vhdl/lib/uart/juart_tx.vhd

-- UART Transmitter
--
-- Version : 1.00 
-- Version Date : 15 December 2016
--
------------------------------------------------------------------------------------
--
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.numeric_std.ALL;
--
------------------------------------------------------------------------------------
--
-- Main Entity
--
entity juart_tx is
    Port (        data_in : in unsigned(7 downto 0);
           send_character : in std_logic;
             en_16_x_baud : in std_logic;
               serial_out : out std_logic;
              Tx_complete : out std_logic;
                      clk : in std_logic;
                      rst : in std_logic
                      );
    end juart_tx;
--
------------------------------------------------------------------------------------
--
-- Start of Main Architecture
--	 
architecture behavior of juart_tx is
--
------------------------------------------------------------------------------------
--
------------------------------------------------------------------------------------
--
-- Signals used
--
------------------------------------------------------------------------------------
--
signal dinreg				: unsigned (7 downto 0);
signal shiftreg				: unsigned (9 downto 0);
signal busy					: std_logic;
signal start				: std_logic;
signal shift_enable			: std_logic;
signal shift_enable_count	: unsigned (3 downto 0);
signal bit_count			: unsigned (3 downto 0);

--
--
begin
serial_out	<= shiftreg(shiftreg'left);
Tx_complete	<= not busy;

shift_enable_counter:
process(clk)
begin
	if rising_edge(clk) then
		shift_enable <= '0';
		if rst = '1' then
			shift_enable_count <= (others => '1');
		elsif en_16_x_baud = '1' then
			if shift_enable_count = 0 then
				shift_enable	<= '1';
				shift_enable_count <= (others => '1');
			else
				shift_enable_count <= shift_enable_count - 1;
			end if;
		end if;
	end if;
end process;
	
bit_counter:
process(clk)
begin
	if rising_edge(clk) then
		if start = '1' then
			bit_count <= to_unsigned(shiftreg'length-1, bit_count'length);
		elsif shift_enable = '1' then
			if not (bit_count = 0) then
				bit_count <= bit_count - 1;
			end if;
		end if;
	end if;
end process;

transmit:
process(clk)
begin
	if rising_edge(clk) then
		if rst = '1' then
			shiftreg <= (others => '1');
			busy	<= '0';
			start	<= '0';
		elsif busy = '0' then
			if send_character = '1' then
				busy		<= '1';
				start		<= '1';
				dinreg		<= data_in;
			end if;
		elsif shift_enable = '1' then
			if start = '1' then
				start		<= '0';
				shiftreg	<= '0' & dinreg & '1';
			elsif bit_count /= 0 then
				shiftreg	<= shiftreg(shiftreg'left-1 downto 0) & '1';
				if bit_count = 1 then
					busy	<= '0';
				end if;

			end if;
		end if;
	end if;
end process;

end behavior;

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--
-- END OF FILE
--
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