# HDL Model of Combinational Circuits

• Last Updated : 15 Sep, 2022

Any language from a class of computer languages and/or programming languages used for the formal description of digital logic and electronic circuits is known as a hardware description language, or HDL, in the field of electronics.

To create executable hardware specifications, HDLs are employed.

The capacity to simulate a piece of hardware before it is built physically is given to the hardware designer via simulation software that is written to embody the underlying semantics of the language statements and simulate time.

Combinational Circuit:

A combinational circuit is one in which the output is solely dependent on the inputs’ current configuration at any one time, completely disregarding the inputs’ prior configuration. The most fundamental component of combinational logic is the logic gate. A group of Boolean expressions completely specify the logical task carried out by a combinational circuit.

Sequential logic circuits, the other subcategory of logic circuits, include both logic gates and memory components like flip-flops. The output of a sequential circuit depends on both the current and the previous state of the inputs due to the presence of memory elements. Input variables, logic gates, and output variables make up a combinational circuit.

Combinational logic gates convert binary information from the provided input data to the necessary output data by responding to the values of the signals at their inputs and producing the value of the output signal.

HDL Models

Any one of the following modeling approaches can be used to describe the Verilog HDL model of a combinational circuit:

1. Gate level modeling using instantiations of predefined and user-defined primitive gates.
2. Data flow modeling using continuous assignment with the keyword assign.
3. Behavioral modeling using procedural assignment statements with the keyword always.

Gate level modeling:

A circuit of this type is identified by its logic gates and how they are connected. A schematic diagram’s textual description is provided through gate-level modeling. Twelve fundamental gates are preset primitives in the Verilog HDL. They consist of and, NAND, OR, NOR, XOR, XNOR, NOT, & Buffer.

Data flow modeling:

Combinational logic dataflow modeling employs a variety of operators that work on operands to yield desired outcomes. Around 30 distinct operators are offered by Verilog HDL. Continuous assignments and the phrase assign are used in dataflow modeling. A statement that gives a value to a net is known as a continuous assignment. Using the data type family net, a physical link between circuit components can be represented.

Behavioral modeling :

A functional and algorithmic level representation of digital circuits is provided by behavioral modeling. Although it can also be used to describe combinational circuits, it is most frequently employed to explain sequential circuits. A list of procedural assignment statements is followed by an optional event control expression and the word always in behavioral descriptions.

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