module ring_ctr  #(parameter WIDTH=4) 
  (  
    input clk,                
    input rstn,
      output reg [WIDTH-1:0] out
  );    
 
  always @ (posedge clk) begin
      if (!rstn)
         out <= 1;
      else begin
        out[WIDTH-1] <= out[0];
        for (int i = 0; i < WIDTH-1; i=i+1) begin
          out[i] <= out[i+1];
        end
      end
  end
endmodule

module tb;
  parameter WIDTH = 4;
  
  reg clk;
  reg rstn;
  wire [WIDTH-1:0] out;
  
  ring_ctr     u0 (.clk (clk),
                .rstn (rstn),
                .out (out));
  
  always #10 clk = ~clk;
  
  initial begin
    {clk, rstn} <= 0;

    $monitor ("T=%0t out=%b", $time, out);
    repeat (2) @(posedge clk);
    rstn <= 1;
    repeat (15) @(posedge clk);
    $finish;
  end
endmodule