Logic Design Interview Question: 3-Input Priority Encoder

A priority encoder has inputs that are assigned in some predetermined order. The output is the binary encoding of the first "1" valued input from the ordered list, and it is zero otherwise.

1. Give the truth table for a 3-input priority encoder.
2. Give a sum of products realization of this priority encoder.

Solution:

1. 
   
   Assume the inputs are A, B, C with A having priority 3, B priority 2 and C priority 1:

   A  B  C | P1 P0
   ========|=========
   0  0  0 |  0  0
   0  0  1 |  0  1
   0  1  0 |  1  0
   0  1  1 |  1  0
   1  0  0 |  1  1
   1  0  1 |  1  1
   1  1  0 |  1  1
   1  1  1 |  1  1