Library of reusable VHDL components
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-- ======================================================================
-- UART transmitter
-- Copyright (C) 2020 Torsten Meissner
-------------------------------------------------------------------------
-- This program 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.
--
-- This program 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 this program; if not, write to the Free Software Foundation,
-- Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA
-- ======================================================================
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
entity UartTx is
generic (
DATA_LENGTH : positive range 5 to 9 := 8;
PARITY : boolean := true;
CLK_DIV : natural := 10
);
port (
reset_n_i : in std_logic; -- async reset
clk_i : in std_logic; -- clock
data_i : in std_logic_vector(DATA_LENGTH-1 downto 0); -- data input
valid_i : in std_logic; -- input data valid
accept_o : out std_logic; -- inpit data accepted
tx_o : out std_logic -- uart tx data output
);
end entity UartTx;
architecture rtl of UartTx is
function odd_parity (data : in std_logic_vector(DATA_LENGTH-1 downto 0)) return std_logic is
variable v_data : std_logic := '0';
begin
for i in data'range loop
v_data := v_data xor data(i);
end loop;
return not v_data;
end function odd_parity;
function to_integer (data : in boolean) return integer is
begin
if data then
return 1;
else
return 0;
end if;
end function to_integer;
type t_uart_state is (IDLE, SEND);
signal s_uart_state : t_uart_state;
signal s_data : std_logic_vector(DATA_LENGTH+1+to_integer(PARITY) downto 0);
signal s_clk_en : boolean;
begin
ClkDivP : process (clk_i, reset_n_i) is
variable v_clk_cnt : natural range 0 to CLK_DIV-1;
begin
if (reset_n_i = '0') then
s_clk_en <= false;
v_clk_cnt := CLK_DIV-1;
elsif (rising_edge(clk_i)) then
if (s_uart_state = IDLE) then
v_clk_cnt := CLK_DIV-2;
s_clk_en <= false;
elsif (s_uart_state = SEND) then
if (v_clk_cnt = 0) then
v_clk_cnt := CLK_DIV-1;
s_clk_en <= true;
else
v_clk_cnt := v_clk_cnt - 1;
s_clk_en <= false;
end if;
end if;
end if;
end process ClkDivP;
TxP : process (clk_i, reset_n_i) is
variable v_bit_cnt : natural range 0 to s_data'length-1;
begin
if (reset_n_i = '0') then
s_uart_state <= IDLE;
s_data <= (0 => '1', others => '0');
accept_o <= '0';
v_bit_cnt := 0;
elsif (rising_edge(clk_i)) then
FsmL : case s_uart_state is
when IDLE =>
accept_o <= '1';
v_bit_cnt := s_data'length-1;
if (valid_i = '1' and accept_o = '1') then
accept_o <= '0';
if (PARITY) then
s_data <= '1' & odd_parity(data_i) & data_i & '0';
else
s_data <= '1' & data_i & '0';
end if;
s_uart_state <= SEND;
end if;
when SEND =>
if (s_clk_en) then
s_data <= '1' & s_data(s_data'length-1 downto 1);
if (v_bit_cnt = 0) then
accept_o <= '1';
s_uart_state <= IDLE;
else
v_bit_cnt := v_bit_cnt - 1;
end if;
end if;
end case;
end if;
end process TxP;
tx_o <= s_data(0);
end architecture rtl;