9. Verilog - Introduction

9.1. Introduction

Verilog is a HDL, it is very similar to the C programming language and it was developed in 1985. Verilog became popular because it is easy to learn if you have some programming experience with C. With Verilog you can describe the system in a sequential or in a combinational way and the most popular tools that are used to develop digital systems support Verilog.

In Verilog the system is described with inputs, outputs and modules that implement some logic function. The system designer will implement modules and the software will optimize and implement the system to use less logic cells as possible, in Red Pitaya case the software used is Vivado. Modules can be implemented inside other modules, a module output can be another module input. A module is declared using the words module and endmodule, the function, the inputs and outputs are declared inside it. Some characteristics of Verilog are:

  • It is case sensitive, for example an input named adc_in is different from an input named Adc_in.
  • All statements of Verilog language, like module, are in lowercase.
  • The semicolon is used to end a line of code.
  • Line comments are made with // code, and block comments with /* code block */.

9.2. Verilog Example

Next there is an example of a project that multiplies two inputs with 18 bits resulting in an output with 36 bits. First is created a module named projtest and inside parenthesis is declared the input and output ports. The // are a comment so the line of code is invisible to the compilation. Next it is assigned to the output the mathematical multiplication of the two inputs. It is important to know the word lengths to not occur overflow.

module projtest(
    input [17:0] in_a,
    input [17:0] in_b,
    output [35:0] out_f
//   design_1 instanciation1( .input_a(in_a), .input_b(in_b),
.clk(clock), .outp(out_f)  );
assign out_f = in_a * in_b;