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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

-- DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES

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

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

-- ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT

-- (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS

-- SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

--

-- VHDL Units : o8_crc16_ccitt

-- Description: Implements the 16-bit CCITT CRC on byte-wide data suitable for

--            :  use with the Open8 CPU. Logic equations were taken from

--            :  Intel/Altera app note AN049.

--

-- Notes      :  Writing to the byte counter will reset all registers, and to

--            :   should be used to clear the CRC accumulator/byte counter

--            :   between frames.

--

-- Register Map:

-- Offset  Bitfield Description                        Read/Write

--   0x0   AAAAAAAA Data Input register (calc on write)(R/W)

--   0x1   AAAAAAAA Byte Counter (clear all on write)  (R/W)

--   0x2   AAAAAAAA B0 of calculated CRC               (RO)

--   0x3   AAAAAAAA B1 of calculated CRC               (RO)

--

-- Revision History

-- Author          Date     Change

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

-- Seth Henry      12/19/19 Design Start

library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_unsigned.all;

library work;

  use work.open8_pkg.all;

entity o8_crc16_ccitt is

generic(

  Reset_Level                : std_logic := '1';

  Address                    : ADDRESS_TYPE

);

port(

  Clock                      : in  std_logic;

  Reset                      : in  std_logic;

  --

  Bus_Address                : in  ADDRESS_TYPE;

  Wr_Enable                  : in  std_logic;

  Wr_Data                    : in  DATA_TYPE;

  Rd_Enable                  : in  std_logic;

  Rd_Data                    : out DATA_TYPE

);

end entity;

architecture behave of o8_crc16_ccitt is

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

  constant User_Addr         : std_logic_vector(15 downto 2)

                               := Address(15 downto 2);

  alias  Comp_Addr           is Bus_Address(15 downto 2);

  alias  Reg_Addr            is Bus_Address(1 downto 0);

  signal Reg_Sel             : std_logic_vector(1 downto 0);

  signal Addr_Match          : std_logic;

  signal Wr_En               : std_logic;

  signal Wr_Data_q           : DATA_TYPE;

  signal Rd_En               : std_logic;

  signal Next_Byte           : DATA_TYPE;

  signal Byte_Count          : DATA_TYPE;

  signal Calc_En             : std_logic;

  signal Buffer_En           : std_logic;

  signal Data                : DATA_TYPE;

  signal Exr                 : DATA_TYPE;

  signal Reg                 : std_logic_vector(15 downto 0);

  signal Comp_Data           : std_logic_vector(15 downto 0);

begin

  Addr_Match                 <= '1' when Comp_Addr = User_Addr else '0';

  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

      Reg_Sel                <= "00";

      Wr_En                  <= '0';

      Wr_Data_q              <= x"00";

      Rd_En                  <= '0';

      Rd_Data                <= OPEN8_NULLBUS;

      Byte_Count             <= x"00";

      Calc_En                <= '0';

      Buffer_En              <= '0';

      Data                   <= x"00";

      Reg                    <= x"0000";

    elsif( rising_edge(Clock) )then

      Reg_Sel                <= Reg_Addr;

      Wr_En                  <= Addr_Match and Wr_Enable;

      Wr_Data_q              <= Wr_Data;

      if( Wr_En = '1' )then

        case( Reg_Sel )is

          when "00" => -- Load next byte

            Data             <= Wr_Data_q;

            Calc_En          <= '1';

          when "01" => -- Clear accumulator and byte counter

            Byte_Count       <= x"00";

            Reg              <= Poly_Init;

          when others => null;

        end case;

      end if;

      Rd_En                  <= Addr_Match and Rd_Enable;

      Rd_Data                <= OPEN8_NULLBUS;

      if( Rd_En = '1' )then

        case( Reg_Sel )is

          when "00" => -- Read last byte

            Rd_Data          <= Data;

          when "01" => -- Read the byte counter

            Rd_Data          <= Byte_Count;

          when "10" => -- Read the lower byte of the calculated CRC

            Rd_Data          <= Comp_Data(7 downto 0);

          when "11" => -- Read the upper byte of the calculated CRC

            Rd_Data          <= Comp_Data(15 downto 8);

          when others => null;

        end case;

      end if;

      Calc_En                <= '0';

      Buffer_En              <= Calc_En;

      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( Buffer_En = '1' )then

        Byte_Count           <= Byte_Count + 1;

        Comp_Data            <= Reg xor x"FFFF";

      end if;

    end if;

  end process;

end architecture;