5.2 DESIGN PROCEDURE

Any combinational circuit can be designed by the following steps of design procedure.

1. The problem is stated.

2. Identify the input variables and output functions.

3. The input and output variables are assigned letter symbols.

4. The truth table is prepared that completely defines the relationship between the input variables and output functions.

5. The simplified Boolean expression is obtained by any method of minimization algebraic method, Karnaugh map method, or tabulation method.

6. A logic diagram is realized from the simplified expression using logic gates.

It is very important that the design problem or the verbal specifications be interpreted correctly to prepare the truth table. Sometimes the designer must use his intuition and experience to arrive at the correct interpretation. Word specification are very seldom exact and complete. Any wrong interpretation results in incorrect truth table and combinational circuit.

Varieties of simplification methods are available to derive the output Boolean functions from the truth table, such as the algebraic method, the Karnaugh map, and the tabulation method. However, one must consider different aspects, limitations, restrictions, and criteria for a particular design application to arrive at suitable algebraic expression. A practical design approach should consider constraints like (1) minimum number of gates, (2) minimum number of outputs, (3) minimum propagation time of the signal through a circuit, (4) minimum number of interconnections, and (5) limitations of the driving capabilities of each logic gate. Since the importance of each constraint is dictated by the particular application, it is difficult...