在ISE下仿真老是通不过！！为什么！！

这是一个二进制的8X8的乘法器，请哪位帮忙看看！
module mul_ser (clk, x, a, y);

input   clk;
input[7:0]x, a;
output [15:0] y;
reg  [15:0] y;


always @(posedge clk)
begin : States
  parameter s0=0, s1=1, s2=2;
  reg [2:0] count;
  reg [1:0] state;
  reg[15:0] p, t;
  reg[7:0] a_reg;
  case (state)
  s0 : begin
  a_reg <= a;
  state <= s1;
  count = 0;
  p <= 0;   
  t <= {{8{x[7]}},x};
  end              
  s1 : begin
  if (count == 7)
    state <= s2;
  else   
    begin
    if (a_reg[0] == 1)
    p <= p + t;
    a_reg <= a_reg >> 1;
    t <= t << 1;
    count = count + 1;
    state <= s1;
  end
  end
  s2 : begin   
  y <= p;  
  state <= s0;
  end
  endcase
end

endmodule