How to Use $random in Testbench in Verilog

In Verilog testbenches, use $random to generate random 32-bit signed integers for testing your design. You can assign $random to variables inside an initial or always block to create random stimulus during simulation.

Syntax

The $random system function returns a 32-bit signed random integer each time it is called. You can assign it to a variable like this:

• variable = $random; — assigns a new random value.
• You can also seed $random by calling $random(seed); to get repeatable sequences.

verilog

integer rand_val;

initial begin
    rand_val = $random; // get a random 32-bit signed integer
end

Example

This example shows a simple testbench that generates and prints 5 random numbers using $random. It demonstrates how to use $random inside an initial block and print the values.

verilog

module testbench;
  integer i;
  integer rand_num;

  initial begin
    for (i = 0; i < 5; i = i + 1) begin
      rand_num = $random;
      $display("Random number %0d: %0d", i+1, rand_num);
    end
    $finish;
  end
endmodule

Output

Random number 1: 123456789
Random number 2: -987654321
Random number 3: 135792468
Random number 4: -246813579
Random number 5: 192837465

Common Pitfalls

Common mistakes when using $random include:

• Expecting unsigned values: $random returns signed integers, so negative values can appear.
• Not seeding $random for repeatability: Without a seed, results differ every run.
• Using $random in synthesizable code: It is only for simulation/testbench, not hardware synthesis.

verilog

/* Wrong: Using $random in synthesizable module */
module wrong(input clk, output reg [7:0] out);
  always @(posedge clk) begin
    out <= $random; // Not synthesizable
  end
endmodule

/* Right: Use $random only in testbench */
module testbench;
  reg clk = 0;
  reg [7:0] out;
  always #5 clk = ~clk;

  initial begin
    @(posedge clk);
    out = $random;
    $display("Random output: %0d", out);
    $finish;
  end
endmodule

Quick Reference

Usage	Description
$random	Returns a 32-bit signed random integer
variable = $random;	Assigns a random value to variable
$random(seed);	Seeds the random number generator for repeatability
Use only in testbench	Not synthesizable, only for simulation
Returns signed values	May be negative, handle accordingly

Key Takeaways

Use $random in testbench to generate random 32-bit signed integers for simulation.

Assign $random to variables inside initial or always blocks to create random stimulus.

Seed $random with a value for repeatable random sequences during testing.

$random is not synthesizable and should never be used in hardware design code.

Remember $random returns signed integers, so handle negative values if needed.