//
// synchro.v
//
// Asynchronous input synchronizer
//
//
// These attributes tell the compiler and fitter respectively
// to treat these registers as low-level constructs and turn
// them into a synchronizer chain. No inferring RAMs or anything like that,
// and pack them close together.
//
module synchronizer
  #(parameter width = 1,
    parameter stages = 2,
    parameter noioregs = 0)
   (
    input rst_n,
    input clk,

    input [width-1:0] d,
    output [width-1:0] q
    );

   // SYNCHRONIZER_IDENTIFICATION FORCED identifies the *beginning* of
   // the synchro; it needs to be used with AUTO for the other stages or
   // the chains will be broken up for each stage.
   //
   // Because of different attributes, this is not simply qreg[0].
`ifdef ALTERA_RESERVED_QIS
   (*
    syn_preserve = 1,
    dont_replicate = 1,
    useioff = noioregs,
    altera_attribute =
	{"-name SYNCHRONIZER_IDENTIFICATION FORCED ; ",
	 "-name SYNCHRONIZATION_REGISTER_CHAIN_LENGTH ", tostr(stages-1)}
    *)
`endif
   reg [width-1:0] qreg0;

`ifdef ALTERA_RESERVED_QIS
   (*
    syn_preserve = 1,
    dont_replicate = 1,
    useioff = 1,
    altera_attribute =
	{"-name SYNCHRONIZER_IDENTIFICATION AUTO ; ",
	 "-name SYNCHRONIZATION_REGISTER_CHAIN_LENGTH ", tostr(stages-1)}
    *)
`endif
   reg [width-1:0] qreg[stages-1:1];

   always @(posedge clk or negedge rst_n)
     if (~rst_n)
       qreg0 <= {width{1'b0}};
     else
       qreg0 <= d;

   always @(posedge clk or negedge rst_n)
     if (~rst_n)
       qreg[1] <= {width{1'b0}};
     else
       qreg[1] <= qreg0;

   generate
      genvar i;

      for (i = 2; i < stages; i = i + 1)
	begin : stage
	   always @(posedge clk or negedge rst_n)
	     if (~rst_n)
	       qreg[i] <= {width{1'b0}};
	     else
	       qreg[i] <= qreg[i-1];
	end
   endgenerate

   assign q = qreg[stages-1];
endmodule // synchronizer