// ***************************************************************************
// ***************************************************************************
// Copyright 2013(c) Analog Devices, Inc.
//  Author: Lars-Peter Clausen <lars@metafoo.de>
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/*
 * Helper module for synchronizing a counter from one clock domain to another
 * using gray code. To work correctly the counter must not change its value by
 * more than one in one clock cycle in the source domain. I.e. the value may
 * change by either -1, 0 or +1.
 */
module sync_gray (
  input in_clk,
  input in_resetn,
  input [DATA_WIDTH-1:0] in_count,
  input out_resetn,
  input out_clk,
  output [DATA_WIDTH-1:0] out_count
);

// Bit-width of the counter
parameter DATA_WIDTH = 1;
// Whether the input and output clock are asynchronous, if set to 0 the
// synchronizer will be bypassed and out_count will be in_count.
parameter ASYNC_CLK = 1;

reg [DATA_WIDTH-1:0] cdc_sync_stage0 = 'h0;
reg [DATA_WIDTH-1:0] cdc_sync_stage1 = 'h0;
reg [DATA_WIDTH-1:0] cdc_sync_stage2 = 'h0;
reg [DATA_WIDTH-1:0] out_count_m = 'h0;

function [DATA_WIDTH-1:0] g2b;
  input [DATA_WIDTH-1:0] g;
  reg   [DATA_WIDTH-1:0] b;
  integer i;
  begin
    b[DATA_WIDTH-1] = g[DATA_WIDTH-1];
    for (i = DATA_WIDTH - 2; i >= 0; i =  i - 1)
      b[i] = b[i + 1] ^ g[i];
    g2b = b;
  end
endfunction

function [DATA_WIDTH-1:0] b2g;
  input [DATA_WIDTH-1:0] b;
  reg [DATA_WIDTH-1:0] g;
  integer i;
  begin
    g[DATA_WIDTH-1] = b[DATA_WIDTH-1];
    for (i = DATA_WIDTH - 2; i >= 0; i = i -1)
        g[i] = b[i + 1] ^ b[i];
    b2g = g;
  end
endfunction

always @(posedge in_clk) begin
  if (in_resetn == 1'b0) begin
    cdc_sync_stage0 <= 'h00;
  end else begin
    cdc_sync_stage0 <= b2g(in_count);
  end
end

always @(posedge out_clk) begin
  if (out_resetn == 1'b0) begin
    cdc_sync_stage1 <= 'h00;
    cdc_sync_stage2 <= 'h00;
    out_count_m <= 'h00;
  end else begin
    cdc_sync_stage1 <= cdc_sync_stage0;
    cdc_sync_stage2 <= cdc_sync_stage1;
    out_count_m <= g2b(cdc_sync_stage2);
  end
end

assign out_count = ASYNC_CLK ? out_count_m : in_count;

endmodule