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-- Copyright 2015, Jürgen Defurne

--

-- This file is part of the Experimental Unstable CPU System.

--

-- The Experimental Unstable CPU System Is free software: you can redistribute

-- it and/or modify it under the terms of the GNU Lesser General Public License

-- as published by the Free Software Foundation, either version 3 of the

-- License, or (at your option) any later version.

--

-- The Experimental Unstable CPU System 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 Lesser

-- General Public License for more details.

--

-- You should have received a copy of the GNU Lesser General Public License

-- along with Experimental Unstable CPU System. If not, see

-- http://www.gnu.org/licenses/lgpl.txt.

LIBRARY IEEE;

USE IEEE.STD_LOGIC_1164.ALL;

USE work.system_package.ALL;

ENTITY tty_in IS

  PORT (

    reset    : IN  STD_LOGIC;

    clock    : IN  STD_LOGIC;

    rd       : IN  STD_LOGIC;

    wr       : IN  STD_LOGIC;

    data_in  : IN  STD_LOGIC_VECTOR(15 DOWNTO 0);

    data_out : OUT STD_LOGIC_VECTOR(15 DOWNTO 0);

    address  : IN  STD_LOGIC_VECTOR(14 DOWNTO 0);

    rx       : IN  STD_LOGIC);

END tty_in;

ARCHITECTURE Behavioral OF tty_in IS

  SIGNAL data   : STD_LOGIC_VECTOR(7 DOWNTO 0);

  SIGNAL status : STD_LOGIC_VECTOR(7 DOWNTO 0);

  SIGNAL shifter : STD_LOGIC_VECTOR(9 DOWNTO 0);

  SIGNAL count   : INTEGER RANGE 0 TO 9;

BEGIN

  -- purpose: Serially read in data

  -- type   : sequential

  -- inputs : clock, reset

  -- outputs: 

  serial_input : PROCESS (clock, reset)

  BEGIN  -- PROCESS read

    IF reset = '0' THEN                 -- asynchronous reset (active low)

      data    <= X"00";

      status  <= X"00";

      shifter <= "0000000000";

      count   <= 0;

    ELSIF rising_edge(clock) THEN       -- rising clock edge

      IF count = 9 THEN

        data    <= shifter(8 DOWNTO 1);

        status  <= X"01";

        count   <= 0;

        shifter <= rx & shifter(9 DOWNTO 1);

      ELSE

        IF address = "111111100000000" AND rd = '1' THEN

          status <= X"00";

        ELSE

          status <= status;

        END IF;

        data    <= data;

        count   <= count + 1;

        shifter <= rx & shifter(9 DOWNTO 1);

      END IF;

    END IF;

  END PROCESS serial_input;

  data_out <= status & data;

END Behavioral;