Verilog cylon using Icestorm framework and targeting the tinyFPGA-B2 board. Properly uses a register and the shift operator.

cylon.v

// cylon effect - top.v
//
// pins are relative to the tinyFPGA-B2

module top (
   output pin1_usb_dp,// USB pull-up enable, set low to disable
   output pin2_usb_dn,
   input  pin3_clk_16mhz,   // 16 MHz on-board clock
   output pin13,
   output pin12,
   output pin11,
   output pin10,
   output pin9,
   output pin8,
   output pin7,
   output pin6,
   output pin5,
   output pin4
   );

   reg [22:0] clk_1hz_counter = 23'b0;  // Hz clock generation counter
   reg        clk_1hz = 1'b0;           // Hz clock

   reg[9:0] shift;

   reg direction = 0;  // Initial direction is from pin4 to pin13
   reg[3:0] counter;

   // 2 Hz clock
   always @(posedge pin3_clk_16mhz) begin
      if (clk_1hz_counter < 23'd7_999_999)
         clk_1hz_counter <= clk_1hz_counter + 23'd16;
      else begin
         clk_1hz_counter <= 23'b0;
         clk_1hz <= ~clk_1hz;
      end
   end

   initial begin
      shift[0] = 1;
      counter = 0;
   end

   always @(posedge clk_1hz) begin
      if (counter == 9) begin
         direction = ~direction;
         counter = 0;
      end

      if (direction == 0) begin
         shift <= shift << 1;
      end
      else begin
         shift <= shift >> 1;
      end

      counter <= #1 counter + 1;
   end

   assign pin13 = shift[9];
   assign pin12 = shift[8];
   assign pin11 = shift[7];
   assign pin10 = shift[6];
   assign pin9  = shift[5];
   assign pin8  = shift[4];
   assign pin7  = shift[3];
   assign pin6  = shift[2];
   assign pin5  = shift[1];
   assign pin4  = shift[0];

   assign pin1_usb_dp = 1'b0;
   assign pin2_usb_dn = 1'b0;

endmodule  // top

In action.