder circuit that includes the priority function. The operation of
the priority encoder is such that if two or more inputs are equal to 1 at the same time, the
input having the highest priority will take precedence.
 In addition to the two outputs x and y, the circuit has a third output designated by V; this is a
valid bit indicator that is set to 1 when one or more inputs are equal to 1. If all inputs are 0,
there is no valid input and V is equal to 0.
 The other two outputs are not inspected when V equals 0 and are specified as don’t-care
conditions.

Priority Encoder
Designing of 4X2 Priority Encoder

Priority Encoder

Priority Encoder

Decoder
What is a Decoder?
 A combinational circuit that has ‘n’ input lines and maximum of output lines.
 One of these outputs will be active High based on the combination of inputs present, when the
decoder is enabled.
 The outputs of the decoder are min terms of ‘n’ input variables lines when it is enabled
Parallel Decoder
BCD to Decimal
BCD to Seven Segment

2x4 Decoder

2x4 Decoder

2x4 Decoder
Therefore, the outputs of 2 to 4 decoder are nothing but

the min terms of two input variables A1 & A0, when enable,
E is equal to one. If enable, E is zero, then all the outputs of
decoder will be equal to zero.
3x8 Decoder

Home assignment 3x8 Decoder
E S2 S1 S0 DO D1 D2 D3 D4 D5 D6 D7
0 X X X
1 0 0 0
1 0 0 1
1
1
1
1
1
1

3x8 Decoder: Active High
Output is represents with high (logic 1)

3x8 Decoder: Active Low

Full Adder Design using 3:8 Decoder
74138 IC 3:8 Decoder

Full Subtractor Design using 3:8 Decoder

Function Design using 3:8 Decoder
F1=m(1,2,5,7)
F2=m(0,1,3,6)

Decoder
Tree 3:8 Decoder using 2:4 Decoder
A2 A1 A0 HIGH OUTPUT
0 0 0 Y0
0 0 1 Y1
0 1 0 Y2
0 1 1 Y3
1 0 0 Y4
1 0 1 Y5
1 1 0 Y6
1 1 1 Y7

3:8 Decoder using 2:4 Decoder
Question :Design 3:8 decoder using 2:4 Decoder Solution
A2 A1 A0 HIGH OUTPUT
0 0 0 Y0
0 0 1 Y1
0 1 0 Y2
0 1 1 Y3
1 0 0 Y4
1 0 1 Y5
1 1 0 Y6
1 1 1 Y7

4:16 Decoder
A3 Y0
A2
A1
4:16
Decoder
Y15
A0
EN


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Design Procedure

Prepared and Delivered By: Pawandeep Kaur

Simplification Design the

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Therefore, the outputs of 2 to 4 decoder are nothing but

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

74138 IC 3:8 Decoder

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

Prepared and Delivered By: Pawandeep Kaur

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