Verilog Module - Part 6 of our Verilog Journey

2024-02-28 | By DWARAKAN RAMANATHAN

What is a Module?

In Verilog, a module is a fundamental building block used to describe and define the behavior and structure of digital hardware components. Modules are a way to encapsulate and organize various elements of a digital circuit, making it easier to design and understand complex systems. They serve as the equivalent of functions or subroutines in software programming, allowing for modularity and reusability in hardware design.

A Verilog module typically consists of the following components:

Module Name: A user-defined name that identifies the module. This name is used to instantiate (use) the module in other parts of the design.

Ports: Ports are the interface between the module and the outside world. They define how data and signals enter and exit the module. Ports can be categorized as input (input ports), output (output ports), or bidirectional (inout ports.)
Internal Logic: This is the core of the module, where you describe the behavior and functionality of the hardware component using a combination of logic gates, sequential elements (such as flip-flops), and other Verilog constructs.

Here's a basic example of a Verilog module that represents a 2-input AND gate:

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module AND2 (  
input A,  
input B,  
output Y );  
assign Y = A & B;  
endmodule

In this example:

"AND2" is the module name.
"A" and "B" are input ports.
"Y" is an output port.
The internal logic uses the "assign" statement to compute the AND operation of inputs "A" and "B" and assigns the result to the output "Y."

Modules can be instantiated multiple times within a larger Verilog design, facilitating the creation of complex digital circuits by connecting modules together in a hierarchical fashion. This modularity and hierarchical design approach are key features of Verilog, making it a powerful language for describing digital systems.

The module is used to synthesize a hardware schematic. A Verilog code must be written only inside the module.

The hardware schematic of a half adder is shown below:

Verilog Module - Part 6 of our Verilog Journey

Why should we need a module?

Modules are an essential concept in hardware description languages like Verilog for several important reasons:

Modularity: Modules allow you to break down a complex digital system into smaller, manageable, and reusable components. Each module can represent a specific function or hardware block, making it easier to design, test, and understand. This modularity promotes code reusability, as you can use the same module in different parts of your design or in future projects.
Abstraction: Modules provide a level of abstraction that hides the internal details of a hardware component. This abstraction makes it easier to work with and reason about complex systems. Designers can focus on the high-level behavior and connectivity of modules without needing to understand their internal workings.
Hierarchical Design: Modules enable hierarchical design, where you can build larger systems by connecting smaller modules together. This hierarchical approach mirrors how real-world electronic systems are constructed, with various components and subsystems interconnected to create a complete system. It promotes a structured and organized design methodology.
Readability and Maintainability: Breaking a design into modules enhances the readability and maintainability of the code. Modules with clear names and well-defined interfaces make it easier for designers to collaborate and understand each other's contributions. Maintenance and debugging become more manageable because issues can be isolated to specific modules.
Testing and Verification: Modules facilitate the testing and verification of hardware designs. You can test individual modules in isolation, which simplifies debugging and ensures that each component functions correctly. This testing process can be done before integrating modules into the larger system, reducing the complexity of debugging at the system level.
Parallel Development: In large design projects, different teams or individuals can work on separate modules concurrently. This parallel development approach accelerates the overall design process and promotes teamwork.
Scalability: Modules provide a scalable approach to design. You can reuse modules across various projects and easily scale up your designs by adding more instances of existing modules or by creating new modules that adhere to established interfaces.
Design Reuse: Once you've created and thoroughly tested a module for a specific function (e.g., an adder, a multiplexer, a memory controller), you can reuse it in multiple projects without reinventing the wheel. This leads to time savings and consistency in designs.

What is a Top-Level Module?

In Verilog or other hardware description languages, a top-level module is the highest-level module in a design hierarchy. It represents the overall system or the top-level view of a digital circuit. The top-level module typically encapsulates and connects lower-level modules and describes the behavior of the entire system.

Explanation: We'll design a top-level module that instantiates a 4-bit binary adder module. The top-level module will define the inputs (two 4-bit binary numbers) and the output (the sum of the two numbers). This is a basic example to illustrate how a top-level module connects lower-level modules for a specific purpose.

Example: Here's the Verilog code for the top-level module:

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module TopLevelAdder (  
input wire [3:0] A, // 4-bit input A  
input wire [3:0] B, // 4-bit input B  
output wire [4:0] Sum // 4-bit output Sum );  
‎// Instantiate a 4-bit binary adder module  
‎  FourBitAdder adder (  
‎  .A(A),  
‎  .B(B),  
‎  .Sum(Sum) );  
endmodule

In this example:

TopLevelAdder is the top-level module, representing a 4-bit binary adder system.
It has input ports for two 4-bit binary numbers A and B and an output port Sum to represent the sum of the two numbers.
Inside the top-level module, we instantiate a lower-level module called FourBitAdder. This lower-level module is responsible for performing the binary addition of two 4-bit numbers.

Here's a simple representation of the FourBitAdder module:

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module FourBitAdder (  
input wire [3:0] A,  
input wire [3:0] B,  
output wire [4:0] Sum );  
assign Sum = A + B;  
endmodule

In this FourBitAdder module:

It takes two 4-bit inputs A and B.
The assign statement computes the sum Sum of inputs A and B.

The top-level module TopLevelAdder instantiates this lower-level FourBitAdder module, connecting the inputs and outputs accordingly. This top-level module represents the complete 4-bit binary adder system and can be synthesized to create actual hardware for adding 4-bit binary numbers.