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