Design Procedure

The design of combinational circuits starts from the verbal outline of the problem and ends in a logic circuit diagram or a set of Boolean functions from which the logic diagram can be easily obtained. The design procedure involves the following steps:

1. Specification

• Write a specification for the circuit if one is not already available.
• Specification such as names for inputs and outputs.

2. Formulation

• Derive a truth table or initial Boolean equations that define the required relationships between the inputs and outputs

3. Optimization

• Apply different optimization methods, such as algebraic manipulation, K-map method.
• Draw a logic diagram for the resulting circuit using ANDs, ORs, and inverters

4. Technology Mapping

• Map the logic diagram to the implementation technology selected.

5. Verification

• Verify the correctness of the final design manually or using simulation.

In simple words, we can list out the design procedure of combinational circuits as:
1. The problem is stated.
2. The number of available input variables and required output variables is determined.
3. The input and output variables are assigned letter symbols.
4. The truth table that defines the required relationships between inputs and outputs is derived.
5. The simplified Boolean function for each output is obtained.
6. The logic diagram is drawn.
7. Verify the correctness of the design.

Example: Design of Combinational Circuit

Q. Design a combinational circuit with three inputs and one output. The output must be logic 1 when the binary value of the inputs is less than 110 and logic 0 otherwise. Use only NAND gates.

Input: 3 (let x, y and z)
Output: 1 (let F)

Truth Table:

Simplification:

Logic Diagram:

This is a simple example of designing a combinational logic. We can design other combinational logics in similar ways.