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Encoder (digital)

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Digital electronic circuit
A General encoder's block diagram.
A General encoder's block diagram.

An encoder (or "simple encoder") in digital electronics is a one-hot to binary converter. That is, if there are 2 input lines, and at most only one of them will ever be high, the binary code of this 'hot' line is produced on the n-bit output lines. A binary encoder is the dual of a binary decoder.

If the input circuit can guarantee at most a single-active input, a simple encoder is a better choice than a priority encoder, since it requires less logic to implement. However, a simple encoder can generate an incorrect output when more than a single input is active, so a priority encoder is required in such cases.

Types of encoder

2-to-n encoders

A 2 n {\displaystyle 2^{n}} -to-n encoder has n number of outputs in correspondence to the 2 n {\displaystyle 2^{n}} number of inputs. It thus reduces the number of transmission lines and can be compared to a multiplexer. Only one of the inputs become "high" (logic state "1") at a time.

For example, a 4-to-2 simple encoder takes 4 input bits and produces 2 output bits. The illustrated gate level example implements the simple encoder defined by the truth table, but it must be understood that for all the non-explicitly defined input combinations (i.e., inputs containing 0, 2, 3, or 4 high bits) the outputs are treated as don't cares.

4-to-2 encoder

Gate level schematic of a simple 4:2 line encoder
Gate level schematic of a simple 4:2 line encoder
Truth Table
Input Output
I3 I2 I1 I0 O1 O0
0 0 0 0 x
0 0 0 1 0 0
0 0 1 0 0 1
0 1 0 0 1 0
1 0 0 0 1 1

8-to-3 encoder

Gate level schematic of a simple 8:3 encoder.
Gate level schematic of a simple 8:3 encoder. 4-input OR gates have been used for simplicity.
Truth Table
Input Output
I7 I6 I5 I4 I3 I2 I1 I0 O2 O1 O0
0 0 0 0 0 0 0 0 x
0 0 0 0 0 0 0 1 0 0 0
0 0 0 0 0 0 1 0 0 0 1
0 0 0 0 0 1 0 0 0 1 0
0 0 0 0 1 0 0 0 0 1 1
0 0 0 1 0 0 0 0 1 0 0
0 0 1 0 0 0 0 0 1 0 1
0 1 0 0 0 0 0 0 1 1 0
1 0 0 0 0 0 0 0 1 1 1

See also

References

  1. "Binary Encoders And Their Applications". Electronics Hub. 2015-06-29. Retrieved 2017-05-01.
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