Introduction to Verilog: Basics, Syntax, Design Examples, and Learning Resources for Beginners

By 佐川 直弘 | Naohiro Sagawa (運営者)

※記事内に広告を含みます。Amazonアソシエイトとして適格販売により収入を得ています。

1. What Is Verilog? Overview and Use Cases

Basic Definition of Verilog

The Role of Verilog in Digital Circuit Design

• FPGA design: FPGAs are highly flexible devices. Verilog allows engineers to freely define FPGA functionality and reprogram the device as needed.
• ASIC design: ASICs are specialized integrated circuits for specific applications. Verilog enables custom IC design.
• Simulation: Circuits described in Verilog can be simulated in software to verify behavior in advance, enabling early detection and correction of design errors.

What You Will Learn in This Article

1. Basic syntax and foundational concepts of Verilog
2. Practical design examples and debugging techniques
3. Helpful resources and tool recommendations

• Beginners who want to learn Verilog or HDL
• Engineers starting FPGA or ASIC design
• Designers or students reviewing foundational knowledge

2. Basic Syntax and Concepts of Verilog

Verilog Syntax and Structure

Defining and Using Modules

module AND_gate (
    input wire a, // input a
    input wire b, // input b
    output wire y // output y
);
    assign y = a & b; // AND operation
endmodule

Choosing Data Types (wire vs. reg)

• wire: Represents a physical wire. Used for connecting signals.
• reg: Represents a register. Used for storing values based on clock events.

module Example (
    input wire clk,    // clock input
    input wire rst,    // reset input
    input wire a,      // input a
    output reg y       // output y
);
    always @(posedge clk or posedge rst) begin
        if (rst)
            y <= 0;    // output = 0 on reset
        else
            y <= a;    // assign input a to output y on clock edge
    end
endmodule

Control Structures (if, case) and Simulation Notes

Using if Statements

always @(posedge clk) begin
    if (a == 1'b1)
        y <= 1'b0;  // set y = 0 when a is 1
    else
        y <= 1'b1;  // otherwise set y = 1
end

Using case Statements

always @(state) begin
    case (state)
        2'b00: y = 1'b0;
        2'b01: y = 1'b1;
        2'b10: y = 1'b0;
        2'b11: y = 1'b1;
        default: y = 1'bx; // unknown state
    endcase
end

Essential Concepts for Beginners

Blocking vs. Non-Blocking Assignments

• Blocking (=): Executes sequentially.
• Non-blocking (<=): Executes concurrently.

always @(posedge clk) begin
    a = b;     // blocking
    c <= d;    // non-blocking
end

Thinking in Parallel

always @(posedge clk) begin
    a <= b + 1;
end

always @(posedge clk) begin
    c <= d - 1;
end

Difference Between Simulation and Synthesis

• Simulation: Verify behavior in software.
• Synthesis: Convert design into hardware.

3. Key Features of Verilog

Strengths and Comparison with Other HDLs

Strengths of Verilog

1. Simple syntax

• Verilog resembles C, making it easy for programmers to learn.
• Its foundational elements—modules, data types, operators—are intuitive.

1. Extensive support

• Standard in FPGA/ASIC tools like Vivado and ModelSim.
• Large learning communities and resources.

1. Flexible design methodologies

• Supports RTL design and more.

Comparing Verilog, VHDL, and SystemVerilog

• Difference with VHDL: VHDL emphasizes strict syntax, while Verilog prioritizes concise expression.
• Difference with SystemVerilog: SystemVerilog adds object-oriented features and advanced verification capabilities.

Practical Use Cases of Verilog

The Role of Verilog in FPGA Design

1. Prototyping

• Used to verify circuit behavior early in product development.
• Rapid prototyping with easy adaptation to specification changes.

1. Behavioral Verification

• Simulation with Verilog helps identify design issues early.
• Tools like ModelSim allow engineers to build testbenches and observe system behavior.

Basic Flow of Circuit Simulation

1. Describe the circuit

• Implement the target circuit using Verilog.

1. Create a testbench

• Testbenches define the environment for verifying circuit behavior.
• Example:

module Testbench;
    reg a, b;
    wire y;

    // Instantiate the module under test
    AND_gate uut (
        .a(a),
        .b(b),
        .y(y)
    );

    initial begin
        // Apply test patterns
        a = 0; b = 0;
        #10; a = 0; b = 1;
        #10; a = 1; b = 0;
        #10; a = 1; b = 1;
        #10;
    end
endmodule

3. Run the simulation

• Execute the testbench in a simulator and verify expected behavior.

4. Analyze the results

• Examine waveform outputs and identify design issues.

4. Practical Design Examples Using Verilog

Learning Through Sample Verilog Code

Counter Design Example (with Code Explanation)

module Counter (
    input wire clk,    // clock input
    input wire rst,    // reset input
    output reg [3:0] count // 4-bit counter output
);
    always @(posedge clk or posedge rst) begin
        if (rst)
            count <= 4'b0000; // reset counter to 0
        else
            count <= count + 1; // increment on rising edge
    end
endmodule

1. clk controls timing.
2. rst initializes the counter.
3. The counter increases from 0 to 15 synchronously with the clock.

FSM (Finite State Machine) Example and Applications

module FSM (
    input wire clk,    // clock input
    input wire rst,    // reset input
    input wire in,     // trigger input
    output reg [1:0] state // current state
);
    // State definitions
    localparam STATE0 = 2'b00,
               STATE1 = 2'b01,
               STATE2 = 2'b10;

    always @(posedge clk or posedge rst) begin
        if (rst)
            state <= STATE0; // initial state
        else begin
            case (state)
                STATE0: state <= (in) ? STATE1 : STATE0;
                STATE1: state <= (in) ? STATE2 : STATE0;
                STATE2: state <= (in) ? STATE0 : STATE1;
                default: state <= STATE0;
            endcase
        end
    end
endmodule

1. States are defined using localparam.
2. case handles transitions based on in.
3. This structure can be applied to controllers or signal generation circuits.

Simple Adder Design (Beginner-Friendly)

module Adder (
    input wire [1:0] a, // 2-bit input a
    input wire [1:0] b, // 2-bit input b
    output wire [2:0] sum // 3-bit output to handle carry
);
    assign sum = a + b; // addition
endmodule

1. Uses assign for combinational logic.
2. 3-bit output considers carry-out.
3. This is a foundational building block for arithmetic logic.

Common Issues and Solutions

Typical Errors (Simulation & Synthesis)

1. Simulation Errors

• Symptom: Signal becomes undefined (x).
• Cause: Missing initialization or incorrect module connections.
• Solution: Define initial values or initialize via a testbench.

1. Synthesis Errors

• Symptom: Using non-synthesizable constructs (e.g., initial).
• Cause: Inclusion of simulation-only code.
• Solution: Use synthesizable structures (such as always).

Using Debugging Tools Effectively

1. Simulators (e.g., ModelSim)

• Inspect signal behavior and verify timing via waveform displays.

1. Waveform Viewers

• Visually analyze input/output signals to identify design issues.

1. Debug Messages

• Use $display to print values during simulation:

initial begin
    $display("Initial state: %b", state);
end

5. Resources and Tools for Learning Verilog

Recommended Learning Resources

Books & Tutorials for Beginners

1. Books

• “Introduction to Digital Design with HDL”
  • Explains basic concepts of both Verilog and VHDL.
• “Verilog HDL: A Guide to Digital Design and Synthesis”
  • A comprehensive English-language book covering fundamentals to advanced design.
• “Digital Design and Verilog HDL Fundamentals”
  • Suited for beginners to intermediate learners, with a strong focus on Verilog-based design.

1. Online Tutorials

• YouTube
  • Free tutorials available in both English and Japanese.
  • Allows learners to follow along with real code.
• Websites
  • EDA Playground: A browser-based environment for running Verilog.
  • ASIC World: Offers tutorials from syntax to practical design examples.

Video Courses and Hands-On Learning

1. Udemy

• Courses like “Verilog for Beginners” and “Learn FPGA Design” provide structured content.

1. Coursera

• University-level courses focused on hardware design.

Tools That Support Development

Modeling Tools (ModelSim, Vivado)

1. ModelSim

• A leading Verilog simulation tool.
• Features:
  • User-friendly interface.
  • Intuitive waveform viewer for debugging.

1. Vivado

• Xilinx’s FPGA design suite.
• Features:
  • Integrated support from RTL to implementation.
  • Seamless connection with Xilinx FPGA boards.

Choosing and Installing Synthesis Tools

1. Quartus Prime

• Intel’s FPGA development suite, with free editions available.

1. ISE Design Suite

• Used for older Xilinx FPGA devices.

6. FAQ About Learning Verilog

How Should Beginners Start Learning Verilog?

Question: I just started learning Verilog. What is the best way to begin?

Answer:

1. Start with the fundamentals

• Begin by designing simple circuits such as AND/OR gates. Understanding basic syntax and module structure is essential.

1. Use simulation tools

• Tools like ModelSim or Vivado help verify your code. Running simulations deepens your understanding.

1. Use reliable resources

• Refer to books and online tutorials to build a strong foundation (see Section 5 for recommendations).

1. Attempt small projects

• After learning the basics, try building small projects such as 4-bit counters or simple FSMs.

Should I Choose VHDL or Verilog?

Question: Which language should I learn—VHDL or Verilog? When should each be used?

Answer:

1. When to choose Verilog

• Verilog’s simple syntax makes it beginner-friendly, especially for those with C programming experience.
• Widely used for prototyping and FPGA development.

1. When to choose VHDL

• Ideal for mission-critical systems requiring strict design validation.
• Strong type checking reduces the chance of design errors.

1. Selection criteria

• Ease of learning: Verilog is typically easier for beginners.
• Project requirements: Follow the language used in the design environment.
• Tool support: Most tools support both, but selection depends on target FPGA/ASIC.

Common Mistakes Beginners Should Avoid

Question: What mistakes do beginners commonly make when learning Verilog, and how can they avoid them?

Answer:

1. Lack of initialization

• Signals may appear as x (undefined) during simulation.
• Solution: Always initialize signals or set values in your testbench.

initial begin
    signal = 0; // initialize signal
end

2. Confusing blocking and non-blocking assignments

• Misusing = (blocking) and <= (non-blocking) can cause unexpected behavior.
• Solution: Use non-blocking assignments in clocked always blocks.

3. Mixing synthesizable and simulation-only code

• Including simulation-only constructs (e.g., initial) in synthesizable logic causes errors.
• Solution: Separate synthesizable logic from simulation-only blocks.

4. Misunderstanding parallel execution

• Verilog describes parallel hardware behavior, not sequential software logic.
• Solution: Understand that each always block runs independently.

7. Moving to the Next Step with Verilog

Reviewing Your Verilog Learning Progress

Review Checklist

• Basic syntax understanding: Ensure you can describe modules, data types, and control structures.
• Practical design experience: Counters, FSMs, and similar designs should feel comfortable.
• Tool usage: You should be able to simulate and debug with ModelSim or Vivado.

Preparing for the Next Step

• If you understand the basics, you are ready to move on to more advanced topics.
• Use the steps below to expand your technical skills.

Advancing to FPGA Design

Learn FPGA Fundamentals

1. Prepare an FPGA development board

• Recommended beginner boards: Basys 3, Nexys A7 (Digilent)
• Reason: Strong community support and easy integration with Vivado.

1. Start with simple projects

• Begin with LED blinking or switch-controlled circuits.
• Learn clock division and basic control logic.

1. Move to more complex systems

• Design multi-FSM controllers, memory interfaces, and integrated systems.

Transitioning to SystemVerilog

Why Learn SystemVerilog?

• SystemVerilog extends Verilog with advanced verification and object-oriented features.
• Useful for building sophisticated testbenches and large-scale systems.

Key Topics to Study

1. Class-based testbenches

• Enables randomized testing and coverage analysis.

1. Using interfaces

• Simplifies communication between modules.

1. Extended control structures

• Learn features such as unique and priority to improve design safety.

Working on Real-World Projects

Join Open-Source Projects

• RISC-V processor designs
• Simple DSP (Digital Signal Processing) modules

Start Your Own Projects

• Create original designs based on your interests.
• Examples: digital clocks, audio processors, signal filtering circuits.

Recommended Next Learning Steps

Acquire Advanced Design Skills

1. Pipeline design

• Learn the fundamentals used in high-performance processors.

1. Understanding clock domains

• Master techniques for handling signals across multiple clock domains.

1. Low-power design

• Apply power-efficient design methods for real-world applications.