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In the early days of integrated circuits, engineers had to sit down and physically draw transistors and their connections on paper to design them such that it can be fabricated on silicon. Bigger and complex circuits demanded more engineers, time and other resources and soon enough there was a need to have a better way of designing integrated circuits.

VHDL was soon developed to enhance the design process by allowing engineers to describe functionality of the desired hardware and let automation tools convert that behavior into actual hardware elements like combinational gates and sequential logic. Verilog was developed to simplify the process and make the Hardware Description Language (HDL) more robust and flexible. Today, Verilog is the most popular HDL used and practiced throughout the semiconductor industry.

Contents

Ch#1: Introduction Learn the essence of Verilog, how it fits into the design flow and why it is required in design of digital circuits
Ch#2: Data Types Learn all about basic data types like reg, wire and nets with simple examples
Ch#3: Building Blocks Learn about modules, ports, initial, always and assign blocks and what they actually mean in hardware circuits
Ch#4: Behavioral Modeling Learn how to model behavior of sequential and combinational logic using always and assign constructs
Ch#5: Gate/Switch Modeling Learn how to represent delays in combinational gates, and model using primitive logic gates like AND, OR, etc
Ch#6: Simulation Basics Learn basics of simulation, concept of timescales, scheduling regions and display statements
Ch#7: Examples Learn real simple examples with complete testbench code and analysis

How is Verilog useful ?

Verilog creates a level of abstraction that helps hide away the details of its implementation and technology.

For example, the design of a D flip-flop would require the knowledge of how the transistors need to be arranged to achieve a positive-edge triggered FF and what the rise, fall and clk-Q times required to latch the value onto a flop among many other technology oriented details. Power dissipation, timing and the ability to drive nets and other flops would also require a more thorough understanding of the physical characteristics of a transistor.

Verilog helps us to focus on the behavior and leave the rest to be sorted out later.

Example

The following code illustrates how a Verilog code looks like. We will delve into more details of the code in the next article. For the time being, let us simply understand that the behavior of a counter is described. The code essentially makes the counter count up if the up_down signal is 1, and down if its value is 0. It also resets the counter if the signal rstn becomes 0 making this an active-low reset.

 
  module ctr (input          up_down,
                  clk,
                  rstn,
              output reg [2:0]   out);
 
    always @ (posedge clk)
      if (!rstn)
        out <= 0;
      else begin
        if (up_down)
          out <= out + 1;
        else
          out <= out - 1;
      end
  endmodule
 

The simple example shown above illustrates how all the physical implementation details have been hidden while still providing a clear idea of how the counter functions.

ctr is a module that represents an up/down counter, and it is possible to choose the actual physical implementation of the design from a wide variety of different styles of flops optimized for area, power and performance. They are usually compiled into libraries and will be available for us to select within EDA tools at a later stage in the design process.

Now that you know what Verilog is, let's start learning Verilog !

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