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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_async_serial

-- Description:  Provides a single 8-bit, asynchronous transceiver. While the

--               width is fixed at 8-bits, the bit rate and parity controls

--               are settable via generics.

--

-- Register Map:

-- Offset  Bitfield Description                        Read/Write

--   0x00  AAAAAAAA TX Data (WR) RX Data (RD)             (RW)

--   0x01  DCBA---- FIFO Status                           (RO)

--                  A: RX FIFO Empty

--                  B: RX FIFO almost full (922/1024)

--                  C: TX FIFO Empty

--                  D: TX FIFO almost full (922/1024)

--

-- Note: The baud rate generator will produce an approximate frequency. The

--        final bit rate should be within +/- 1% of the true bit rate to

--        ensure the receiver can successfully receive. With a sufficiently

--        high core clock, this is generally achievable for common PC serial

--        data rates.

--

-- Revision History

-- Author          Date     Change

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

-- Seth Henry      12/20/19 Design Start

-- Seth Henry      04/10/20 Code cleanup and register documentation

library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_unsigned.all;

use ieee.std_logic_arith.all;

use ieee.std_logic_misc.all;

library work;

  use work.open8_pkg.all;

entity o8_async_serial is

generic(

  Disable_Transmit           : boolean := FALSE;

  Disable_Receive            : boolean := FALSE;

  Bit_Rate                   : real;

  Enable_Parity              : boolean;

  Parity_Odd_Even_n          : std_logic;

  Sys_Freq                   : real;

  Reset_Level                : std_logic;

  Address                    : ADDRESS_TYPE

);

port(

  Clock                      : in  std_logic;

  Reset                      : in  std_logic;

  --

  Open8_Bus                  : in  OPEN8_BUS_TYPE;

  Rd_Data                    : out DATA_TYPE;

  --

  TX_Out                     : out std_logic;

  CTS_In                     : in  std_logic;

  RX_In                      : in  std_logic;

  RTS_Out                    : out std_logic

);

end entity;

architecture behave of o8_async_serial is

  signal FIFO_Reset          : std_logic := '0';

  constant User_Addr         : std_logic_vector(15 downto 1) :=

                                Address(15 downto 1);

  alias  Comp_Addr           is Open8_Bus.Address(15 downto 1);

  signal Addr_Match          : std_logic := '0';

  alias  Reg_Addr            is Open8_Bus.Address(0);

  signal Reg_Sel             : std_logic := '0';

  signal Rd_En               : std_logic := '0';

  signal TX_FIFO_Wr_En       : std_logic := '0';

  alias  TX_FIFO_Wr_Data     is Open8_Bus.Wr_Data;

  signal TX_FIFO_Rd_En       : std_logic := '0';

  signal TX_FIFO_Empty       : std_logic := '0';

  signal TX_FIFO_AFull       : std_logic := '0';

  signal TX_FIFO_Rd_Data     : DATA_TYPE := x"00";

  alias  Tx_Data             is TX_FIFO_Rd_Data;

  type TX_CTRL_STATES is (IDLE, TX_BYTE, TX_START, TX_WAIT );

  signal TX_Ctrl             : TX_CTRL_STATES := IDLE;

  signal TX_Xmit             : std_logic := '0';

  signal TX_Done             : std_logic := '0';

  constant BAUD_RATE_DIV     : integer := integer(Sys_Freq / Bit_Rate);

  signal CTS_sr              : std_logic_vector(3 downto 0) := "0000";

  alias  CTS_Okay            is CTS_sr(3);

  signal RX_FIFO_Wr_En       : std_logic := '0';

  signal RX_FIFO_Wr_Data     : DATA_TYPE := x"00";

  signal RX_FIFO_Rd_En       : std_logic := '0';

  signal RX_FIFO_Empty       : std_logic := '0';

  signal RX_FIFO_AFull       : std_logic := '0';

  signal RX_FIFO_Rd_Data     : DATA_TYPE := x"00";

begin

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

  io_reg: process( Clock, Reset )

  begin

    if( Reset = Reset_Level )then

      Rd_En                  <= '0';

      Rd_Data                <= OPEN8_NULLBUS;

      RTS_Out                <= '0';

    elsif( rising_edge( Clock ) )then

      Rd_Data                <= OPEN8_NULLBUS;

      Rd_En                  <= Addr_Match and Open8_Bus.Rd_En;

      Reg_Sel                <= Reg_Addr;

      if( Rd_En = '1' and Reg_Sel = '1' )then

        Rd_Data(4)           <= RX_FIFO_Empty;

        Rd_Data(5)           <= RX_FIFO_AFull;

        Rd_Data(6)           <= TX_FIFO_Empty;

        Rd_Data(7)           <= TX_FIFO_AFull;

      end if;

      if( Rd_En = '1' and Reg_Sel = '0' )then

        Rd_Data              <= RX_FIFO_Rd_Data;

      end if;

      RTS_Out                <= not RX_FIFO_AFull;

    end if;

  end process;

TX_Disabled : if( Disable_Transmit )generate

  TX_FIFO_Empty              <= '1';

  TX_FIFO_AFull              <= '0';

  TX_Out                     <= '1';

end generate;

TX_Enabled : if( not Disable_Transmit )generate

  TX_FIFO_Wr_En              <= Open8_Bus.Wr_En and

                                Addr_Match and

                                (not Reg_Addr);

  FIFO_Reset                 <= '1' when Reset = Reset_Level else '0';

  U_TX_FIFO : entity work.fifo_1k_core

  port map(

    aclr                     => FIFO_Reset,

    clock                    => Clock,

    data                     => TX_FIFO_Wr_Data,

    rdreq                    => TX_FIFO_Rd_En,

    wrreq                    => TX_FIFO_Wr_En,

    empty                    => TX_FIFO_Empty,

    almost_full              => TX_FIFO_AFull,

    q                        => TX_FIFO_Rd_Data

  );

  tx_FSM: process( Clock, Reset )

  begin

    if( Reset = Reset_Level )then

      TX_Ctrl                <= IDLE;

      TX_Xmit                <= '0';

      TX_FIFO_Rd_En          <= '0';

      CTS_sr                 <= (others => '0');

    elsif( rising_edge(Clock) )then

      TX_Xmit                <= '0';

      TX_FIFO_Rd_En          <= '0';

      CTS_sr                 <= CTS_sr(2 downto 0) & CTS_In;

      case( TX_Ctrl )is

        when IDLE =>

          if( TX_FIFO_Empty = '0' and CTS_Okay = '1' )then

            TX_FIFO_Rd_En    <= '1';

            TX_Ctrl          <= TX_BYTE;

          end if;

        when TX_BYTE =>

          TX_Xmit            <= '1';

          TX_Ctrl            <= TX_START;

        when TX_START =>

          if( Tx_Done = '0' )then

            TX_Ctrl          <= TX_WAIT;

          end if;

        when TX_WAIT =>

          if( Tx_Done = '1' )then

            TX_Ctrl          <= IDLE;

          end if;

        when others => null;

      end case;

    end if;

  end process;

  U_TX : entity work.async_ser_tx

  generic map(

    Reset_Level              => Reset_Level,

    Enable_Parity            => Enable_Parity,

    Parity_Odd_Even_n        => Parity_Odd_Even_n,

    Clock_Divider            => BAUD_RATE_DIV

  )

  port map(

    Clock                    => Clock,

    Reset                    => Reset,

    --

    Tx_Data                  => Tx_Data,

    Tx_Valid                 => TX_Xmit,

    --

    Tx_Out                   => TX_Out,

    Tx_Done                  => Tx_Done

  );

end generate;

RX_Disabled : if( Disable_Transmit )generate

  RX_FIFO_Empty              <= '1';

  RX_FIFO_AFull              <= '0';

  RX_FIFO_Rd_Data            <= x"00";

end generate;

RX_Enabled : if( not Disable_Receive )generate

  U_RX : entity work.async_ser_rx

  generic map(

    Reset_Level              => Reset_Level,

    Enable_Parity            => Enable_Parity,

    Parity_Odd_Even_n        => Parity_Odd_Even_n,

    Clock_Divider            => BAUD_RATE_DIV

  )

  port map(

    Clock                    => Clock,

    Reset                    => Reset,

    --

    Rx_In                    => RX_In,

    --

    Rx_Data                  => RX_FIFO_Wr_Data,

    Rx_Valid                 => RX_FIFO_Wr_En,

    Rx_PErr                  => open

  );

  RX_FIFO_Rd_En              <= Open8_Bus.Rd_En and

                                Addr_Match and

                                (not Reg_Addr);

  U_RX_FIFO : entity work.fifo_1k_core

  port map(

    aclr                     => FIFO_Reset,

    clock                    => Clock,

    data                     => RX_FIFO_Wr_Data,

    rdreq                    => RX_FIFO_Rd_En,

    wrreq                    => RX_FIFO_Wr_En,

    empty                    => RX_FIFO_Empty,

    almost_full              => RX_FIFO_AFull,

    q                        => RX_FIFO_Rd_Data

  );

end generate;

end architecture;