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Introduction to Verilog: Basics for Digital Design

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Kerim Turak
Hardware Design Verilog FPGA ASIC RTL Design Hardware Description Language
Hardware Design
Author
Kerim Turak
Digital IC Design & Verification Engineer
Table of Contents
Verilog HDL Series - This article is part of a series.
Part 1: This Article

🔧 What is Verilog?
#

Verilog is a Hardware Description Language (HDL) designed to describe, simulate, and implement digital electronic systems — from simple logic gates to entire processors. Originally developed in the 1980s by Gateway Design Automation and later standardized by IEEE as IEEE 1364, Verilog enables designers to describe hardware circuits using a textual, programming-like syntax.

Unlike traditional programming languages that execute on CPUs, Verilog describes circuits that are synthesized onto physical hardware like FPGAs or ASICs. It allows designers to model:

  • Logic gates and combinational circuits
  • Sequential logic (like flip-flops and registers)
  • State machines
  • Complete processor architectures


🕰️ What Came Before Verilog?
#

Before Verilog, digital hardware was typically designed using schematic entry tools and low-level hardware description languages like:

  • Switch-level modeling or transistor-level schematics, which required describing circuits gate-by-gate or even transistor-by-transistor.
  • ABEL (Advanced Boolean Expression Language) – used mostly for PAL and CPLD programming in the early 1980s.
  • PALASM (Programmable Array Logic Assembler) – one of the first hardware languages used to describe logic equations for PAL devices.
  • VHDL (VHSIC Hardware Description Language) – developed by the U.S. Department of Defense in 1981 as a more formal HDL, and standardized as IEEE 1076 in 1987.

Note : VHDL was more verbose and strongly typed, whereas Verilog offered a more concise and C-like alternative, especially for RTL modeling.


🔧 Origin and Early Versions of Verilog
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1983–1985:
Verilog was developed by Phil Moorby and Prabhu Goel at Automated Integrated Design Systems (later Gateway Design Automation) between 1983 and 1984. It was first used as a hardware modeling language in 1985.

1990:
Cadence Design Systems acquired Gateway, obtaining all rights to Verilog.

1995 – Verilog-95 (IEEE 1364-1995):
To counter the rise of VHDL, Cadence initiated Open Verilog International (OVI) to standardize Verilog. This led to Verilog’s adoption as an IEEE standard (IEEE 1364-1995), the first official release.

2001 – Verilog-2001 (IEEE 1364-2001):

  • Introduced signed variables, generate blocks, always @*, and file I/O.
  • Greatly improved readability and synthesis support.

2005 – Verilog-2005 (IEEE 1364-2005):

  • Included minor fixes and enhancements.
  • Integration efforts with Verilog-AMS began.
  • Still a separate language from SystemVerilog.

🧠 Emergence of SystemVerilog
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2002 – SystemVerilog 3.0:
Developed by Accellera as a superset of Verilog-2001.

  • Aimed to unify design and verification under one language.
  • Introduced class, interface, constraint, covergroup, assertion, randomization, logic, and more.

2003–2004 – SystemVerilog 3.1 / 3.1a:

  • Improved DPI (Direct Programming Interface) with C.
  • Enhanced verification support.

2005 – IEEE 1800-2005:

  • SystemVerilog became an official IEEE standard.
  • Designed to support both RTL design and advanced verification.

🔁 Unification of Verilog & SystemVerilog
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2009 – IEEE 1800-2009:

🔗 Verilog (IEEE 1364-2005) and SystemVerilog were officially merged into a single standard: IEEE 1800-2009.

Consequences of this unification:

  • Development of Verilog under IEEE 1364 was discontinued.
  • SystemVerilog became a superset of Verilog-2005.
  • HDL (Hardware Description) and HVL (Hardware Verification) languages were unified.
  • SystemVerilog-2009 enhanced interoperability with Verilog-AMS, SystemC, and VHDL.

🧬 Modern SystemVerilog Standards
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YearVersionDescription
2012IEEE 1800-2012Simplified syntax, clarification, and fixes
2017IEEE 1800-2017Enhancements for modern verification practices
2023IEEE 1800-2023Latest update, includes new features for assertions and DPI

📊 Timeline Summary
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YearEvent / ReleaseStandard No.
1983–1985Initial development of Verilog
1995Verilog’s first IEEE standardIEEE 1364-1995
2001Verilog-2001 (data types, readability)IEEE 1364-2001
2005Verilog-2005IEEE 1364-2005
2005First SystemVerilog IEEE standardIEEE 1800-2005
2009Verilog & SystemVerilog mergedIEEE 1800-2009
2012Language improvementsIEEE 1800-2012
2017Widespread industry adoptionIEEE 1800-2017

🚀 What Did Verilog Bring to the Table?
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Verilog introduced a C-like, concise syntax that made hardware design more approachable and readable compared to older HDL languages like VHDL or schematic-based workflows.

Key contributions of Verilog:

  • RTL Modeling: Simplified Register Transfer Level design using always, assign, and initial blocks.
  • Mixed Abstraction: Allowed both behavioral and gate-level descriptions.
  • Modular Design: Supported module-based hierarchical design, encouraging reusable components.
  • Widespread Tool Support: Quickly adopted in industry thanks to simplicity and synthesis compatibility.

💻 What is an HDL (Hardware Description Language)?
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An HDL is a programming-like language used to model and describe the behavior and structure of digital circuits. Popular HDLs include Verilog and VHDL. Unlike software, HDL code is intended to be synthesized into real hardware on FPGAs or ASICs.


🧠 HDL vs. Software Languages: What Makes Verilog Different?
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At first glance, Verilog may look like a programming language – it uses if, case, for, and semicolons. But in reality, Verilog is fundamentally not a software language.

Here’s how HDLs like Verilog differ from traditional programming languages such as C, Java, or Python:

FeatureVerilog (HDL)C / Python / Java (Software)
Execution ModelEvent-driven, parallelInstruction-driven, sequential
Time AwarenessTime is explicit (#10, @posedge)Time is implicit or nonexistent
Output BehaviorDescribes hardware connectionsComputes software behavior
ConcurrencyMultiple blocks run simultaneouslyCode executes one line at a time
Final DestinationMapped to silicon (FPGA/ASIC)Runs on CPUs or VMs

Verilog simulates real-world circuits. It doesn’t just “run” — it waits for events, reacts to signal changes, and models physical timing.

This event-driven nature is what makes Verilog powerful yet different. For example:

module incrementer (
    input        clk,
    input  [7:0] data_in,
    output [7:0] data_out
);

    reg [7:0] data_out_reg;
    assign data_out = data_out_reg;

    always @(posedge clk) begin
        data_out_reg <= data_in + 1;
    end

endmodule

This block only triggers when clk has a rising edge, not sequentially like software. That’s why understanding time, delay, and signal sensitivity is essential in HDL design.


⚡ Software vs. Hardware Mindset: A Simple Comparison
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Understanding the difference between sequential software execution and concurrent hardware description is essential when learning Verilog.

Let’s compare a simple example:

👨‍💻 Software Perspective (C code – sequential)
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int a = 5;
int b = a + 3;

In C, the CPU executes these lines in order. First, it assigns 5 to a, then calculates a + 3 and stores the result in b. Once executed, these values stay constant unless explicitly changed later in the code.


🔌 Hardware Perspective (Verilog – concurrent)
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assign b = a + 3;

In Verilog, this is not a one-time execution. Instead, it defines a continuous assignment:

Whenever a changes, b automatically updates in real-time — as if a wire is connecting them.

There’s no instruction pointer. No order of execution. Just signal propagation, like in actual circuits.


💡 Tip for Beginners
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Don’t treat Verilog like C or Python. Instead:

  • Think in terms of hardware structure, not software flow.
  • Ask yourself: “What real hardware am I describing?”
  • Master the concepts of time, edge, and signal dependency.

Once you make that mindset shift, Verilog becomes a powerful design tool.



📚 References
#

The list of references is extensive; below are some core sources.
For the full list, see the PDF version or source page.


This article is part of the Axolot Logic series on digital design history.
For more tutorials and insights, check the homepage.

Verilog HDL Series - This article is part of a series.
Part 1: This Article