Subversion Repositories light8080

--##############################################################################

-- RS-232 transmitter, parametrizable bit rate through generics.

-- Bit rate defaults to 19200 bps @50MHz.

-- WARNING: Hacked up for light8080 demo. Poor performance, no formal testing!

-- I don't advise using this in for any general purpose.

--##############################################################################

library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_arith.all;

use ieee.std_logic_unsigned.all;

entity rs232_tx is

  generic (

    BAUD_RATE     : integer := 19200;

    CLOCK_FREQ    : integer := 50000000);

  port (

      clk         : in std_logic;

      reset       : in std_logic;

      rdy         : out std_logic;

      load        : in std_logic;

      data_i      : in std_logic_vector(7 downto 0);

      txd         : out std_logic);

end rs232_tx;

architecture hardwired of rs232_tx is

-- Bit period expressed in master clock cycles

constant BIT_PERIOD : integer := (CLOCK_FREQ / BAUD_RATE);

signal counter :      std_logic_vector(13 downto 0);

signal data :         std_logic_vector(10 downto 0);

signal ctr_bit :      std_logic_vector(3 downto 0);

signal tx :           std_logic;

begin

process(clk)

begin

if clk'event and clk='1' then

  if reset='1' then

    data <= "10111111111";

    tx <= '0';

    ctr_bit <= "0000";

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

  elsif load='1' and tx='0' then

    data <= "1"&data_i&"01";

    tx <= '1';

  else

    if tx='1' then

      if conv_integer(counter) = BIT_PERIOD then --e.g. 5200 for 9600 bps

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

        data(9 downto 0) <= data(10 downto 1);

        data(10) <= '1';

        if ctr_bit = "1010" then

           tx <= '0';

           ctr_bit <= "0000";

        else

           ctr_bit <= ctr_bit + 1;

        end if;

      else

        counter <= counter + 1;

      end if;

    end if;

  end if;

end if;

end process;

rdy <= not tx;

txd <= data(0);

end hardwired;