library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity vhd is
    Port (
        clk_i : in std_logic;
        TXD_o : out std_logic := '1';
        RXD_i : in std_logic
        );
end vhd;

architecture Behavioral of vhd is
signal data : std_logic_vector(7 downto 0) := "00000000";
begin
process (clk_i)

variable counter : integer range 0 to 5208 := 0;
variable index : integer range -1 to 8 := 0;
variable receive : boolean := false;
variable send : boolean := false;
variable half : boolean := false;

begin

if rising_edge(clk_i) then
    counter := counter + 1;

    if (receive = false and RXD_i = '0') then
        counter := 0;
        index := 0;
        receive := true;
    end if;

    if (receive = true and half = false and counter > 2000 and counter < 3000) then
        half := true;
        counter := 0;
    end if;

    if (receive = true) then
        if (counter = 5208 and index < 8) then
            data(index) <= RXD_i;
            counter := 0;
            index := index + 1;
        elsif (counter = 5208 and index = 8 and RXD_i = '1') then
            counter := 0;
            index := -1;
            send := true;
            receive := false;
            --data <= data + x"20";
            half := false;
        elsif (counter = 5208 and index = 8 and RXD_i /= '1') then
            counter := 0;
            index := 0;
            receive := false;
        end if;
    end if;

    if (send = true) then
        if(index = -1) then
            TXD_o <= '0';
            index := index + 1;
            counter := 0;
        elsif(counter = 5208 and index < 8 and index > -1) then
            TXD_o <= data(index);
            counter := 0;
            index := index + 1;
        elsif (counter = 5208 and index = 8) then
            TXD_o <= '1';
            counter := 0;
            index := 0;
            send := false;
        end if;
    end if;

end if;
end process;
end Behavioral;