Why Verilog? Learn HDL for VLSI Design

Verilog is a specialized language used to describe and implement hardware behavior. Unlike traditional programming languages like C and C++, which are designed for software development, Verilog was created to address the specific requirements of hardware design. This article explores why existing programming languages are insufficient for hardware modeling and how Verilog provides solutions for structural representation, timing, state handling, and concurrency.

Need for a Hardware Description Language

Q. Why Not Use C or C++ for Hardware Behavior?

• C and C++ are well-known programming languages used for software development.
• However, they are not suitable for implementing hardware behavior.
• Hardware differs from software in multiple fundamental ways, requiring specialized language constructs.

Q. How Hardware Differs from Software?

1. Structure of Hardware vs Software

• Hardware has an inherent structure that must be defined in a design.
• Example: Computer has multiple ports like USB, Mouse, Keyboard, etc.
• Software, on the other hand, is simply a piece of code and does not possess an explicit structure.
• C and C++ cannot define hardware input and output ports.

2. Concept of State in Hardware

• Hardware operates with states that change with time.
• Example: A traffic light controller has three states (Red, Green, Yellow), which transition over time.
• Software execution completes instantly and does not have the concept of time.
• A C program cannot model the behavior of a traffic light controller.

3. Concurrent Running Processes in Hardware

• Hardware supports multiple processes running simultaneously. Example: Computer can handle tasks like typing, using social media, projecting a presentation, and attending an online meet.
• Software generally follows sequential execution, without built-in concurrency.
• C and C++ can only handle sequential execution, making them inadequate for hardware modeling.

4. Need for Storage Elements 

• Hardware requires storage elements to retain information.
• Software execution completes instantly, eliminating the need for storage elements.

Thus, a specialized language is needed that supports:

• Hardware structure modeling
• Time and state implementation
• Concurrent processing

Emergence of Hardware Description Languages

To address these challenges, new languages were developed specifically for hardware design:

• Verilog (widely used in US-based companies)
• VHDL (commonly used in Europe-based companies)

These languages offer essential constructs needed for hardware-specific requirements.

Key Features of Verilog

Hardware Structure with Port Directions

Verilog provides constructs to define the structure of hardware, including port directions and sizes.

Example:  

module keyboard_ctrl_signal(
    input pclk_in,            // Clock signal for synchronization
    input [7:0] paddr_in,     // 8-bit address input
    input [7:0] pwdata_in,    // 8-bit data input
    output [7:0] kbd_data_out  // 8-bit output for keyboard data
);
endmodule

Time and State Implementation

• Verilog enables precise measurement of time and state transitions using constructs like always blocks.
• Time Handling Example: always @(posedge clk) or always @(negedge clk)
• If a clock frequency is 1KHz with a time period (TP) of 1ms, then counting 2000 positive edges will measure 2 seconds.
• State Handling Example: reg variables are used to define stateful elements.

 Concurrent Processing Implementation

Verilog supports the concurrent execution of multiple processes using always blocks.

Example:

A single always block implements one process.

Multiple always blocks implement multiple concurrent processes.

Verilog: A Hardware Description Language (HDL)

Unlike programming languages like C, C++, and Java, which are used for software development, Verilog was specifically designed for hardware behavior implementation.

Thus, Verilog is classified as a Hardware Description Language (HDL) rather than a traditional programming language.

Verilog was developed to address the limitations of conventional programming languages for hardware design. It provides necessary constructs for defining hardware structure, handling time and state transitions, executing concurrent processes, and implementing storage elements. As a result, Verilog has become a widely used HDL for digital system design across industries.

For students learning hardware design, understanding these fundamental differences between software and hardware is crucial. Mastering Verilog will allow them to efficiently model, simulate, and implement digital systems in real-world applications.