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-------------------------------------------------------------------------------
-- 
-- Copyright (C) 2009, 2010 Dr. Juergen Sauermann
-- 
--  This code 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 code 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 code (see the file named COPYING).
--  If not, see http://www.gnu.org/licenses/.
--
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
--
-- Module Name:    uart_tx - Behavioral 
-- Create Date:    14:21:59 11/07/2009 
-- Description:    a UART receiver.
--
-------------------------------------------------------------------------------
--
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;
 
entity uart_tx is 
    port(   I_CLK       : in  std_logic;    
            I_CLR       : in  std_logic;            -- RESET
            I_CE_1      : in  std_logic;            -- BAUD rate clock enable
            I_DATA      : in  std_logic_vector(7 downto 0);   -- DATA to be sent
            I_FLAG      : in  std_logic;            -- toggle to send data
            Q_TX        : out std_logic;            -- Serial output line
            Q_FLAG      : out std_logic);           -- Transmitting Flag
end uart_tx;
 
architecture Behavioral of uart_tx is
 
signal L_BUF            : std_logic_vector(7 downto 0);
signal L_TODO           : std_logic_vector(3 downto 0);     -- bits to send
signal L_FLAG           : std_logic;
 
begin
 
    process(I_CLK)
    begin
        if (rising_edge(I_CLK)) then
            if (I_CLR = '1') then
                Q_TX   <= '1';
                L_BUF  <= "11111111";
                L_TODO <= "0000";
                L_FLAG <= I_FLAG;                   -- idle
            elsif (I_CE_1 = '1') then
                if (L_TODO /= "0000") then          -- transmitting
                    Q_TX <= L_BUF(0);               -- next bit
                    L_BUF     <= '1' & L_BUF(7 downto 1);
                    if (L_TODO = "0001") then
                        L_FLAG <= I_FLAG;
                    end if;
                    L_TODO <= L_TODO - "0001";
                elsif (L_FLAG /= I_FLAG) then       -- new byte
                    Q_TX <= '0';                    -- start bit
                    L_BUF <= I_DATA;                -- data bits
                    L_TODO <= "1001";
                end if;
            end if;
        end if;
    end process; 
 
    Q_FLAG <= L_FLAG;
 
end Behavioral;