The University of Texas at Austin
Department of Electrical and Computer Engineering

Verilog Manual

Table of Contents

  1. 1. Introduction
  2. 2. Syntax
  3. 3. Data Types
  4. 4. Module Definitions and Instances
  5. 5. Continuous Assignments
  6. 6. Procedural Assignments
    1. 6.1. initial
    2. 6.2. always
    3. 6.3. repeat
  7. 7. Compiler Directives
    1. 7.1. `define
    2. 7.2. `include
  8. 8. System Tasks and Functions
    1. 8.1. $finish
    2. 8.2. $time
    3. 8.3. $strobe
    4. 8.4. $readmemb and $readmemh
    5. 8.5. Dumping Waveforms
  9. 9. Timing Information
  10. 10. A Complete TOP Module
  11. 11. Behavioral Verilog

1. Introduction

This semester, you will design your project using Verilog HDL, a hardware description language commonly used in industry. While behavioral Verilog can be used to describe designs at a high level of abstraction, you will design your processor at the gate level in order to quantify the complexity and timing requirements of your design. Hence you will use structural Verilog only.

You will be provided Verilog libraries containing modules that will be the basic building blocks for your design. These library parts include simple logic gates, registers, and memory modules, for example. While the library parts are designed behaviorally, they incorporate some timing information that will be used in your simulations. Using the class libraries ensures a uniform timing standard for everyone.

Structural Verilog allows designers to describe a digital system as a hierarchical interconnection of modules. The Verilog code for your project will consist only of module definitions and their instances, although you may decide to use some behavioral Verilog for debugging purposes. This manual will cover all aspects of the Verilog language that you will need to be familiar with.

2. Syntax

Verilog uses a C-like syntax. It is case sensitive, and all keywords are in lower case letters. Declarations, assignments, and statements end with a semicolon. It also uses C-style comments:

// comment to end of line
/* closed comment */

Numbers are represented as <number_of_bits>'<base><number>, where base can be b, o, d, or h, for binary, octal, decimal, or hex, respectively. Some examples:

8'hFF     //8-bit hex number
5'b101    //5-bit binary number 00101
1         //decimal number 1.  (decimal is the default base)
3'o5      //3-bit octal number 5

3. Data Types

The data types that you will use are reg (register) and wire. Wires cannot hold a value; they are used to connect modules of combinational logic. Registers are used to store values. Since regs keep state, a reg cannot be the output of combinational logic.

Note: The register datatype is not what you use to make flip-flops and registers. When implemented at the structural level, flip-flops and registers are still made out of wires and logic gates or other modules, as you will see in the D latch example in section 4. Registers are only used in behavioral Verilog.

You can declare regs and wires as follows:

reg a,b;             //two scalar registers
reg [0:7] Byte;      //8-bit vector, bit 0 is MSB
wire [31:0] Word;    //32-bit vector, bit 31 is MSB

You can reference a subset of bits in a vector as Word[0] or Byte[3:0], for example.

You can use braces for concatenating two or more signals, like this:

{Word[7:0], Word[31:8]} is the 32-bit vector obtained by rotating Word to the right by one byte. {24'b0, Word[7:0]} is the 32-bit vector obtained by zero extending the first byte of Word.

You can specify repetition by putting a number before the opening brace, like this: 4{Byte[0]} is the same thing as {Byte[0], Byte[0], Byte[0], Byte[0]}.

A 2-dimensional memory of type reg can be declared as follows:

reg [7:0] memory [0:1023];

You can reference bytes of memory as memory[5], for example. If you want to reference an individual bit, you must first copy the byte into another variable.

We are using 4-value logic; that is, each bit can take on one of four possible values:

0logic zero
1logic one
zhigh impedance (for tri-state driver output)
xunknown or undefined

4. Module Definitions and Instances

The module is the basic logic entity in Verilog.

A module definition is delimited by the keywords module and endmodule, as shown in the example below.

The module header consists of the module keyword, the name of the module, and the port list in parenthesis, followed by a semicolon:

module d_latch (d, q, qbar, wen);

Following the module header are the port declarations. Ports may be of type input, output, or inout, for input, output, or bidirectional ports. Ports may be either scalar or vector. The port names in the declarations do not have to occur in the same order as they did in the port list.

The body of the module (consisting of wire and reg declarations and module instances) follows the port declarations.

Here is an example of a module definition for a gated D latch:


module d_latch (d, q, qbar, wen);
   input d, wen;
   output q, qbar;

   wire dbar, r, s;

   inv1$ inv1 (dbar, d);
   nand2$ nand1 (s, d, wen);
   nand2$ nand2 (r, dbar, wen);

   nand2$ nand3 (q, s, qbar);
   nand2$ nand4 (qbar, r, q);


In this example, there are 5 module instances. inv1$ and nand2$ (1-input inverter and 2-input NAND gate) are modules that are defined in the class libraries. For both of these gates, the first port is the output of the gate, and the remaining ports are inputs.

A module instance consists of the module name followed by an instance identifier, port list, and semicolon. Several instantiations of the same type of module can be made in a list like this:

nand2$ nand1 (s, d, wen),
       nand2 (r, dbar, wen),

       nand3 (q, s, qbar),
       nand4 (qbar, r, q);

When instantiating a module, you can omit ports from the port list. For example, if we wanted to instantiate d_latch in another module but did not care about the qbar output, we could instantiate it as follows:

d_latch latch1 (din, q_out, , wen);

In this example, din and wen could be either wires, registers, or ports declared in the higher-level module, but q_out cannot be a register.

You should know that any undeclared identifiers are implicitly declared as scalar wires. So watch out for typos in your port lists.

5. Continuous Assignments

The assign keyword can be used to tie two wires together or to continually assign a behavioral expression to a wire. Examples:

wire a, b, c;

assign a = b;
assign c = ~b;  //c is assigned to NOT b

With a continuous assignment, whenever the value of a variable on the right-hand side changes, the expression is re-evaluated and the value of the left-hand side is updated. The left-hand side of the assignment must be a wire.

6. Procedural Assignments

6.1. initial

You will use registers primarily in testing your code. Unlike wires, regs will hold a value until it is re-assigned. In other words, it maintains state. Delays, indicated by a number following a pound sign, can be used to specify an amount of time before the value held by the reg changes. Here is an example.

reg [3:0] A;

    A = 4'hf;
    #10        //delay 10 time units
    A = 0;
    #10        //delay 10 time units
    A = 4'h5;

    end  //end initial description

This example makes procedural assignments to register A. Any code in a begin-end block following the initial keyword executes only once, starting at the beginning (time 0) of a simulation. The first assignment to register A is made at time 0. The #10 indicates a delay of 10 time units before the next assignments are made. A is set to 0 at time 10, and A is set to 5 at time 20.

An initial description may be used without a block delimited by begin and end if there is only one statement (see example in next section).

Note that your code can have multiple procedural assignments. If you have more than one initial block in your code, they will be executed concurrently.

6.2. always

The always descriptions will execute repeatedly throughout a simulation. To initialize and cycle a clock at 10 time units, you could do the following:

reg clk;

initial clk = 1'b0;

always #5 clk = ~clk;  //clock cycle is 10 ns

6.3. repeat

The repeat statement can be used to specify how many times a begin-end block is executed. The block following the repeat statement is executed the number of times indicated by the expression in parenthesis following the repeat keyword. In the following example, the block is executed four times. Note that I added a time delay after the last statement in the block to avoid a race condition.

reg [3:0] A;

        A = 4'hf;
        #10        //delay 10 time units
        A = 0;
        #10        //delay 10 time units
        A = 4'h5;
        #10        //delay 10 time units
    end  //end repeat(4)

end //initial description

7. Compiler Directives

You can use an accent grave (`) followed by a keyword to control compilation and simulation. This compiler directive is not followed by a semicolon.

7.1. `define

The define keyword can be used for making text macros. During compilation, when the macro name is preceded by an accent grave, the compiler substitutes the macro text. Here are a couple of examples:

reg clk;
reg [1:0] a;

`define half_cycle 5
`define set_a #10 a = 2'b

always #(`half_cycle) clk = ~clk;


    clk = 1'b0;
    `set_a 00;
    `set_a 01;
    `set_a 10;
    `set_a 11;

    end    //initial begin

In the example above, the value of register a is changed every clock cycle.

7.2. `include

This is used to include more source files, for example:

`include "file.v"

8. System Tasks and Functions

System tasks are commands built into the simulator. The system task calls end with a semicolon. You may find some of these useful:

8.1. $finish

Use $finish to end the simulation after a specified amount of time. For example,

initial #1000 $finish;

stops the simulation after 1000 time units. Unless you are stepping through the simulation interactively using VirSim, you will need to use $finish.

8.2. $time

Returns the current simulation time.

8.3. $strobe

Use $strobe to display a message on your screen. The format is similar to that of printf in C. Note that if the $strobe command is called at the same simulation time that other registers or wires are changing values, the value changes will happen before the strobe command is executed. Example:

always @(posedge clk)
    $strobe ("at time %0d, a = %b", $time, a);

This statement is executed every time clk transitions from 0 to 1, printing the simulation time in decimal and the value of a in binary. If a changed right at the clock edge, the new value would be printed.

8.4. $readmemb and $readmemh

The $readmemb and $readmemh tasks are used for reading in data from a file of binary or hex numbers to initialize a memory module. You can use it like this:


The starting_address parameter is the address of the memory module where you begin initialization. If starting_address is greater than zero, the memory locations at the beginning will remain unitialized (logic value x). The data file is just a text file that contains a list of binary or hex numbers. An example of a data file will be given later in the semester.

8.5. Dumping Waveforms

You can dump out a a waveform file in text format using VCS the following tasks:

$dumpfile is used to specify the name of the waveform file:

$dumpfile ("file.dump");

$dumpvars specifies the wires and registers whose values you want to record. The syntax is:

$dumpvars (number_of_levels, instance_name);

If number_of_levels is 1, it will dump all the signals instantiated in the top level of the instance specified. If number_of_levels is greater than 1, it will dump all signals instantiated number_of_levels levels hierarchically below the specified instance. If number_of_levels is 0, it will dump all signals instantiated under instance_name at any level. In the examples below, suppose you have an instance called reg_file in an instance called data_path in an instance called exec_core in your TOP module.

$dumpvars (0, TOP.exec_core.data_path.reg_file);
$dumpvars (2, TOP.exec_core);

The first statement will dump all wires and registers instantiated at any level under the module called reg_file. The second statement will dump all wires and registers instantiated in the top level of exec_core, and the top level of data_path, or any other modules instantiated in exec_core.

9. Timing Information

You will need to examine the library files to figure out the timing delays of the library modules. Below is an example of a module from the library. The delays between the rise or fall of any input and the change of the output are specified. Two sets of three numbers are specified for each input. The first set are the minimum, typical, and maximum delays when the output transitions from 0 to 1. The second set are the delays when the output transitions from 1 to 0. For this module, the delays are the same regardless of which way the inputs are changing.

In this class we will always use the typical delays, so you need only use the middle number (0.2 in this case for both rise and fall times) when making your critical path and cycle time calculations.

module  nand2$(out, in0, in1);
        input   in0, in1;
        output  out;

        nand (out, in0, in1);

            (in0 *> out) = (0.18:0.2:0.22, 0.18:0.2:0.22);
            (in1 *> out) = (0.18:0.2:0.22, 0.18:0.2:0.22);

10. A Complete TOP Module

Here is a small example of a complete program illustrating use of the D latch. You can read the VCS Manual to learn how to simulate it. When simulated, it will dump a waveform file of all signals instantiated under TOP. It will also print any transitions of the d register to standard output.

11. Behavioral Verilog

To clarify the difference between structural and behavioral verilog: structural verilog is composed of module instances and their interconnections (by wires) only. The use of regs, explicit time delays, arithmetic expressions, procedural assignments, or other verilog control flow structures are considered behavioral verilog.

As stated earlier, your project code will consist primarily of structural verilog. You may use behavioral statements for debugging purposes only. In fact, you will probably only instantiate two regs in your whole design: one for the clock and one for a reset signal that is asserted at the beginning of your simulation.

This document has described all of the Verilog functionality you will need for your final project. If you want more information on behavioral Verilog, try reading the Bucknell CSCI Verilog Manual or the Verilog manual at The University of Edinburgh.