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--===========================================================================--

--                                                                           --

--                  Synthesizable 6850 compatible ACIA                       --

--                                                                           --

--===========================================================================--

--

--  File name      : acia6850.vhd

--

--  Entity name    : acia6850

--

--  Purpose        : Implements a RS232 6850 compatible 

--                   Asynchronous Communications Interface Adapter (ACIA)

--                  

--  Dependencies   : ieee.std_logic_1164

--                   ieee.numeric_std

--                   ieee.std_logic_unsigned

--

--  Author         : John E. Kent

--

--  Email          : dilbert57@opencores.org      

--

--  Web            : http://opencores.org/project,system09

--

--  Origins        : miniUART written by Ovidiu Lupas olupas@opencores.org

--

--  Registers      :

--

--  IO address + 0 Read - Status Register

--

--     Bit[7] - Interrupt Request Flag

--     Bit[6] - Receive Parity Error (parity bit does not match)

--     Bit[5] - Receive Overrun Error (new character received before last read)

--     Bit[4] - Receive Framing Error (bad stop bit)

--     Bit[3] - Clear To Send level

--     Bit[2] - Data Carrier Detect (lost modem carrier)

--     Bit[1] - Transmit Buffer Empty (ready to accept next transmit character)

--     Bit[0] - Receive Data Ready (character received)

-- 

--  IO address + 0 Write - Control Register

--

--     Bit[7]     - Rx Interupt Enable

--          0     - disabled

--          1     - enabled

--     Bits[6..5] - Transmit Control

--        0 0     - TX interrupt disabled, RTS asserted

--        0 1     - TX interrupt enabled,  RTS asserted

--        1 0     - TX interrupt disabled, RTS cleared

--        1 1     - TX interrupt disabled, RTS asserted, Send Break

--     Bits[4..2] - Word Control

--      0 0 0     - 7 data, even parity, 2 stop

--      0 0 1     - 7 data, odd  parity, 2 stop

--      0 1 0     - 7 data, even parity, 1 stop

--      0 1 1     - 7 data, odd  parity, 1 stop

--      1 0 0     - 8 data, no   parity, 2 stop

--      1 0 1     - 8 data, no   parity, 1 stop

--      1 1 0     - 8 data, even parity, 1 stop

--      1 1 1     - 8 data, odd  parity, 1 stop

--     Bits[1..0] - Baud Control

--        0 0     - Baud Clk divide by 1

--        0 1     - Baud Clk divide by 16

--        1 0     - Baud Clk divide by 64

--        1 1     - Reset

--

--  IO address + 1 Read - Receive Data Register

--

--     Read when Receive Data Ready bit set

--     Read resets Receive Data Ready bit 

--

--  IO address + 1 Write - Transmit Data Register

--

--     Write when Transmit Buffer Empty bit set

--     Write resets Transmit Buffer Empty Bit

--

--

--  Copyright (C) 2002 - 2010 John Kent

--

--  This program is free software: you can redistribute it and/or modify

--  it under the terms of the GNU General Public License as published by

--  the Free Software Foundation, either version 3 of the License, or

--  (at your option) any later version.

--

--  This program is distributed in the hope that it will be useful,

--  but WITHOUT ANY WARRANTY; without even the implied warranty of

--  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

--  GNU General Public License for more details.

--

--  You should have received a copy of the GNU General Public License

--  along with this program.  If not, see <http://www.gnu.org/licenses/>.

--

--===========================================================================--

--                                                                           --

--                              Revision  History                            --

--                                                                           --

--===========================================================================--

--

-- Version Author        Date         Changes

--

-- 0.1     Ovidiu Lupas  2000-01-15   New model

-- 1.0     Ovidiu Lupas  2000-01      Synthesis optimizations

-- 2.0     Ovidiu Lupas  2000-04      Bugs removed - the RSBusCtrl did not

--                                    process all possible situations

--

-- 3.0     John Kent     2002-10      Changed Status bits to match MC6805

--                                    Added CTS, RTS, Baud rate control & Software Reset

-- 3.1     John Kent     2003-01-05   Added Word Format control a'la mc6850

-- 3.2     John Kent     2003-07-19   Latched Data input to UART

-- 3.3     John Kent     2004-01-16   Integrated clkunit in rxunit & txunit

--                                    TX / RX Baud Clock now external

--                                    also supports x1 clock and DCD. 

-- 3.4     John Kent     2005-09-13   Removed LoadCS signal. 

--                                    Fixed ReadCS and Read 

--                                    in miniuart_DCD_Init process

-- 3.5     John Kent     2006-11-28   Cleaned up code.

--

-- 4.0     John Kent     2007-02-03   Renamed ACIA6850

-- 4.1     John Kent     2007-02-06   Made software reset synchronous

-- 4.2     John Kent     2007-02-25   Changed sensitivity lists

--                                    Rearranged Reset process.

-- 4.3     John Kent     2010-06-17   Updated header

-- 4.4     John Kent     2010-08-27   Combined with ACIA_RX & ACIA_TX

--                                    Renamed to acia6850

--

library ieee;

  use ieee.std_logic_1164.all;

  use ieee.numeric_std.all;

  use ieee.std_logic_unsigned.all;

--library unisim;

--  use unisim.vcomponents.all;

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

-- Entity for ACIA_6850                                              --

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

entity acia6850 is

  port (

    --

    -- CPU Interface signals

    --

    clk      : in  std_logic;                     -- System Clock

    rst      : in  std_logic;                     -- Reset input (active high)

    cs       : in  std_logic;                     -- miniUART Chip Select

    addr     : in  std_logic;                     -- Register Select

    rw       : in  std_logic;                     -- Read / Not Write

    data_in  : in  std_logic_vector(7 downto 0);  -- Data Bus In 

    data_out : out std_logic_vector(7 downto 0);  -- Data Bus Out

    irq      : out std_logic;                     -- Interrupt Request out

    --

    -- RS232 Interface Signals

    --

    RxC   : in  std_logic;              -- Receive Baud Clock

    TxC   : in  std_logic;              -- Transmit Baud Clock

    RxD   : in  std_logic;              -- Receive Data

    TxD   : out std_logic;              -- Transmit Data

    DCD_n : in  std_logic;              -- Data Carrier Detect

    CTS_n : in  std_logic;              -- Clear To Send

    RTS_n : out std_logic               -- Request To send

    );

end acia6850;  --================== End of entity ==============================--

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

-- Architecture for ACIA_6850 Interface registees

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

architecture rtl of acia6850 is

  type DCD_State_Type is (DCD_State_Idle, DCD_State_Int, DCD_State_Reset);

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

  -- Signals

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

  --

  -- Reset signals

  --

  signal ac_rst   : std_logic;          -- Reset (Software & Hardware)

  signal rx_rst   : std_logic;          -- Receive Reset (Software & Hardware)

  signal tx_rst   : std_logic;          -- Transmit Reset (Software & Hardware)

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

  --  Status Register: StatReg 

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

  --

  -- IO address + 0 Read

  --

  -----------+--------+-------+--------+--------+--------+--------+--------+

  --  Irq    | PErr   | OErr  | FErr   |  CTS   |  DCD   | TxRdy  | RxRdy  |

  -----------+--------+-------+--------+--------+--------+--------+--------+

  --

  -- Irq   - Bit[7] - Interrupt request

  -- PErr  - Bit[6] - Receive Parity error (parity bit does not match)

  -- OErr  - Bit[5] - Receive Overrun error (new character received before last read)

  -- FErr  - Bit[4] - Receive Framing Error (bad stop bit)

  -- CTS   - Bit[3] - Clear To Send level

  -- DCD   - Bit[2] - Data Carrier Detect (lost modem carrier)

  -- TxRdy - Bit[1] - Transmit Buffer Empty (ready to accept next transmit character)

  -- RxRdy - Bit[0] - Receive Data Ready (character received)

  -- 

  signal StatReg : std_logic_vector(7 downto 0) := (others => '0'); -- status register

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

  --  Control Register: CtrlReg

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

  --

  -- IO address + 0 Write

  --

  -----------+--------+--------+--------+--------+--------+--------+--------+

  --  RXIEnb |TxCtl(1)|TxCtl(0)|WdFmt(2)|WdFmt(1)|WdFmt(0)|BdCtl(1)|BdCtl(0)|

  -----------+--------+--------+--------+--------+--------+--------+--------+

  -- RxIEnb - Bit[7]

  -- 0       - Rx Interrupt disabled

  -- 1       - Rx Interrupt enabled

  -- TxCtl - Bits[6..5]

  -- 0 1     - Tx Interrupt Enable

  -- 1 0     - RTS high

  -- WdFmt - Bits[4..2]

  -- 0 0 0   - 7 data, even parity, 2 stop

  -- 0 0 1   - 7 data, odd  parity, 2 stop

  -- 0 1 0   - 7 data, even parity, 1 stop

  -- 0 1 1   - 7 data, odd  parity, 1 stop

  -- 1 0 0   - 8 data, no   parity, 2 stop

  -- 1 0 1   - 8 data, no   parity, 1 stop

  -- 1 1 0   - 8 data, even parity, 1 stop

  -- 1 1 1   - 8 data, odd  parity, 1 stop

  -- BdCtl - Bits[1..0]

  -- 0 0     - Baud Clk divide by 1

  -- 0 1     - Baud Clk divide by 16

  -- 1 0     - Baud Clk divide by 64

  -- 1 1     - reset

  signal CtrlReg : std_logic_vector(7 downto 0) := (others => '0'); -- control register

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

  -- Receive Register

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

  --

  -- IO address + 1     Read

  --

  signal RxReg : std_logic_vector(7 downto 0) := (others => '0');

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

  -- Transmit Register

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

  --

  -- IO address + 1     Write

  --

  signal TxReg    : std_logic_vector(7 downto 0) := (others => '0');

  signal TxDat    : std_logic := '1';   -- Transmit data bit

  signal TxRdy    : std_logic := '0';   -- Transmit buffer empty

  signal RxRdy    : std_logic := '0';   -- Receive Data ready

  --

  signal FErr     : std_logic := '0';   -- Frame error

  signal OErr     : std_logic := '0';   -- Output error

  signal PErr     : std_logic := '0';   -- Parity Error

  --

  signal TxIE     : std_logic := '0';   -- Transmit interrupt enable

  signal RxIE     : std_logic := '0';   -- Receive interrupt enable

  --

  signal RxRd     : std_logic := '0';   -- Read receive buffer

  signal TxWr     : std_logic := '0';   -- Write Transmit buffer

  signal StRd     : std_logic := '0';   -- Read status register

  --

  signal DCDState : DCD_State_Type;     -- DCD Reset state sequencer

  signal DCDDel   : std_logic := '0';   -- Delayed DCD_n

  signal DCDEdge  : std_logic := '0';   -- Rising DCD_N Edge Pulse

  signal DCDInt   : std_logic := '0';   -- DCD Interrupt

  signal BdFmt    : std_logic_vector(1 downto 0) := "00";   -- Baud Clock Format

  signal WdFmt    : std_logic_vector(2 downto 0) := "000";  -- Data Word Format

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

  -- RX Signals

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

  type RxStateType is ( RxState_Wait, RxState_Data, RxState_Parity, RxState_Stop );

  signal RxState    : RxStateType;                  -- receive bit state

  signal RxDatDel0  : Std_Logic := '0';             -- Delayed Rx Data

  signal RxDatDel1  : Std_Logic := '0';             -- Delayed Rx Data

  signal RxDatDel2  : Std_Logic := '0';             -- Delayed Rx Data

  signal RxDatEdge  : Std_Logic := '0';             -- Rx Data Edge pulse

  signal RxClkDel   : Std_Logic := '0';             -- Delayed Rx Input Clock

  signal RxClkEdge  : Std_Logic := '0';             -- Rx Input Clock Edge pulse

  signal RxStart    : Std_Logic := '0';             -- Rx Start request

  signal RxEnable   : Std_Logic := '0';             -- Rx Enabled

  signal RxClkCnt   : Std_Logic_Vector(5 downto 0) := (others => '0'); -- Rx Baud Clock Counter

  signal RxBdClk    : Std_Logic := '0';             -- Rx Baud Clock

  signal RxBdDel    : Std_Logic := '0';             -- Delayed Rx Baud Clock

  signal RxReq      : Std_Logic := '0';             -- Rx Data Valid

  signal RxAck      : Std_Logic := '0';             -- Rx Data Valid

  signal RxParity   : Std_Logic := '0';             -- Calculated RX parity bit

  signal RxBitCount : Std_Logic_Vector(2 downto 0) := (others => '0');  -- Rx Bit counter

  signal RxShiftReg : Std_Logic_Vector(7 downto 0) := (others => '0');  -- Shift Register

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

  -- TX Signals

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

  type TxStateType is ( TxState_Idle, TxState_Start, TxState_Data, TxState_Parity, TxState_Stop );

  signal TxState     : TxStateType;                   -- Transmitter state

  signal TxClkDel    : Std_Logic := '0';              -- Delayed Tx Input Clock

  signal TxClkEdge   : Std_Logic := '0';              -- Tx Input Clock Edge pulse

  signal TxClkCnt    : Std_Logic_Vector(5 downto 0) := (others => '0');  -- Tx Baud Clock Counter

  signal TxBdClk     : Std_Logic := '0';              -- Tx Baud Clock

  signal TxBdDel     : Std_Logic := '0';              -- Delayed Tx Baud Clock

  signal TxReq       : std_logic := '0';              -- Request transmit start

  signal TxAck       : std_logic := '0';              -- Acknowledge transmit start

  signal TxParity    : Std_logic := '0';              -- Parity Bit

  signal TxBitCount  : Std_Logic_Vector(2 downto 0) := (others => '0');  -- Data Bit Counter

  signal TxShiftReg  : Std_Logic_Vector(7 downto 0) := (others => '0');  -- Transmit shift register

begin

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

  -- ACIA Reset may be hardware or software

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

  acia_reset : process( clk, rst, ac_rst, dcd_n )

  begin

    --

    -- ACIA reset Synchronous 

    -- Includes software reset

    --

    if falling_edge(clk) then

      ac_rst <= (CtrlReg(1) and CtrlReg(0)) or rst;

    end if;

    -- Receiver reset

    rx_rst <= ac_rst or DCD_n;

    -- Transmitter reset

    tx_rst <= ac_rst;

  end process;

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

  -- Generate Read / Write strobes.

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

  acia_read_write : process(clk, ac_rst)

  begin

    if falling_edge(clk) then

      if rst = '1' then

        CtrlReg(1 downto 0) <= "11";

        CtrlReg(7 downto 2) <= (others => '0');

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

        RxRd  <= '0';

        TxWr  <= '0';

        StRd  <= '0';

      else

        RxRd  <= '0';

        TxWr  <= '0';

        StRd  <= '0';

        if cs = '1' then

          if Addr = '0' then              -- Control / Status register

            if rw = '0' then              -- write control register

              CtrlReg <= data_in;

            else                          -- read status register

              StRd <= '1';

            end if;

          else                            -- Data Register

            if rw = '0' then              -- write transmiter register

              TxReg <= data_in;

              TxWr  <= '1';

            else                          -- read receiver register

              RxRd <= '1';

            end if;

          end if;

        end if;

      end if;

    end if;

  end process;

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

  -- ACIA Status Register

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

  acia_status : process( clk )

  begin

    if falling_edge( clk ) then

      StatReg(0) <= RxRdy;                 -- Receive Data Ready

      StatReg(1) <= TxRdy and (not CTS_n); -- Transmit Buffer Empty

      StatReg(2) <= DCDInt;                -- Data Carrier Detect

      StatReg(3) <= CTS_n;                 -- Clear To Send

      StatReg(4) <= FErr;                  -- Framing error

      StatReg(5) <= OErr;                  -- Overrun error

      StatReg(6) <= PErr;                  -- Parity error

      StatReg(7) <= (RxIE and RxRdy) or

                    (RxIE and DCDInt) or

                    (TxIE and TxRdy);

    end if;

  end process;

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

  -- ACIA Transmit Control

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

  acia_control : process(CtrlReg, TxDat)

  begin

    case CtrlReg(6 downto 5) is

      when "00" =>                      -- Disable TX Interrupts, Assert RTS

        TxD   <= TxDat;

        TxIE  <= '0';

        RTS_n <= '0';

      when "01" =>                      -- Enable TX interrupts, Assert RTS

        TxD   <= TxDat;

        TxIE  <= '1';

        RTS_n <= '0';

      when "10" =>                      -- Disable Tx Interrupts, Clear RTS

        TxD   <= TxDat;

        TxIE  <= '0';

        RTS_n <= '1';

      when "11" =>                      -- Disable Tx interrupts, Assert RTS, send break

        TxD   <= '0';

        TxIE  <= '0';

        RTS_n <= '0';

      when others =>

        null;

    end case;

    RxIE  <= CtrlReg(7);

    WdFmt <= CtrlReg(4 downto 2);

    BdFmt <= CtrlReg(1 downto 0);

  end process;

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

  -- Set Data Output Multiplexer

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

  acia_data_mux : process(Addr, RxReg, StatReg)

  begin

    if Addr = '1' then

      data_out <= RxReg;               -- read receiver register

    else

      data_out <= StatReg;               -- read status register

    end if;

  end process;

  irq <= StatReg(7);

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

  -- Data Carrier Detect Edge rising edge detect

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

  acia_dcd_edge : process( clk, ac_rst )

  begin

    if falling_edge(clk) then

      if ac_rst = '1' then

        DCDDel  <= '0';

        DCDEdge <= '0';

      else

        DCDDel  <= DCD_n;

        DCDEdge <= DCD_n and (not DCDDel);

      end if;

    end if;

  end process;

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

  -- Data Carrier Detect Interrupt

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

  -- If Data Carrier is lost, an interrupt is generated

  -- To clear the interrupt, first read the status register

  --      then read the data receive register

  acia_dcd_int : process( clk, ac_rst )

  begin

    if falling_edge(clk) then

      if ac_rst = '1' then

        DCDInt   <= '0';

        DCDState <= DCD_State_Idle;

      else

        case DCDState is

        when DCD_State_Idle =>

          -- DCD Edge activates interrupt

          if DCDEdge = '1' then

            DCDInt   <= '1';

            DCDState <= DCD_State_Int;

          end if;

        when DCD_State_Int =>

          -- To reset DCD interrupt, 

          -- First read status

          if StRd = '1' then

            DCDState <= DCD_State_Reset;

          end if;

        when DCD_State_Reset =>

          -- Then read receive register

          if RxRd = '1' then

            DCDInt   <= '0';

            DCDState <= DCD_State_Idle;

          end if;

        when others =>

          null;

        end case;

      end if;

    end if;

  end process;

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

  -- Receiver Clock Edge Detection

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

  -- A rising edge will produce a one clock cycle pulse

  acia_rx_clock_edge : process( clk, rx_rst )

  begin

    if falling_edge(clk) then

      if rx_rst = '1' then

        RxClkDel  <= '0';

        RxClkEdge <= '0';

      else

        RxClkDel  <= RxC;

        RxClkEdge <= (not RxClkDel) and RxC;

      end if;

    end if;

  end process;

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

  -- Receiver Data Edge Detection

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

  -- A falling edge will produce a pulse on RxClk wide

  acia_rx_data_edge : process( clk, rx_rst )

  begin

    if falling_edge(clk) then

      if rx_rst = '1' then

        RxDatDel0 <= '0';

        RxDatDel1 <= '0';

        RxDatDel2 <= '0';

        RxDatEdge <= '0';

      else

        RxDatDel0 <= RxD;

        RxDatDel1 <= RxDatDel0;

        RxDatDel2 <= RxDatDel1;

        RxDatEdge <= RxDatDel0 and (not RxD);

      end if;

    end if;

  end process;

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

  -- Receiver Start / Stop

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

  -- Enable the receive clock on detection of a start bit

  -- Disable the receive clock after a byte is received.

  acia_rx_start_stop : process( clk, rx_rst )

  begin

    if falling_edge(clk) then

      if rx_rst = '1' then

        RxEnable <= '0';

        RxStart  <= '0';

      elsif (RxEnable = '0') and (RxDatEdge = '1') then

        -- Data Edge detected 

        RxStart  <= '1';                    -- Request Start and

        RxEnable <= '1';                    -- Enable Receive Clock

      elsif (RxStart = '1') and (RxAck = '1') then

        -- Data is being received

        RxStart <= '0';                     -- Reset Start Request

      elsif (RxStart = '0') and (RxAck = '0') then

        -- Data has now been received

        RxEnable <= '0';                    -- Disable Receiver until next Start Bit

      end if;

    end if;

  end process;

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

  -- Receiver Clock Divider

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

  -- Hold the Rx Clock divider in reset when the receiver is disabled

  -- Advance the count only on a rising Rx clock edge

  acia_rx_clock_divide : process( clk, rx_rst )

  begin

    if falling_edge(clk) then

      if rx_rst = '1' then

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

      elsif RxDatEdge = '1' then

        -- reset on falling data edge

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

      elsif RxClkEdge = '1' then

        -- increment count on Clock edge

        RxClkCnt <= RxClkCnt + "000001";

      end if;

    end if;

  end process;

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

  -- Receiver Baud Clock Selector

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

  -- BdFmt

  -- 0 0     - Baud Clk divide by 1

  -- 0 1     - Baud Clk divide by 16

  -- 1 0     - Baud Clk divide by 64

  -- 1 1     - Reset

  --

  acia_rx_baud_clock_select : process( BdFmt, RxC, RxClkCnt )

  begin

    case BdFmt is

      when "00" =>                              -- Div by 1

        RxBdClk <= RxC;

      when "01" =>                              -- Div by 16

        RxBdClk <= RxClkCnt(3);

      when "10" =>                              -- Div by 64

        RxBdClk <= RxClkCnt(5);

      when others =>                         -- Software Reset

        RxBdClk <= '0';

    end case;

  end process;

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

  -- Receiver process

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

  -- WdFmt - Bits[4..2]

  -- 0 0 0   - 7 data, even parity, 2 stop

  -- 0 0 1   - 7 data, odd  parity, 2 stop

  -- 0 1 0   - 7 data, even parity, 1 stop

  -- 0 1 1   - 7 data, odd  parity, 1 stop

  -- 1 0 0   - 8 data, no   parity, 2 stop

  -- 1 0 1   - 8 data, no   parity, 1 stop

  -- 1 1 0   - 8 data, even parity, 1 stop

  -- 1 1 1   - 8 data, odd  parity, 1 stop

  acia_rx_receive : process( clk, rst )

  begin

    if falling_edge( clk ) then

      if rx_rst = '1' then

        FErr       <= '0';

        OErr       <= '0';

        PErr       <= '0';

        RxShiftReg <= (others => '0');       -- Reset Shift register

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

        RxParity   <= '0';                   -- reset Parity bit

        RxAck      <= '0';                   -- Receiving data

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

        RxState    <= RxState_Wait;

      else

        RxBdDel      <= RxBdClk;

        if RxBdDel = '0' and RxBdClk = '1' then

          case RxState is

          when RxState_Wait =>

            RxShiftReg <= (others => '0');   -- Reset Shift register

            RxParity   <= '0';               -- Reset Parity bit

            if WdFmt(2) = '0' then           -- WdFmt(2) = '0' => 7 data bits

              RxBitCount <= "110";

            else                             -- WdFmt(2) = '1' => 8 data bits

              RxBitCount <= "111";

            end if;

            if RxDatDel2 = '0' then          -- look for start bit

              RxState <= RxState_Data;       -- if low, start reading data

            end if;

          when RxState_Data =>               -- Receiving data bits 

            RxShiftReg <= RxDatDel2 & RxShiftReg(7 downto 1);

            RxParity   <= RxParity xor RxDatDel2;

            RxAck      <= '1';               -- Flag receive in progress

            RxBitCount <= RxBitCount - "001";

            if RxBitCount = "000" then

              if WdFmt(2) = '0' then         -- WdFmt(2) = '0' => 7 data

                RxState <= RxState_Parity;   -- 7 bits always has parity

              elsif WdFmt(1) = '0' then      -- WdFmt(2) = '1' => 8 data                          

                RxState <= RxState_Stop;     -- WdFmt(1) = '0' => no parity

                PErr <= '0';               -- Reset Parity Error

              else

                RxState <= RxState_Parity;   -- WdFmt(1) = '1' => 8 data + parity

              end if;

            end if;

          when RxState_Parity =>             -- Receive Parity bit

            if WdFmt(2) = '0' then           -- if 7 data bits, shift parity into MSB