Question

Design an 8-input priority encoder with inputs a7:0?

Answer 1

An 8-input priority encoder encodes the highest-order active input into a three-bit binary number. The design involves creating a truth table, deriving logical expressions, and implementing the circuit with logic gates, including 'valid' and 'zero' outputs to indicate active input or none.

Step-by-step explanation:

An 8-input priority encoder takes eight input signals and encodes them into a three-bit binary number based on the highest-order active input. The design process involves assigning priority to the highest-order bit, meaning if two or more inputs are high (logic 1), the output will represent the highest-order input. Here is a step-by-step approach to designing such a device:

1. Label the inputs a7, a6, a5, a4, a3, a2, a1, and a0, with a7 being the highest priority and a0 the lowest.
2. Create truth tables that map each input to a unique 3-bit output code, with 'don't care' conditions for lower priority inputs when a higher priority input is active.
3. Using the truth table, derive the logical expression for each output bit of the encoder using Karnaugh maps or Boolean algebra for simplification.
4. Implement the logic circuit using AND, OR, NOT, and other required logic gates based on the simplified expressions.
5. Include additional output to indicate when any input is active, often referred to as 'valid' output.

For example, if a7 is active, the output should be 111, regardless of the state of the other inputs. This continues in descending order of priority through the inputs.

Remember to account for the possibility that no inputs may be active by including an output that indicates when none of the inputs are high, typically called the 'zero' or 'invalid' output.