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-- Copyright (c)2020 Jeremy Seth Henry

-- All rights reserved.

--

-- Redistribution and use in source and binary forms, with or without

-- modification, are permitted provided that the following conditions are met:

--     * Redistributions of source code must retain the above copyright

--       notice, this list of conditions and the following disclaimer.

--     * Redistributions in binary form must reproduce the above copyright

--       notice, this list of conditions and the following disclaimer in the

--       documentation and/or other materials provided with the distribution,

--       where applicable (as part of a user interface, debugging port, etc.)

--

-- THIS SOFTWARE IS PROVIDED BY JEREMY SETH HENRY ``AS IS'' AND ANY

-- EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED

-- WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE

-- DISCLAIMED. IN NO EVENT SHALL JEREMY SETH HENRY BE LIABLE FOR ANY

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-- (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;

-- LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND

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--

-- VHDL Units :  crc16_ccitt

-- Description:  Implements the 16-bit CCITT CRC on byte-wide data. Logic

--  equations were taken from Intel/Altera app note AN049.

--

-- Revision History

-- Author          Date     Change

------------------ -------- ---------------------------------------------------

-- Seth Henry      12/09/20 Created from original version

library ieee;

use ieee.std_logic_1164.all;

entity sdlc_crc16_ccitt is

generic(

  Poly_Init                  : std_logic_vector(15 downto 0) := x"0000";

  Reset_Level                : std_logic := '1'

);

port(

  Clock                      : in  std_logic;

  Reset                      : in  std_logic;

  --

  Clear                      : in  std_logic;

  Wr_En                      : in  std_logic;

  Wr_Data                    : in  std_logic_vector(7 downto 0);

  --

  CRC16_Valid                : out std_logic;

  CRC16_Out                  : out std_logic_vector(15 downto 0)

);

end entity;

architecture behave of sdlc_crc16_ccitt is

  signal Calc_En             : std_logic    := '0';

  signal Buffer_En           : std_logic    := '0';

  signal Data                : std_logic_vector(7 downto 0)  := x"00";

  signal Exr                 : std_logic_vector(7 downto 0)  := x"00";

  signal Reg                 : std_logic_vector(15 downto 0) := x"0000";

begin

  Exr(0)                     <= Reg(0) xor Data(0);

  Exr(1)                     <= Reg(1) xor Data(1);

  Exr(2)                     <= Reg(2) xor Data(2);

  Exr(3)                     <= Reg(3) xor Data(3);

  Exr(4)                     <= Reg(4) xor Data(4);

  Exr(5)                     <= Reg(5) xor Data(5);

  Exr(6)                     <= Reg(6) xor Data(6);

  Exr(7)                     <= Reg(7) xor Data(7);

  CRC16_Calc: process( Clock, Reset )

  begin

    if( Reset = Reset_Level )then

      Calc_En                <= '0';

      Buffer_En              <= '0';

      Data                   <= x"00";

      Reg                    <= Poly_Init;

      CRC16_Out              <= x"0000";

      CRC16_Valid            <= '0';

    elsif( rising_edge(Clock) )then

      Calc_En                <= Wr_En;

      if( Wr_En  = '1' )then

        Data                 <= Wr_Data;

      end if;

      if( Calc_En = '1' )then

        Reg(0)               <= Reg(8)  xor            Exr(4) xor Exr(0);

        Reg(1)               <= Reg(9)  xor            Exr(5) xor Exr(1);

        Reg(2)               <= Reg(10) xor            Exr(6) xor Exr(2);

        Reg(3)               <= Reg(11) xor Exr(0) xor Exr(7) xor Exr(3);

        Reg(4)               <= Reg(12) xor Exr(1)                      ;

        Reg(5)               <= Reg(13) xor Exr(2)                      ;

        Reg(6)               <= Reg(14) xor Exr(3)                      ;

        Reg(7)               <= Reg(15) xor Exr(4)            xor Exr(0);

        Reg(8)               <= Exr(0)  xor Exr(5)            xor Exr(1);

        Reg(9)               <= Exr(1)  xor Exr(6)            xor Exr(2);

        Reg(10)              <= Exr(2)  xor Exr(7)            xor Exr(3);

        Reg(11)              <= Exr(3)                                  ;

        Reg(12)              <= Exr(4)                        xor Exr(0);

        Reg(13)              <= Exr(5)                        xor Exr(1);

        Reg(14)              <= Exr(6)                        xor Exr(2);

        Reg(15)              <= Exr(7)                        xor Exr(3);

      end if;

      if( Clear = '1' )then

        Reg                  <= Poly_Init;

      end if;

      Buffer_En              <= Calc_En;

      if( Buffer_En = '1' )then

        CRC16_Out            <= Reg xor x"FFFF";

      end if;

      CRC16_Valid            <= Buffer_En;

    end if;

  end process;

end architecture;