0000
    0001
    0010
    ...
    1110
    1111
           rolls over
    0000
    0001
    ...

module counter (input clk,      // Declare input port for the clock to allow counter to count up
                  input rstn,              // Declare input port for the reset to allow the counter to be reset to 0 when required
                  output reg[3:0] out);    // Declare 4-bit output port to get the counter values

 // This always block will be triggered at the rising edge of clk (0->1)
  // Once inside this block, it checks if the reset is 0, then change out to zero 
  // If reset is 1, then the design should be allowed to count up, so increment the counter 

  always @ (posedge clk) begin
    if (! rstn)
      out <= 0;
    else
      out <= out + 1;
  end
endmodule

module tb_counter;
  reg clk;                     // Declare an internal TB variable called clk to drive clock to the design
  reg rstn;                    // Declare an internal TB variable called rstn to drive active low reset to design
  wire [3:0] out;              // Declare a wire to connect to design output

  // Instantiate counter design and connect with Testbench variables
  counter   c0 ( .clk (clk),
                 .rstn (rstn),
                 .out (out));

  // Generate a clock that should be driven to design
  // This clock will flip its value every 5ns -> time period = 10ns -> freq = 100 MHz
 
 always #5 clk = ~clk;

  // This initial block forms the stimulus of the testbench
  initial begin
    // Initialize testbench variables to 0 at start of simulation
    clk <= 0;
    rstn <= 0;

    // Drive rest of the stimulus, reset is asserted in between
    #20   rstn <= 1;
    #80   rstn <= 0;
    #50   rstn <= 1;

    // Finish the stimulus after 200ns
    #20 $finish;
  end
endmodule