Difference between revisions of "Priority encoder"

Latest revision as of 17:11, 9 December 2011

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are to the left and "x" indicates an irrelevant value - i.e. any input value there yields the same output since it is superseded by higher-priority input.

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 take the output of the first four as input.^{[citation needed]}

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 is one-hot to binary converter. That is, if there are of 2ⁿ input lines at the input, whereas only one of them is in 1, binary code of this 'hot' line is produced a on the n'-bit output.

For example a single bit 4 to 2 encoder takes in 4 bits and outputs 2 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 bits) the outputs are treated as don't cares. It makes no sense to infer the priority encoder if input circuit guarantees one and only one bit high in the input or if it is understood that non-single-active-inputs will produce garbage encodings.