The document discusses different types of encoders and decoders used in digital circuits. It describes encoders that convert parallel inputs into binary codes, such as 4-to-2 and priority encoders. It also explains decoders that perform the reverse operation of encoders, such as 2-to-4 and 3-to-8 decoders that convert binary codes into activated outputs. Priority, BCD, and 7-segment decoders are discussed along with their applications in digital systems.

1. Encoder & Decoder
21BCA2T312:Digital Electronics
SIMMI S
Assistant Professor
Department Of Computer Science(UG)
Kristu Jayanti College, Autonomous
(Reaccredited A++ Grade by NAAC with CGPA 3.78/4)
Bengaluru -560077,India

2. Encoder
• An Encoder is a combinational circuit that performs the reverse operation of
a Decoder. It has a maximum of 2^n input lines and ‘n’ output lines. It is a
digital circuit that converts a set of binary inputs into a unique binary code.
• The binary code represents the position of the input and is used to identify
the specific input that is active.
• Encoders are commonly used in digital systems to convert a parallel set of
inputs into a serial code.
• Used to reduce the number of data lines
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3. Types OF Encoders
• Priority Encoders: which assign a priority to each input.
• Binary-weighted Encoders: which use a binary weighting system to
assign binary codes to inputs
• 4 to 2 Encoder
• Octal to Binary Encoder (8 to 3 Encoder)
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4. 4 to 2 Encoder
• The 4 to 2 Encoder consists of four inputs Y3, Y2, Y1 & Y0, and two
outputs A1 & A0. At any time, only one of these 4 inputs can be ‘1’ in order
to get the respective binary code at the output. The figure below shows the
logic symbol of the 4 to 2 encoder.
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5. 4 to 2 Encoder
Logicial expression for A1 and A0:
A1 = Y3 + Y2
A0 = Y3 + Y1
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6. Priority Encoder
• The priority encoder is a combinational logic circuit that contains 2^n input
lines and n output lines and represents the highest priority input among all
the input lines.
• When multiple input lines are active high at the same time, then the input
that has the highest priority is considered first to generate the output.
• In most digital applications, these encoders are used to select the inputs,
which have the highest priority level, This process of selecting the input is
called arbitration.
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7. Decimal to BCD Priority Encoder
• This type of priority encoder performs the function of encoding the
decimal digits into 4-bit BCD (Binary Coded Decimal) outputs.
• As it is a decimal to BCD priority encoder, therefore, it produces a BCD
corresponding to the decimal digit of highest priority among all the
inputs and ignores all others.
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8. Decimal to BCD Priority Encoder
DECIMAL INPUTS BCD OUTPUTS
Y1 Y2 Y3 Y4 Y5 Y6 Y7 Y8 Y9 A3 A2 A1 A0
1 1 1 1 1 1 1 1 1 1 1 1 1
X X X X X X X X 0 0 1 1 0
X X X X X X X 0 1 0 1 1 1
X X X X X X 0 1 1 1 0 0 0
X X X X X 0 1 1 1 1 0 0 1
X X X X 0 1 1 1 1 1 0 1 0
X X X 0 1 1 1 1 1 1 0 1 1
X X 0 1 1 1 1 1 1 1 1 0 0
X 0 1 1 1 1 1 1 1 1 1 0 1
0 1 1 1 1 1 1 1 1 1 1 1 0
DECIMAL INPUTS BCD OUTPUTS
Y1 Y2 Y3 Y4 Y5 Y6 Y7 Y8 Y9 A3 A2 A1 A0
1 1 1 1 1 1 1 1 1 1 1 1 1
X X X X X X X X 0 0 1 1 0
X X X X X X X 0 1 0 1 1 1
X X X X X X 0 1 1 1 0 0 0
X X X X X 0 1 1 1 1 0 0 1
X X X X 0 1 1 1 1 1 0 1 0
X X X 0 1 1 1 1 1 1 0 1 1
X X 0 1 1 1 1 1 1 1 1 0 0
X 0 1 1 1 1 1 1 1 1 1 0 1
0 1 1 1 1 1 1 1 1 1 1 1 0
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9. Decimal to BCD Priority Encoder
DECIMAL INPUTS BCD OUTPUTS
Y1 Y2 Y3 Y4 Y5 Y6 Y7 Y8 Y9 A3 A2 A1 A0
1 1 1 1 1 1 1 1 1 1 1 1 1
X X X X X X X X 0 0 1 1 0
X X X X X X X 0 1 0 1 1 1
X X X X X X 0 1 1 1 0 0 0
X X X X X 0 1 1 1 1 0 0 1
X X X X 0 1 1 1 1 1 0 1 0
X X X 0 1 1 1 1 1 1 0 1 1
X X 0 1 1 1 1 1 1 1 1 0 0
X 0 1 1 1 1 1 1 1 1 1 0 1
0 1 1 1 1 1 1 1 1 1 1 1 0
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10. Decoder
• Decoder is a combinational circuit that has ‘n’ input lines and maximum of
2n output lines. One of these outputs will be active High based on the
combination of inputs present, when the decoder is enabled. That means
decoder detects a particular code.
• Decoder performs the reverse operation of the Encoder.
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11. 2 to 4 Decoder
• In the 2 to 4 line decoder, there is a total of three inputs, i.e., A0, and A1 and
E and four outputs, i.e., Y0, Y1, Y2, and Y3. For each combination of inputs,
when the enable 'E' is set to 1, one of these four outputs will be 1.
• The block diagram and the truth table of the 2 to 4 line decoder are given
below.
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12. 2 to 4 Decoder
Y3=E.A1.A0
Y2=E.A1.A0'
Y1=E.A1'.A0
Y0=E.A1'.A0
The logical expression of the term Y0, Y0, Y2, and Y3 is
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13. 2 to 4 Decoder
Y3=E.A1.A0
Y2=E.A1.A0'
Y1=E.A1'.A0
Y0=E.A1'.A0
Logical Expression
Circuit Diagram
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14. The 3 to 8 line Decoder
• The 3 to 8 line decoder is also known as Binary to Octal Decoder.
• In a 3 to 8 line decoder, there is a total of eight outputs, i.e., Y0, Y1, Y2, Y3, Y4,
Y5, Y6, and Y7 and three outputs, i.e., A0, A1, and A2.
• This circuit has an enable input 'E'. Just like 2 to 4 line decoder, when enable
'E' is set to 1, one of these four outputs will be 1.
• The block diagram and the truth table of the 3 to 8 line encoder are given
below.
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15. The 3 to 8 line Decoder
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16. The 3 to 8 line Decoder
Enabl
e
Inputs Outputs
E A2 A1 A0 Y7 Y6 Y5 Y4 Y3 Y2 Y1 Y0
0 X X X 0 0 0 0 0 0 0 0
1 0 0 0 0 0 0 0 0 0 0 1
1 0 0 1 0 0 0 0 0 0 1 0
1 0 1 0 0 0 0 0 0 1 0 0
1 0 1 1 0 0 0 0 1 0 0 0
1 1 0 0 0 0 0 1 0 0 0 0
1 1 0 1 0 0 1 0 0 0 0 0
1 1 1 0 0 1 0 0 0 0 0 0
1 1 1 1 1 0 0 0 0 0 0 0
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17. BCD to 7-segment decoder
• In Binary Coded Decimal (BCD) encoding scheme each of the decimal
numbers(0-9) is represented by its equivalent binary pattern(which is
generally of 4-bits).
• Seven segment display is an electronic device which consists of seven
Light Emitting Diodes (LEDs) arranged in a some definite pattern (common
cathode or common anode type), which is used to display Hexadecimal
numerals(in this case decimal numbers, as input is BCD i.e., 0-9).
• A BCD to 7-segment decoder is a circuit that converts a BCD input into a
display with seven segments each containing an LED.
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18. BCD to 7-segment decoder
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19. BCD to 7-Segment Decoder
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20. Simmi S, Department of Computer Science