module synchronizer #(
   parameter WIDTH            = 1,
   parameter STAGES           = 2,
   parameter INITIAL_VAL      = 0,
   parameter FALSE_PATH_TO_IN = 1
)(
   input              clk,
   input              rst,
   input  [WIDTH-1:0] in,
   output [WIDTH-1:0] out
);

   //Q: Why do we have a separate impl and instantiate
   //it with a different instance name based on this
   //arbitrary parameter FALSE_PATH_TO_IN?
   //A: To make constraining these synchronizers easier.
   //We would like to write a single false path constraint
   //for all synchronizers when the input is truly async.
   //However other cases might require constraining the input
   //of this module.
   //To enable this, all clients that hook up async signals to
   //the "in" port can set FALSE_PATH_TO_IN=1 (or use the default)
   //and all clients that want the "in" delay to be constrained can
   //set FALSE_PATH_TO_IN=0.
   //In the XDC we can write the following async constraint:
   //set_false_path -to [get_pins */synchronizer_false_path/stages[0].value_reg[0]/D]
   //and this will take care of all instances of this module with FALSE_PATH_TO_IN==1

   generate if (FALSE_PATH_TO_IN == 1) begin
      synchronizer_impl #(
        .WIDTH(WIDTH), .STAGES(STAGES), .INITIAL_VAL(INITIAL_VAL)
      ) synchronizer_false_path (
         .clk(clk), .rst(rst), .in(in), .out(out)
      );
   end else begin
      synchronizer_impl #(
        .WIDTH(WIDTH), .STAGES(STAGES), .INITIAL_VAL(INITIAL_VAL)
      ) synchronizer_constrained (
         .clk(clk), .rst(rst), .in(in), .out(out)
      );
   end endgenerate

endmodule   //synchronizer