Priority encoder

A priority encoder is a circuit or algorithm that compresses multiple binary inputs into a smaller number of outputs. The output of a priority encoder is the binary representation of the ordinal number starting from zero of the most significant input bit. They are often used to control interrupt requests by acting on the highest priority request.

If two or more inputs are given at the same time, the input having the highest priority will take precedence. An example of a single bit 4 to 2 encoder is shown, where highest-priority inputs are to the left and "x" indicates an irrelevant value - i.e. any input value there yields the same output since it is superseded by higher-priority input.

I3	I2	I1	I0	O1	O0
0	0	0	x	0	0
0	0	1	x	0	1
0	1	x	x	1	0
1	x	x	x	1	1

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Priority encoders can be easily connected in arrays to make larger encoders, such as one 16-to-4 encoder made from six 4-to-2 priority encoders - four 4-to-2 encoders having the signal source connected to their inputs, and the two remaining encoders that the output of the first four as input.

The priority encoder is an improvement on a simple encoder circuit, in terms of handling all possible input configurations.

Simple encoder

A simple encoder circuit can receive a single active input out of 2ⁿ input lines generate a binary code on n parallel output lines.

For example a single bit 4 to 2 encoder takes in 4 bits and outputs 2 bits. The illustrated example will sometimes fail as a priority encoder if more than one input line is active at a time.

Gate level circuit diagram of a single bit 4-to-2 line encoder

I3	I2	I1	I0	O1	O0
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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References