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

-- Package of UART components

--

-- This UART uses a squarewave input for the BAUDRATE clock.  In other

-- words, the BAUD rate is exactly the same as the frequency of the

-- incoming clock.  This is in contrast to other UARTs which need a

-- Baud rate clock which is some multiple of the actual Baud rate

-- desired.  Because of the 1x nature of the Baud clock, the receiver

-- needs at least one Baud Clock interval in which to measure the

-- Baud clock versus the system clock, before it can start working.

-- Also, the system clock must be somewhat higher than the Baud clock

-- in order for the receiver to work.

--

-- This package contains the UART, plus individual async_tx and async_rx

-- modules, which are the transmit and receive sections of the UART.

--

--

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use IEEE.NUMERIC_STD.ALL;

package uart_sqclk_pack is

  component uart_sqclk

    port (

      sys_rst_n     : in std_logic;

      sys_clk       : in std_logic;

      sys_clk_en    : in std_logic;

      -- rate and parity

      parity_i      : in unsigned(1 downto 0); -- 0=none, 1=even, 2 or 3=odd

      rate_clk_i    : in std_logic;

      -- serial I/O

      tx_stream     : out std_logic;

      rx_stream     : in std_logic;

      --control and status

      tx_wr_i       : in  std_logic;        -- Starts Transmit

      tx_dat_i      : in  unsigned(7 downto 0);

      tx_done_o     : out std_logic;

      rx_restart_i  : in  std_logic;        -- High clears error flags, clears rx_done_o

      rx_dat_o      : out unsigned(7 downto 0);

      rx_wr_o       : out std_logic;        -- High pulse means store rx_dat_o.

      rx_done_o     : out std_logic;        -- Remains high after receive, until clk edge with rx_restart_i=1

      frame_err_o   : out std_logic;        -- High = error.  Reset when rx_restart_i asserted.

      parity_err_o  : out std_logic         -- High = error.  Reset when rx_restart_i asserted.

    );

  end component;

  component async_tx_sqclk

    port (

      sys_rst_n    : in std_logic;

      sys_clk      : in std_logic;

      sys_clk_en   : in std_logic;

      -- rate and parity

      tx_parity_i  : in unsigned(1 downto 0); -- 0=none, 1=even, 2 or 3=odd

      tx_clk_i     : in std_logic;

      -- serial output

      tx_stream    : out std_logic;

      -- control and status

      tx_wr_i      : in  std_logic;        -- Starts Transmit

      tx_dat_i     : in  unsigned(7 downto 0);

      tx_done_o    : out std_logic

    );

  end component;

  component async_rx_sqclk

    port (

      sys_rst_n    : in std_logic;

      sys_clk      : in std_logic;

      sys_clk_en   : in std_logic;

      -- rate and parity

      rx_parity_i  : in unsigned(1 downto 0); -- 0=none, 1=even, 2 or 3=odd

      rx_clk_i     : in std_logic;

      -- serial input

      rx_stream    : in std_logic;

      -- control and status

      rx_restart_i : in  std_logic;        -- High clears error flags, clears rx_done_o

      rx_dat_o     : out unsigned(7 downto 0);

      rx_wr_o      : out std_logic;        -- High pulse means store rx_dat_o.

      rx_done_o    : out std_logic;        -- Remains high after receive, until rx_restart_i

      frame_err_o  : out std_logic;        -- High = error.  Reset when rx_restart_i asserted.

      parity_err_o : out std_logic         -- High = error.  Reset when rx_restart_i asserted.

    );

  end component;

end uart_sqclk_pack;

package body uart_sqclk_pack is

end uart_sqclk_pack;

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

-- UART.  Variable Speed, RX Buffer, but no TX buffer

-- High Speed Asynchronous Receiver & Transmitter

-- 

-- Description:

--   This block receives and transmits asynchronous serial bytes.  The Baudrate

-- and parity are selectable through inputs, but the number of bits per character

-- is fixed at eight.

--

-- NOTES:

-- Transmit starts when tx_wr_i is detected high at a rising clock edge.

-- Once the transmit operation is completed, tx_done_o latches high.

--

-- The receive input is passed through two layers of synchronizing flip-flops

-- to help mitigate metastability issues, since this signal can come from

-- outside of the sys_clk clock domain.  All other logic connecting to inputs

-- of this function are assumed to be within the same clock domain.

--

-- The receiver looks for a new start bit immediately following the rx_wr_o

-- pulse, but rx_done_o is delayed until the expected end of the received

-- character.  If a new start bit is detected just prior to the expected end

-- of the received character, then rx_done_o is not asserted.

--

-- Receive begins when the falling edge of the start bit is detected.

-- Then after 10 or 11 bit times have passed (depending on the parity setting)

-- the rx_wr_o signal will pulse high for one clock period, indicating rx_dat_o

-- contains valid receive data.  The rx_wr_o pulse is only issued if there is

-- no parity error, and the stop bit is actually detected high at the sampling

-- time.

-- The rx_dat_o outputs will hold the received data until the next rx_wr_o pulse.

-- The rx_dat_o output provides the received data, even in the presence of a

-- parity or stop-bit error.

-- Error flags are valid during rx_wr_o, but they remain latched, until

-- rx_restart_i.

--

-- The Baud rate is equal to the frequency of the rate_clk_i input.

-- It should be, as nearly as possible, a square wave of the desired

-- communications rate.

--

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use IEEE.NUMERIC_STD.ALL;

entity uart_sqclk is

    port (

      sys_rst_n     : in std_logic;

      sys_clk       : in std_logic;

      sys_clk_en    : in std_logic;

      -- rate and parity

      parity_i      : in unsigned(1 downto 0); -- 0=none, 1=even, 2 or 3=odd

      rate_clk_i    : in std_logic;

      -- serial I/O

      tx_stream     : out std_logic;

      rx_stream     : in std_logic;

      --control and status

      tx_wr_i       : in  std_logic;        -- Starts Transmit

      tx_dat_i      : in  unsigned(7 downto 0);

      tx_done_o     : out std_logic;

      rx_restart_i  : in  std_logic;        -- High clears error flags, clears rx_done_o

      rx_dat_o      : out unsigned(7 downto 0);

      rx_wr_o       : out std_logic;        -- High pulse means store rx_dat_o.

      rx_done_o     : out std_logic;        -- Remains high after receive, until clk edge with rx_restart_i=1

      frame_err_o   : out std_logic;        -- High = error.  Reset when rx_restart_i asserted.

      parity_err_o  : out std_logic         -- High = error.  Reset when rx_restart_i asserted.

    );

end uart_sqclk;

library work;

use work.uart_sqclk_pack.all;

architecture beh of uart_sqclk is

-- Components

begin

  tx1: async_tx_sqclk

    port map (

       sys_rst_n     => sys_rst_n,

       sys_clk       => sys_clk,

       sys_clk_en    => sys_clk_en,

       -- rate and parity

       tx_parity_i   => parity_i, -- 0=none, 1=even, 2 or 3=odd

       tx_clk_i      => rate_clk_i,

       -- serial output

       tx_stream     => tx_stream,

       -- control and status

       tx_wr_i       => tx_wr_i,       -- Starts Transmit

       tx_dat_i      => tx_dat_i,

       tx_done_o     => tx_done_o

    );

  rx1: async_rx_sqclk

    port map (

       sys_rst_n     => sys_rst_n,

       sys_clk       => sys_clk,

       sys_clk_en    => sys_clk_en,

       -- rate and parity

       rx_parity_i   => parity_i, -- 0=none, 1=even, 2 or 3=odd

       rx_clk_i      => rate_clk_i,

       -- serial input

       rx_stream     => rx_stream,

       -- control and status

       rx_restart_i  => rx_restart_i,  -- High clears error flags, clears rx_done_o

       rx_dat_o      => rx_dat_o,

       rx_wr_o       => rx_wr_o,       -- High pulse means store rx_dat_o.

       rx_done_o     => rx_done_o,     -- Remains high after receive, until rx_restart_i

       frame_err_o   => frame_err_o,   -- High = error.  Reset when rx_restart_i asserted.

       parity_err_o  => parity_err_o   -- High = error.  Reset when rx_restart_i asserted.

    );

end beh;

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

-- Asynchronous Receiver With Output Buffer

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

--

-- Author: John Clayton

-- Date  : Aug  05, 2013 Added this change log header, which was missing.

--                       Added first_edge signal to avoid erroneous initial

--                       baud interval measurement (John Clayton & Philip 

--                       Kasavan)

--         Jan.  2, 2014 Added output buffer, changed idle_prep to include

--                       the actual transition to IDLE state.  Added

--                       POST_RECV state, so that the rx_done_o signal will

--                       reflect the true end of the received character.

--                       This helps in applications where a received

--                       asynchronous input is "echoed back" directly,

--                       as the rx_wr_o signal can be used to switch the

--                       signal at the correct time.

--         Feb.  6, 2014 Added requirement for half_baud to be non-zero

--                       before leaving IDLE state.  This prevents leaving

--                       IDLE due to falling edges prior to the first Baud

--                       interval measurement.

--

-- Description

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

-- Squarewave tx_clk_i input determines rate.

-- (tx_clk_i does not really need to be a squarewave.  Only the rising edges

--  are measured and used.)

-- 

-- Description:

--   This block receives asynchronous serial bytes.  The Baudrate and parity

-- are determined by inputs, but the number of bits per character is

-- fixed at eight.

--

-- NOTES:

-- The receive input and baudrate clock are passed through two layers of

-- synchronizing flip-flops to mitigate metastability, since those signals can

-- originate outside of the sys_clk clock domain.  All other logic connecting to 

-- input of this function are assumed to be within the same clock domain.

--

-- The receiver looks for a new start bit immediately following the rx_wr_o

-- pulse, but rx_done_o is delayed until the expected end of the received

-- character.  If a new start bit is detected just prior to the expected end

-- of the received character, then rx_done_o is not asserted.

--

-- Receive begins when the falling edge of the start bit is detected.

-- Then after 10 or 11 bit times have passed (depending on the parity setting)

-- the rx_wr_o signal will pulse high for one clock period, indicating rx_dat_o

-- contains valid receive data.  The rx_wr_o pulse is only issued if there is

-- no parity error, and the stop bit is actually detected high at the sampling

-- time.

-- The rx_dat_o outputs will hold the received data until the next rx_wr_o pulse.

-- The rx_dat_o output provides the received data, even in the presence of a

-- parity or stop-bit error.

-- Error flags are valid during rx_wr_o, but they remain latched, until

-- rx_restart_i.

--

-- Although the receiver immediately restarts itself to receive the next

-- character, the rx_restart_i input can clear the error indicators.  The rx_restart_i

-- input is like a synchronous reset in this respect since it will cause a receive

-- operation to abort.

--

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use IEEE.NUMERIC_STD.ALL;

entity async_rx_sqclk is

    port (

      sys_rst_n    : in std_logic;

      sys_clk      : in std_logic;

      sys_clk_en   : in std_logic;

      -- rate and parity

      rx_parity_i  : in unsigned(1 downto 0); -- 0=none, 1=even, 2 or 3=odd

      rx_clk_i     : in std_logic;

      -- serial input

      rx_stream    : in std_logic;

      -- control and status

      rx_restart_i : in  std_logic;        -- High clears error flags, synchronously resets receiver

      rx_dat_o     : out unsigned(7 downto 0);

      rx_wr_o      : out std_logic;        -- High pulse means store rx_dat_o.

      rx_done_o    : out std_logic;        -- Indicates receiver is idle

      frame_err_o  : out std_logic;        -- High = error.  Reset when rx_restart_i asserted.

      parity_err_o : out std_logic         -- High = error.  Reset when rx_restart_i asserted.

    );

end async_rx_sqclk;

architecture beh of async_rx_sqclk is

-- RX signals

  -- rx_clk_i synchronizing flip-flops and rising edge detector

signal rx_clk_r1       : std_logic;

signal rx_clk_r2       : std_logic;

  -- RX input synchronizing flip flops

signal rx_stream_r1    : std_logic;

signal rx_stream_r2    : std_logic;

  -- RX signals

    -- RX State Machine

type RX_STATE_TYPE is (IDLE, CHECK_START_1, CHECK_START_2, RECV_DATA, POST_RECV);

signal rx_state        : RX_STATE_TYPE;

signal start_bit_start : std_logic;             -- Signals falling edge of rx_stream_i

signal rx_sr           : unsigned(8 downto 0);  -- Shift register

signal rx_bcnt         : unsigned(3 downto 0);  -- Number of bits left, counts down

signal rx_bcnt_start   : unsigned(3 downto 0);  -- Total number of bits

signal rx_parity_good  : std_logic;

  -- Timers have been sized to hold baud interval for speeds as slow as 9600 bps at 100MHz sys_clk

signal rx_timer        : unsigned(13 downto 0); -- Elapsed sys_clks from last bit time start

signal half_baud       : unsigned(13 downto 0); -- One half of full_baud

signal full_baud       : unsigned(13 downto 0); -- Baud interval, as measured from rx_clk_i

signal baud_timer      : unsigned(13 downto 0); -- Used to measure baud interval

signal bit_sampled     : std_logic;             -- High indicates bit is already sampled, don't allow resampling.

signal first_edge      : std_logic;

begin

  -- Synchronizing flip flops to avoid metastability issues...

  rx_stream_syncproc: Process(sys_rst_n,sys_clk)

  BEGIN

    if (sys_rst_n = '0') then

      rx_stream_r1 <= '1';

      rx_stream_r2 <= '1';

    elsif (sys_clk'event AND sys_clk='1') then

      if (sys_clk_en='1') then

        rx_stream_r2 <= rx_stream_r1;

        rx_stream_r1 <= rx_stream;

      end if;

    end if;

  END PROCESS rx_stream_syncproc;

--  start_bit_start <= rx_stream_r2 and not rx_stream_r1;

  -- This signal has been rewritten carefully to avoid "artifacts" which occur

  -- during simulation, when rx_stream_r1 is 'X', in a manner reminiscent of

  -- metastability.

  start_bit_start <= '1' when rx_stream_r2='1' and rx_stream_r1/='1' else '0';

  -- Synchronizing flip flops to avoid metastability issues...

  rx_clk_syncproc: Process(sys_rst_n,sys_clk)

  BEGIN

    if (sys_rst_n = '0') then

      rx_clk_r1 <= '0';

      rx_clk_r2 <= '0';

    elsif (sys_clk'event AND sys_clk='1') then

      if (sys_clk_en='1') then

        rx_clk_r2 <= rx_clk_r1;

        rx_clk_r1 <= rx_clk_i;

      end if;

    end if;

  END PROCESS rx_clk_syncproc;

  -- This is the baud interval measuring process.

  -- Measurements are only made between rising edges

  baud_measure_proc: Process(sys_rst_n,sys_clk)

  BEGIN

    if (sys_rst_n = '0') then

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

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

      first_edge <= '0';

    elsif (sys_clk'event AND sys_clk='1') then

      if (sys_clk_en='1') then

        if(first_edge = '1')then

          if (rx_clk_r1='1' and rx_clk_r2='0') then

            full_baud  <= baud_timer;

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

          else

            baud_timer <= baud_timer+1;

          end if;

        elsif(rx_clk_r1='1' and rx_clk_r2='0') then

          first_edge <= '1';

        end if;

      end if;

    end if;

  END PROCESS baud_measure_proc;

  -- This process handles the incoming bits

  uart_rx_bits: Process(sys_rst_n,sys_clk)

  procedure idle_prep is

  begin

    rx_done_o   <= '1';

    bit_sampled <= '0';

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

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

    rx_state    <= IDLE;

  end idle_prep;

  begin

    if (sys_rst_n = '0') then

      idle_prep;

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

      frame_err_o  <= '0';

      parity_err_o <= '0';

      rx_wr_o      <= '0';

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

    elsif (sys_clk'event AND sys_clk='1') then

      if (sys_clk_en='1') then

        -- Default values

        rx_wr_o      <= '0';           -- Default to no data write

        -- Handle incrementing the sample timer

        rx_timer<=rx_timer+1;

        -- State transitions

        case rx_state is

          when IDLE         =>

            rx_done_o   <= '1';           -- Indicate receive is done.

            bit_sampled <= '0';           -- Indicate bit is not yet sampled.

            if (rx_restart_i='1') then

              idle_prep;

              frame_err_o  <= '0';        -- At rx_restart, also clear error flags

              parity_err_o <= '0';

            elsif (half_baud/=0 and start_bit_start='1') then

              rx_timer  <= (others=>'0'); -- Reset timer back to zero

              rx_bcnt   <= rx_bcnt_start; -- Initialize bit counter

              rx_done_o <= '0';

              rx_state  <= CHECK_START_1;

            end if;

          when CHECK_START_1 =>

            if (rx_restart_i='1') then    -- Restart has very high priority

              idle_prep;

            elsif (rx_stream_r2='1') then -- High during this time is an error

              frame_err_o <= '1';

              idle_prep;

            elsif (rx_timer>=half_baud) then -- Must use >= since threshold may change downward

              rx_state  <= CHECK_START_2;

            end if;

          when CHECK_START_2 =>            -- During second half of start bit, don't verify low level

            if (rx_restart_i='1') then     -- Restart has very high priority

              idle_prep;

            elsif (rx_timer>=full_baud or rx_stream_r2='1') then -- Wait for end of start bit

              rx_timer <= (others=>'0'); -- Reset timer back to zero

              rx_state <= RECV_DATA;

            end if;

          when RECV_DATA     =>

            if (rx_restart_i='1') then     -- Restart has very high priority

              idle_prep;

            elsif (rx_timer>=full_baud) then -- Must use >= since threshold may change downward

              rx_timer <= (others=>'0'); -- Reset timer back to zero

              bit_sampled <= '0';

            elsif (rx_timer>=half_baud and bit_sampled='0') then -- Must use >= since threshold may change downward

              bit_sampled <= '1';

              if (rx_bcnt="0000") then

                rx_state <= POST_RECV;

                rx_dat_o <= rx_sr(7 downto 0);

                if (rx_parity_good='1' and rx_stream_r2='1') then

                  rx_wr_o <= '1';            -- If all is correct, create a one clock long pulse to store rx_dat_o.

                else

                  if (rx_stream_r2='0') then

                    frame_err_o <= '1';        -- Record error if there is a bad stop bit

                  end if;

                  if (rx_parity_good='0') then

                    parity_err_o <= '1';       -- Record error if there is bad parity

                  end if;

                end if;

              else -- Process a new bit

                rx_sr(7 downto 0) <= rx_sr(8 downto 1);

                if (rx_parity_i = "00") then

                  rx_sr(7) <= rx_stream_r2;  -- Store the new incoming bit

                else

                  rx_sr(8) <= rx_stream_r2;

                end if;

                rx_bcnt <= rx_bcnt-1;

              end if;

            end if;

          when POST_RECV => -- Wait out latter half of stop bit, checking for start bits...

            if (rx_restart_i='1') then

              bit_sampled  <= '0';

              frame_err_o  <= '0'; -- At rx_restart, also clear error flags

              parity_err_o <= '0';

              idle_prep;

            elsif (start_bit_start='1') then

              bit_sampled <= '0';

              rx_timer  <= (others=>'0'); -- Reset timer back to zero

              rx_bcnt   <= rx_bcnt_start; -- Initialize bit counter

              rx_done_o <= '0';

              rx_state  <= CHECK_START_1;

            elsif (rx_timer>=full_baud) then -- Wait for end of start bit

              bit_sampled <= '0'; -- Indicate bit is not yet sampled.

              idle_prep; -- Asserts rx_done_o to indicate completion

            end if;

          when others => null;

        end case;

      end if;

    end if;

  end process uart_rx_bits;

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

  -- Assign number of bits to shift in.

  rx_bcnt_start <= "1000" when (rx_parity_i="00") else "1001";

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

  -- Assign half baud period

  half_baud <= ('0' & full_baud(13 downto 1));

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

  -- Parity check process

  rx_parity_check: process(rx_sr, rx_parity_i)

  begin

    if (rx_parity_i="00") then       -- No parity...

      rx_parity_good <= '1'; -- (always good.)

    elsif (rx_parity_i="01") then    -- Even parity...

      rx_parity_good <= not (rx_sr(0) XOR rx_sr(1) XOR rx_sr(2) XOR rx_sr(3) XOR rx_sr(4)

                                      XOR rx_sr(5) XOR rx_sr(6) XOR rx_sr(7) XOR rx_sr(8));

    else                     -- Odd parity...

      rx_parity_good <=     (rx_sr(0) XOR rx_sr(1) XOR rx_sr(2) XOR rx_sr(3) XOR rx_sr(4)

                                      XOR rx_sr(5) XOR rx_sr(6) XOR rx_sr(7) XOR rx_sr(8));

    end if;

  end process;

end beh;

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

-- Asynchronous Transmitter With No Buffering

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

--

-- Author: John Clayton

-- Date  : Aug  08, 2013 Added this change log header, which was missing.

--                       Changed tx_done_o signal so that it pulses after

--                       the stop bit is finished.  How could this have

--                       remained so woefully incorrect for so long?!

--         Jan  02, 2014 Fixed a latent bug in the logic for asserting

--                       tx_done_o.  Prior to this fix, it was possible

--                       for a write that was coincident with do_txbit to

--                       be ignored!  Once again, how could this have

--                       remained so woefully incorrect all this time?!

--         Jan  07, 2014 Rewrote the startup logic to allow for cases

--                       when tx_wr_i='1' and tx_bcnt="0000" and do_txbit='1'

--                       Also rewrote the tx_done_o signal so that it is

--                       asserted earlier - when "tx_almost_done" is high.

--                       This is all calculated to allow the transmitter

--                       to send characters back-to-back using its own

--                       tx_done_o signal as a tx_wr_i signal.  This is

--                       actually getting pretty neat.  The unit sends out

--                       asynchronous characters, but insists on doing it

--                       in synchronism with the tx_clk_i input... so it

--                       isn't really very asynchronous in that sense!

--

-- Description

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

-- Squarewave tx_clk_i input determines rate.

-- (tx_clk_i need not be a squarewave for this module, since only the rising

--  edge is used.  In the accompanying receiver, however, both edges are used.)

-- 

-- Description:

--   This block transmits asynchronous serial bytes.  The Baudrate and parity

-- are determined by inputs, but the number of bits per character is

-- fixed at eight.

--

-- NOTES:

-- Transmit starts when the transmitter is idle and tx_wr_i is detected high

-- at a rising sys_clk edge.

--

-- Once the transmit operation is completed, done_o latches high.

--

-- Since the baud clock might be asynchronous to the sys_clk, there are

-- syncronizing flip-flops on it inside this module.

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use IEEE.NUMERIC_STD.ALL;

entity async_tx_sqclk is

    port (

      sys_rst_n    : in std_logic;

      sys_clk      : in std_logic;

      sys_clk_en   : in std_logic;

      -- rate and parity

      tx_parity_i  : in unsigned(1 downto 0); -- 0=none, 1=even, 2 or 3=odd

      tx_clk_i     : in std_logic;

      -- serial output

      tx_stream    : out std_logic;

      -- control and status

      tx_wr_i      : in  std_logic;        -- Starts Transmit

      tx_dat_i     : in  unsigned(7 downto 0);

      tx_done_o    : out std_logic

    );

end async_tx_sqclk;

architecture beh of async_tx_sqclk is

-- TX signals

  -- TX clock synchronizing flip-flops and rising edge detection

signal tx_clk_r1 : std_logic;

signal tx_clk_r2 : std_logic;

  -- TX clock enable, shift register and bit count

signal do_txbit       : std_logic;

signal tx_sr          : unsigned(9 downto 0);

signal tx_bcnt        : unsigned(3 downto 0); -- Number of bits

signal tx_almost_done : std_logic;

signal tx_done        : std_logic;

begin

  -- This process detects the rising edge of tx_clk_i

  tx_clk_edge_proc: Process(sys_rst_n,sys_clk)

  BEGIN

    if (sys_rst_n = '0') then

      tx_clk_r1 <= '0';

      tx_clk_r2 <= '0';

    elsif (sys_clk'event AND sys_clk='1') then

      if (sys_clk_en='1') then

        tx_clk_r1 <= tx_clk_i;

        tx_clk_r2 <= tx_clk_r1;

      end if;

    end if;

  END PROCESS tx_clk_edge_proc;

  do_txbit <= (tx_clk_r1 and not tx_clk_r2); -- rising edge detect

  -- This process loads the shift register, then counts as the bits transmit out.

  byte_tx: Process(sys_rst_n,sys_clk)

  BEGIN

    if (sys_rst_n = '0') then

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

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

      tx_stream <= '1';

      tx_done   <= '1';

    elsif (sys_clk'event and sys_clk='1') then

      if (sys_clk_en='1') then

        -- Start a new transmission when ready

        -- Case 1 is starting while do_txbit is high

        if tx_bcnt="0000" and do_txbit='1' and tx_wr_i='1' then

          tx_stream <= '0';                             -- Provide start bit

          tx_sr(7 downto 0) <= tx_dat_i;                -- Load the TX data

          tx_sr(8) <= '1';                              -- Default the parity bit to one

          if(tx_parity_i = "00") then                   --If no parity...

            tx_bcnt  <= "1001";                         -- send start, 8 data bits, and stop

          elsif (tx_parity_i = "01") then               --If even parity...

            tx_bcnt  <= "1010";                         -- send start, 8 data bits, parity, and stop

            tx_sr(8) <= tx_dat_i(0) XOR tx_dat_i(1) XOR tx_dat_i(2) XOR tx_dat_i(3) XOR

                        tx_dat_i(4) XOR tx_dat_i(5) XOR tx_dat_i(6) XOR tx_dat_i(7);

          else                                          --If odd parity...

            tx_bcnt  <= "1011";                         --send start, 8 data bits, parity, and stop