--------------------------------------------------------------------------------
-- File       : gig_ethernet_pcs_pma_16_1_sync_block.vhd
-- Author     : Xilinx Inc.
--------------------------------------------------------------------------------
-- Description: Used on signals crossing from one clock domain to
--              another, this is a flip-flop pair, with both flops
--              placed together with RLOCs into the same slice.  Thus
--              the routing delay between the two is minimum to safe-
--              guard against metastability issues.
--------------------------------------------------------------------------------
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--------------------------------------------------------------------------------


library ieee;
use ieee.std_logic_1164.all;

library unisim;
use unisim.vcomponents.all;

entity gig_ethernet_pcs_pma_16_1_sync_block is
  generic (
    INITIALISE : bit_vector(1 downto 0) := "00"
  );
  port (
    clk         : in  std_logic;          -- clock to be sync'ed to
    data_in     : in  std_logic;          -- Data to be 'synced'
    data_out    : out std_logic           -- synced data
    );

end gig_ethernet_pcs_pma_16_1_sync_block;


architecture structural of gig_ethernet_pcs_pma_16_1_sync_block is


  -- Internal Signals
  signal data_sync1 : std_logic;
  signal data_sync2 : std_logic;
  signal data_sync3 : std_logic;
  signal data_sync4 : std_logic;
  signal data_sync5 : std_logic;

  -- These attributes will stop timing errors being reported in back annotated
  -- SDF simulation.
  attribute ASYNC_REG                       : string;
  attribute ASYNC_REG of data_sync_reg1    : label is "true";
  attribute ASYNC_REG of data_sync_reg2    : label is "true";
  attribute ASYNC_REG of data_sync_reg3    : label is "true";
  attribute ASYNC_REG of data_sync_reg4    : label is "true";
  attribute ASYNC_REG of data_sync_reg5    : label is "true";
  attribute ASYNC_REG of data_sync_reg6    : label is "true";

  -- These attributes will stop XST translating the desired flip-flops into an
  -- SRL based shift register.
  attribute shreg_extract                   : string;
  attribute shreg_extract of data_sync_reg1 : label is "no";
  attribute shreg_extract of data_sync_reg2 : label is "no";
  attribute shreg_extract of data_sync_reg3 : label is "no";
  attribute shreg_extract of data_sync_reg4 : label is "no";
  attribute shreg_extract of data_sync_reg5 : label is "no";
  attribute shreg_extract of data_sync_reg6 : label is "no";

  
begin

  data_sync_reg1 : FD
  generic map (
    INIT => INITIALISE(0)
  )
  port map (
    C    => clk,
    D    => data_in,
    Q    => data_sync1
  );

 data_sync_reg2 : FD
  generic map (
    INIT => INITIALISE(0)
  )
  port map (
    C    => clk,
    D    => data_sync1,
    Q    => data_sync2
  );

 data_sync_reg3 : FD
  generic map (
    INIT => INITIALISE(0)
  )
  port map (
    C    => clk,
    D    => data_sync2,
    Q    => data_sync3
  );

 data_sync_reg4 : FD
  generic map (
    INIT => INITIALISE(0)
  )
  port map (
    C    => clk,
    D    => data_sync3,
    Q    => data_sync4
  );

 data_sync_reg5 : FD
  generic map (
    INIT => INITIALISE(0)
  )
  port map (
    C    => clk,
    D    => data_sync4,
    Q    => data_sync5
  );  

  data_sync_reg6 : FD
  generic map (
    INIT => INITIALISE(1)
  )
  port map (
    C    => clk,
    D    => data_sync5,
    Q    => data_out
  );


end structural;