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

--

--  S Y N T H E Z I A B L E    miniUART   C O R E

--

--  www.OpenCores.Org - January 2000

--  This core adheres to the GNU public license  

--

-- Design units   : miniUART core for the OCRP-1

--

-- File name      : TxUnit.vhd

--

-- Purpose        : Implements an miniUART device for communication purposes 

--                  between the OR1K processor and the Host computer through

--                  an RS-232 communication protocol.

--                  

-- Library        : uart_lib.vhd

--

-- Dependencies   : IEEE.Std_Logic_1164

--

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

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

-- Revision list

-- Version   Author                 Date                        Changes

--

-- 0.1      Ovidiu Lupas       15 January 2000                 New model

-- 2.0      Ovidiu Lupas       17 April   2000    unnecessary variable removed

--  olupas@opencores.org

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

-- Description    : 

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

-- Entity for the Tx Unit                                                    --

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

library ieee;

use ieee.std_logic_1164.all;

use ieee.numeric_std.all;

library work;

use work.Uart_Def.all;

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

-- Transmitter unit

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

entity TxUnit is

  port (

     Clk    : in  Std_Logic;  -- Clock signal

     Reset  : in  Std_Logic;  -- Reset input

     Enable : in  Std_Logic;  -- Enable input

     Load   : in  Std_Logic;  -- Load transmit data

     TxD    : out Std_Logic;  -- RS-232 data output

     TRegE  : out Std_Logic;  -- Tx register empty

     TBufE  : out Std_Logic;  -- Tx buffer empty

     DataO  : in  Std_Logic_Vector(7 downto 0));

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

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

-- Architecture for TxUnit

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

architecture Behaviour of TxUnit is

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

  -- Signals

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

  signal TBuff    : Std_Logic_Vector(7 downto 0); -- transmit buffer

  signal TReg     : Std_Logic_Vector(7 downto 0); -- transmit register

  signal BitCnt   : Unsigned(3 downto 0);         -- bit counter

  signal tmpTRegE : Std_Logic;                    -- 

  signal tmpTBufE : Std_Logic;                    --

begin

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

  -- Implements the Tx unit

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

  process(Clk,Reset,Enable,Load,DataO,TBuff,TReg,tmpTRegE,tmpTBufE)

      variable tmp_TRegE : Std_Logic;

      constant CntOne    : Unsigned(3 downto 0):="0001";

  begin

     if Rising_Edge(Clk) then

        if Reset = '0' then

           tmpTRegE <= '1';

           tmpTBufE <= '1';

           TxD <= '1';

           BitCnt <= "0000";

        elsif Load = '1' then

           TBuff <= DataO;

           tmpTBufE <= '0';

        elsif Enable = '1' then

           if ( tmpTBufE = '0') and (tmpTRegE = '1') then

              TReg <= TBuff;

              tmpTRegE <= '0';

--              tmp_TRegE := '0';

              tmpTBufE <= '1';

--           else

--              tmp_TRegE := tmpTRegE;

           end if;

           if tmpTRegE = '0' then

              case BitCnt is

                  when "0000" =>

                          TxD <= '0';

                          BitCnt <= BitCnt + CntOne;

                  when "0001" | "0010" | "0011" |

                       "0100" | "0101" | "0110" |

                       "0111" | "1000" =>

                          TxD <= TReg(0);

                          TReg <= '1' & TReg(7 downto 1);

                          BitCnt <= BitCnt + CntOne;

                  when "1001" =>

                          TxD <= '1';

                          TReg <= '1' & TReg(7 downto 1);

                          BitCnt <= "0000";

                          tmpTRegE <= '1';

                  when others => null;

              end case;

           end if;

        end if;

     end if;

  end process;

      TRegE <= tmpTRegE;

      TBufE <= tmpTBufE;

end Behaviour; --=================== End of architecture ====================--