library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.NUMERIC_STD.ALL;


entity uart_transmit is
	Port ( 
			clk : in  STD_LOGIC;
			chip_enable : in std_logic;
			to_send : in std_logic_vector(7 downto 0);
			send_enable : in std_logic;
			ser_in : in std_logic; 
			ser_out : out STD_LOGIC;
			transmit_finished : out std_logic;
			received_uart : out unsigned(7 downto 0); 
			received_uart_valid : out std_logic
		);		 
end uart_transmit;


architecture Behavioral of uart_transmit is

	signal send_slow : unsigned (5 downto 0) := (others=>'0');
	signal send_counter : unsigned (3 downto 0)  := "1011";		-- "1011" -> idle
	signal ser_out_vector : std_logic_vector (8 downto 0) := (others=>'1');
	signal enabled : std_logic := '0';
	signal direction : std_logic := '0';


begin

	uart_transmit: PROCESS (clk)
	BEGIN
		IF RISING_EDGE(clk) THEN
		  
		  if send_slow = "110110" then -- 115200 baud
			  send_slow <= (others=>'0');
			else
			  send_slow <= send_slow + 1;
			end if;
			  
			if ser_in='0' and direction='0' then -- incoming byte
			  enabled <= '1';
			  direction<='1';
		     send_slow <= "011010"; -- middle of bit
		     send_counter<="1011";
			else
			   if send_counter="1011" and direction='0' and send_enable='1' then  -- send when idle and request
		      enabled <= '1';
		      ser_out_vector <= to_send & '1' ;
			  end if;
			end if;
			 
			if direction = '0' and chip_enable='1' then
					
			 if send_slow = "110101" then
				
			   if send_counter="1011" and enabled='1' then -- start transmit
						send_counter <= "0001";
						enabled <= '0';
				 else
				
				  if not(send_counter="1011") then -- transmitted all bits
						if send_counter="0001" then -- transmit start bit
							ser_out_vector <= ser_out_vector(8 downto 1) & '0';
						else
							ser_out_vector <= '1' & ser_out_vector(8 downto 1); -- transmit next bit
						end if;
						send_counter <= send_counter + 1;
					end if;
					
			   end if; 
			 end if;
					
			else
					
			 if send_slow = "110101" then
				
				 ser_out_vector(7 downto 0) <= ser_in & ser_out_vector(7 downto 1);
				
			   if send_counter="1011" and enabled='1' then -- start receive
						send_counter <= "0001";
						enabled <= '0';
				 else
				
				  if not(send_counter="1010") then -- all bits received  
						send_counter <= send_counter + 1;
					else
					  send_counter <= "1011";
					  direction <= '0';
					end if;
					
			   end if; 
			 end if;
			  
			  
			end if;
							
		END IF;
	END PROCESS uart_transmit;
	
	ser_out <= ser_out_vector(0) when direction='0' and (not(send_counter="1011")) else '1';
	transmit_finished <= '1' when send_counter="1011" and enabled='0' else '0';
	
	received_uart <= unsigned(ser_out_vector(7 downto 0));
	received_uart_valid <= '1' when send_slow = "011010" and send_counter="1001" and direction='1' else '0'; 


end Behavioral;