*3.2.1 Adder circuits*

• *NOT:* The NOT of a variable is 1 if and only if, the variable itself is 0 and vice

• *AND:* The AND of two variables is 1 if and only if *both* the variables are 1.

• *XOR:* The Exclusive-OR of two variables is 1 if *either* of them but *not both* is 1

There are two types of circuits exists in the digital system, Combinational Logic Circuits and Sequential Circuits. A combinational logic circuit is a circuit where the output depends on the combination of present input state. The set of operations which these combinational circuits performs logically by a set of Boolean functions. A sequential logic circuit is a circuit where the output depends on the combination of present input state and past input or previous input values. The previous output

A combinational circuit consist of variables for input and output, and basic logic gates to perform the boolean function. The output signals are generated according to the inputs as well as the logic circuits employed. Here both the input and outputs are binary values either 1 or 0. **Figure 1** shows the simple block diagram of combinational logic circuits with *n* input variables and *m* output variables. If there are *n* number of inputs to the circuit then *2<sup>n</sup>* possible combinations of input states but each combination can produce only one output state [2]. For instance if the combinational logic circuit has 2 inputs A and B then there can be 4 possible input states.

The following are the steps considered while designing a combinational logic

• The first step is the statement of the problem for which this combinational

• Definition of the input and output variables and the variable name for inputs

• The simplified boolean expression is obtained for the output variable with the

• Formation and tabulation of truth table which describes the relationship

help of minimization techniques or Karanaugh map.

• *OR:* The OR of two variables is 1 if *either (or both)* of the variables is 1.

*LabVIEW - A Flexible Environment for Modeling and Daily Laboratory Use*

versa

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circuits.

**Figure 1.**

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**3.1 Design procedure**

and outputs.

*Combinational Logic Circuits.*

circuit need to be designed

between the input and output.

**3. Combinational logic circuits**

values are stored in the memory elements.

A combinational circuit that performs the addition of two bits is called *halfadder*. When the augend and addend numbers contain more significant digits, the carry obtained from the addition of two bits is added to the next higher order pair of significant bits. The combinational circuit that performs the addition of three bits is called a *full-adder*. The full adder can also be obtained by using two half adder circuits.
