Priority Encoder

A priority encoder provide n bits of binary coded output representing the position of the highest order active input of 2n inputs. If two or more inputs are high at the same time, the input having the highest priority will take precedence.

It's applications includes

4 to 2 priority encoder

A 4-to-2 priority encoder takes 4 input bits and produces 2 output bits. In this truth table, for all the non-explicitly defined input combinations (i.e. inputs containing 2, 3, or 4 high bits) the lower priority bits are shown as don't cares (X). Similarly when the inputs are 0000, the outputs are not valid and therefore they are XX.

From the above truth table, we can obtain the full truth table required for our design.

From this truth table, we use the Karnaugh Map to minimise the logic to the following boolean expressions:

  • O1 = I2 + I3
  • O0 = ~I2 * I1 + I3

Implementation of the 4 to 2 priority encoder using combinational logic circuits.

Learn by Doing

Design a 4 to 2 Priority Encoder to deepen your understanding of the circuit.