Digital Electronics| Combinational Logic Circuit-definition,truth table ,block diagram

INTRODUCTION
Generally, digital circuits are divided into two categories: combinational logic circuit and sequential logic circuit.

The combinational logic circuits are the circuits that contain different types of logic gates. Simply, a circuit in which different types of logic gates are combined is known as a combinational logic circuit. The output of the combinational circuit is determined from the present combination of inputs, regardless of the previous input. The input variables, logic gates, and output variables are the basic components of the combinational logic circuit. There are different types of combinational logic circuits, such as Adder, Subtractor, Decoder, Encoder, Multiplexer, and De-multiplexer.

This chapter, concerned with the study of combinational circuit, includes the following topics:

Combinational Logic Circuit Definition

the combinational logic circuits or time-independent logic circuits in digital circuit theory can be defined as a type of digital logic circuit implemented using Boolean circuits, where the output of logic circuit is a pure function of the present inputs only. The combinational logic circuit operation is instantaneous and these circuits do not have the memory or feedback loops.

This combinational logic is in contrast compared to the sequential logic circuit in which the output depends on both present inputs and also on the previous inputs. Thus, we can say that combinational logic does not have memory, whereas sequential logic stores previous input in its memory. Hence, if the input of combinational logic circuit changes, then the output also changes.

The combinational logic design can be done using two methods such as a sum of products and a product of sums. Combinational logic circuits are generally designed by connecting together or combining the basic logic gates such as NAND, NOR, and NOT. Hence, these logic gates are termed as building blocks. These logic circuits can be a very simple circuit or a very complex circuit or huge combinational circuit can be designed using only universal logic gates 

DESIGN PROCEDURE FOR COMBINATION LOGIC CIRCUITS 

There are the following characteristics of the combinational logic circuit:

• At any instant of time, the output of the combinational circuits depends only on the present input terminals.
• The combinational circuit doesn't have any backup or previous memory. The present state of the circuit is not affected by the previous state of the input.
• The n number of inputs and m number of outputs are possible in combinational logic circuits.

The 'n' input variable comes from the external source while the 'm' output variable goes to the external destination. In many applications, the source or destinations are storage registers.

The steps used to design a combinational logic circuit using gates are:
METHODOLOGY: DESIGNING A COMBINATIONAL LOGIC CIRCUITS 
Step 1: Statement of the problem.
Step 2: Identify the input and output variables.
Step 3:The input and output variables are assigned letter symbols.
Step 4:Construct a truth table that defines the required relationship between inputs and outputs is derived.
Step 5:Write Boolean expressions for various output variables in terms of input variables.
Step 6: Simplify the Boolean expression using K-map or Quine- McCluskey method.
Step 7: Implement the simplified Boolean expressions using logic gates.
FUNCTIONS OF COMBINATIONAL LOGIC CIRCUIT
The function of combinational logic circuits can be specified in three main ways such as:

• Boolean Algebra: This forms the algebraic expression showing the operation of the logic circuit for each input variable either True or False that results in a logic "1" output.

• Truth Table: A truth table defines the function of a logic gate by providing a concise list that shows all the output states in tabular form for each possible combination of input variable that the gate could encounter.

• Logic Diagram: This is a graphical representation of a logic circuit that shows the wiring and connections of each individual logic gate, represented by a specific graphical symbol, that implements the logic circuit.