Encoder (digital)

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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.

A ${\displaystyle 2^{n}}$-to-n encoder has n number of outputs in correspondence to the ${\displaystyle 2^{n}}$ 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.

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

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• Binary decoder
• Multiplexer (MUX)
• Priority encoder