Encoder (digital)

An encoder (or "simple encoder") in digital electronics is a one-hot to binary converter. That is, if there are 2ⁿ 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.

A General encoder's block diagram.

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

2ⁿ-to-n encoders

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

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. 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

• Binary decoder
• Multiplexer (MUX)
• Priority encoder