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Verilog Cheatsheet

A comprehensive cheat sheet for Verilog, covering syntax, data types, operators, and common constructs. Useful for digital design and hardware verification.

Syntax and Basic Elements

Modules

Modules are the fundamental building blocks in Verilog.

module module_name (port_list);
  input  input_port;
  output output_port;
  // Internal signals
  wire   internal_signal;
  // Logic implementation
  assign output_port = input_port & internal_signal;
endmodule

Instantiation:

module_name instance_name (
  .input_port  (signal_a),
  .output_port (signal_b)
);

Data Types

wire

Represents a physical wire, cannot store values.

reg

Represents a variable that stores a value. Used in always blocks.

integer

General-purpose variable, typically 32 bits.

real

Floating-point number.

time

Unsigned 64-bit integer for simulation time.

logic

Four-state data type: 0, 1, X (unknown), Z (high impedance). SystemVerilog only.

Number Representation

<size>'<base><value>

  • <size>: Size in bits.
  • <base>: b (binary), o (octal), d (decimal), h (hexadecimal).
  • <value>: The number.

Examples:

  • 4'b1010 - 4-bit binary number 1010.
  • 8'hFF - 8-bit hexadecimal number FF (255 in decimal).
  • 16'd255 - 16-bit decimal number 255.

Operators

Arithmetic Operators

+

Addition

-

Subtraction

*

Multiplication

\

Division

%

Modulo

**

Exponentiation (SystemVerilog)

Logical Operators

&&

Logical AND

||

Logical OR

!

Logical NOT

Bitwise Operators

&

Bitwise AND

|

Bitwise OR

^

Bitwise XOR

~

Bitwise NOT

~&

Bitwise NAND

~|

Bitwise NOR

~^ or ^~

Bitwise XNOR

Reduction Operators

&

Reduction AND

|

Reduction OR

^

Reduction XOR

~&

Reduction NAND

~|

Reduction NOR

~^ or ^~

Reduction XNOR

Control Statements

Always Blocks

Describes sequential logic.

always @(posedge clock or negedge reset) begin
  if (!reset) begin
    // Reset condition
    q <= 0;
  end else begin
    // Sequential logic
    q <= d;
  end
end
  • always @(*): Combinational logic (sensitivity list inferred).
  • always @(posedge clk): Positive edge-triggered sequential logic.
  • always @(negedge clk): Negative edge-triggered sequential logic.

Conditional Statements

if

Basic conditional statement.

if (condition) begin
  // Code to execute if condition is true
end

else

Alternative code block.

if (condition) begin
  // Code if true
end else begin
  // Code if false
end

else if

Multiple conditions.

if (condition1) begin
  // Code if condition1 is true
end else if (condition2) begin
  // Code if condition2 is true
end else begin
  // Default code
end

case

Multi-way branching.

case (expression)
  value1: begin
    // Code for value1
  end
  value2: begin
    // Code for value2
  end
  default: begin
    // Default code
  end
endcase

Loop Statements

for

For loop.

integer i;
always @(posedge clk) begin
  for (i = 0; i < 10; i = i + 1) begin
    // Code to repeat
  end
end

while

While loop (less common in synthesizable code).

integer i;
always @(posedge clk) begin
  i = 0;
  while (i < 10) begin
    // Code to repeat
    i = i + 1;
  end
end

Advanced Topics

Functions and Tasks

Function: Returns a single value. Cannot contain timing controls or event triggers.

function integer my_function (input integer a, b);
  my_function = a + b;
endfunction

Task: Can have multiple outputs and can contain timing controls.

task my_task (input integer a, output integer result);
  #10; // Delay for 10 time units
  result = a * 2;
endtask

Generate Statements

Used for conditional or repetitive instantiation of modules or code blocks at compile time.

generate
  if (parameter_value) begin : label
    // Conditional instantiation
    module_instance instance1 (
      .portA(signalA)
    );
  end else begin : label2
    // Alternative instantiation
    module_instance instance2 (
      .portB(signalB)
    );
  end
endgenerate

generate
  for (i = 0; i < N; i = i + 1) begin : loop_label
    // Repetitive instantiation
    module_instance array_of_instances[i] (
      .portA(signalA[i])
    );
  end
endgenerate

SystemVerilog Constructs (Highlights)

logic Data Type

Four-state data type (0, 1, X, Z), preferred over reg and wire for general use.

Interfaces

Bundles signals for easier module connections and verification.

Assertions

Used to check design properties dynamically during simulation.

Clocking Blocks

Synchronize signals to a specific clock edge for improved timing control.
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