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-- #################################################################################################

-- # << NEORV32 - Universal Asynchronous Receiver and Transmitter (UART0/1) >>                     #

-- # ********************************************************************************************* #

-- # Frame configuration: 1 start bit, 8 bit data, parity bit (none/even/odd), 1 stop bit,         #

-- # programmable BAUD rate via clock pre-scaler and 12-bit BAUD value configuration register,     #

-- # optional configurable RX and TX FIFOs.                                                        #

-- #                                                                                               #

-- # Interrupts: Configurable RX and TX interrupt (both triggered by specific FIFO fill-levels)    #

-- #                                                                                               #

-- # Support for RTS("RTR")/CTS hardware flow control:                                             #

-- # * uart_rts_o = 0: RX is ready to receive a new char, enabled via CTRL.ctrl_rts_en_c           #

-- # * uart_cts_i = 0: TX is allowed to send a new char, enabled via CTRL.ctrl_cts_en_c            #

-- #                                                                                               #

-- # UART0 / UART1:                                                                                #

-- # This module is used for implementing UART0 and UART1. The UART_PRIMARY generic configures the #

-- # interface register addresses and simulation outputs for UART0 (UART_PRIMARY = true) or UART1  #

-- # (UART_PRIMARY = false).                                                                       #

-- #                                                                                               #

-- # SIMULATION MODE:                                                                              #

-- # When the simulation mode is enabled (setting the ctrl.ctrl_sim_en_c bit) any write            #

-- # access to the TX register will not trigger any UART activity. Instead, the written data is    #

-- # output to the simulation environment. The lowest 8 bits of the written data are printed as    #

-- # ASCII char to the simulator console.                                                          #

-- # This char is also stored to the file "neorv32.uartX.sim_mode.text.out" (where X = 0 for UART0 #

-- # and X = 1 for UART1). The full 32-bit write data is also stored as 8-digit hexadecimal value  #

-- # to the file "neorv32.uartX.sim_mode.data.out" (where X = 0 for UART0 and X = 1 for UART1).    #

-- # No interrupts are triggered when in SIMULATION MODE.                                          #

-- # ********************************************************************************************* #

-- # BSD 3-Clause License                                                                          #

-- #                                                                                               #

-- # Copyright (c) 2021, Stephan Nolting. All rights reserved.                                     #

-- #                                                                                               #

-- # Redistribution and use in source and binary forms, with or without modification, are          #

-- # permitted provided that the following conditions are met:                                     #

-- #                                                                                               #

-- # 1. Redistributions of source code must retain the above copyright notice, this list of        #

-- #    conditions and the following disclaimer.                                                   #

-- #                                                                                               #

-- # 2. Redistributions in binary form must reproduce the above copyright notice, this list of     #

-- #    conditions and the following disclaimer in the documentation and/or other materials        #

-- #    provided with the distribution.                                                            #

-- #                                                                                               #

-- # 3. Neither the name of the copyright holder nor the names of its contributors may be used to  #

-- #    endorse or promote products derived from this software without specific prior written      #

-- #    permission.                                                                                #

-- #                                                                                               #

-- # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS   #

-- # OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF               #

-- # MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE    #

-- # COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,     #

-- # EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE #

-- # GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED    #

-- # AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING     #

-- # NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED  #

-- # OF THE POSSIBILITY OF SUCH DAMAGE.                                                            #

-- # ********************************************************************************************* #

-- # The NEORV32 Processor - https://github.com/stnolting/neorv32              (c) Stephan Nolting #

-- #################################################################################################

library ieee;

use ieee.std_logic_1164.all;

use ieee.numeric_std.all;

library neorv32;

use neorv32.neorv32_package.all;

use std.textio.all;

entity neorv32_uart is

  generic (

    UART_PRIMARY : boolean; -- true = primary UART (UART0), false = secondary UART (UART1)

    UART_RX_FIFO : natural; -- RX fifo depth, has to be a power of two, min 1

    UART_TX_FIFO : natural  -- TX fifo depth, has to be a power of two, min 1

  );

  port (

    -- host access --

    clk_i       : in  std_ulogic; -- global clock line

    addr_i      : in  std_ulogic_vector(31 downto 0); -- address

    rden_i      : in  std_ulogic; -- read enable

    wren_i      : in  std_ulogic; -- write enable

    data_i      : in  std_ulogic_vector(31 downto 0); -- data in

    data_o      : out std_ulogic_vector(31 downto 0); -- data out

    ack_o       : out std_ulogic; -- transfer acknowledge

    -- clock generator --

    clkgen_en_o : out std_ulogic; -- enable clock generator

    clkgen_i    : in  std_ulogic_vector(07 downto 0);

    -- com lines --

    uart_txd_o  : out std_ulogic;

    uart_rxd_i  : in  std_ulogic;

    -- hardware flow control --

    uart_rts_o  : out std_ulogic; -- UART.RX ready to receive ("RTR"), low-active, optional

    uart_cts_i  : in  std_ulogic; -- UART.TX allowed to transmit, low-active, optional

    -- interrupts --

    irq_rxd_o   : out std_ulogic; -- uart data received interrupt

    irq_txd_o   : out std_ulogic  -- uart transmission done interrupt

  );

end neorv32_uart;

architecture neorv32_uart_rtl of neorv32_uart is

  -- interface configuration for UART0 / UART1 --

  constant uart_id_base_c      : std_ulogic_vector(data_width_c-1 downto 0) := cond_sel_stdulogicvector_f(UART_PRIMARY, uart0_base_c,      uart1_base_c);

  constant uart_id_size_c      : natural                                    := cond_sel_natural_f(        UART_PRIMARY, uart0_size_c,      uart1_size_c);

  constant uart_id_ctrl_addr_c : std_ulogic_vector(data_width_c-1 downto 0) := cond_sel_stdulogicvector_f(UART_PRIMARY, uart0_ctrl_addr_c, uart1_ctrl_addr_c);

  constant uart_id_rtx_addr_c  : std_ulogic_vector(data_width_c-1 downto 0) := cond_sel_stdulogicvector_f(UART_PRIMARY, uart0_rtx_addr_c,  uart1_rtx_addr_c);

  -- IO space: module base address --

  constant hi_abb_c : natural := index_size_f(io_size_c)-1; -- high address boundary bit

  constant lo_abb_c : natural := index_size_f(uart_id_size_c); -- low address boundary bit

  -- simulation output configuration --

  constant sim_screen_output_en_c : boolean := true; -- output lowest byte as char to simulator console when enabled

  constant sim_text_output_en_c   : boolean := true; -- output lowest byte as char to text file when enabled

  constant sim_data_output_en_c   : boolean := true; -- dump 32-word to file when enabled

  constant sim_uart_text_file_c   : string  := cond_sel_string_f(UART_PRIMARY, "neorv32.uart0.sim_mode.text.out", "neorv32.uart1.sim_mode.text.out");

  constant sim_uart_data_file_c   : string  := cond_sel_string_f(UART_PRIMARY, "neorv32.uart0.sim_mode.data.out", "neorv32.uart1.sim_mode.data.out");

  -- control register --

  signal ctrl : std_ulogic_vector(31 downto 0);

  -- control register bits --

  constant ctrl_baud00_c   : natural :=  0; -- r/w: baud config bit 0

  constant ctrl_baud01_c   : natural :=  1; -- r/w: baud config bit 1

  constant ctrl_baud02_c   : natural :=  2; -- r/w: baud config bit 2

  constant ctrl_baud03_c   : natural :=  3; -- r/w: baud config bit 3

  constant ctrl_baud04_c   : natural :=  4; -- r/w: baud config bit 4

  constant ctrl_baud05_c   : natural :=  5; -- r/w: baud config bit 5

  constant ctrl_baud06_c   : natural :=  6; -- r/w: baud config bit 6

  constant ctrl_baud07_c   : natural :=  7; -- r/w: baud config bit 7

  constant ctrl_baud08_c   : natural :=  8; -- r/w: baud config bit 8

  constant ctrl_baud09_c   : natural :=  9; -- r/w: baud config bit 9

  constant ctrl_baud10_c   : natural := 10; -- r/w: baud config bit 10

  constant ctrl_baud11_c   : natural := 11; -- r/w: baud config bit 11

  constant ctrl_sim_en_c   : natural := 12; -- r/w: UART <<SIMULATION MODE>> enable

  constant ctrl_rx_empty_c : natural := 13; -- r/-: RX FIFO is empty

  constant ctrl_rx_half_c  : natural := 14; -- r/-: RX FIFO is at least half-full

  constant ctrl_rx_full_c  : natural := 15; -- r/-: RX FIFO is full

  constant ctrl_tx_empty_c : natural := 16; -- r/-: TX FIFO is empty

  constant ctrl_tx_half_c  : natural := 17; -- r/-: TX FIFO is at least half-full

  constant ctrl_tx_full_c  : natural := 18; -- r/-: TX FIFO is full

  -- ...

  constant ctrl_rts_en_c   : natural := 20; -- r/w: enable hardware flow control: assert rts_o if ready to receive

  constant ctrl_cts_en_c   : natural := 21; -- r/w: enable hardware flow control: send only if cts_i is asserted

  constant ctrl_pmode0_c   : natural := 22; -- r/w: Parity config (0=even; 1=odd)

  constant ctrl_pmode1_c   : natural := 23; -- r/w: Enable parity bit

  constant ctrl_prsc0_c    : natural := 24; -- r/w: baud prsc bit 0

  constant ctrl_prsc1_c    : natural := 25; -- r/w: baud prsc bit 1

  constant ctrl_prsc2_c    : natural := 26; -- r/w: baud prsc bit 2

  constant ctrl_cts_c      : natural := 27; -- r/-: current state of CTS input

  constant ctrl_en_c       : natural := 28; -- r/w: UART enable

  constant ctrl_rx_irq_c   : natural := 29; -- r/w: RX IRQ mode: 1=FIFO at least half-full; 0=FIFO not empty

  constant ctrl_tx_irq_c   : natural := 30; -- r/w: TX IRQ mode: 1=FIFO less than half-full; 0=FIFO not full

  constant ctrl_tx_busy_c  : natural := 31; -- r/-: UART transmitter is busy

  -- data register flags --

  constant data_lsb_c      : natural :=  0; -- r/-: received char LSB

  constant data_msb_c      : natural :=  7; -- r/-: received char MSB

  -- ...

  constant data_rx_perr_c  : natural := 28; -- r/-: RX parity error

  constant data_rx_ferr_c  : natural := 29; -- r/-: RX frame error

  constant data_rx_overr_c : natural := 30; -- r/-: RX data overrun

  constant data_rx_avail_c : natural := 31; -- r/-: RX data available

  -- access control --

  signal acc_en : std_ulogic; -- module access enable

  signal addr   : std_ulogic_vector(31 downto 0); -- access address

  signal wr_en  : std_ulogic; -- word write enable

  signal rd_en  : std_ulogic; -- read enable

  -- clock generator --

  signal uart_clk : std_ulogic;

  -- numbers of bits in transmission frame --

  signal num_bits : std_ulogic_vector(03 downto 0);

  -- hardware flow-control IO buffer --

  signal uart_cts_ff : std_ulogic_vector(1 downto 0);

  signal uart_rts    : std_ulogic;

  -- UART transmitter --

  type tx_state_t is (S_TX_IDLE, S_TX_GET, S_TX_CHECK, S_TX_TRANSMIT, S_TX_SIM);

  type tx_engine_t is record

    state    : tx_state_t;

    busy     : std_ulogic;

    bitcnt   : std_ulogic_vector(03 downto 0);

    sreg     : std_ulogic_vector(10 downto 0);

    baud_cnt : std_ulogic_vector(11 downto 0);

    cts      : std_ulogic; -- allow new transmission when 1

  end record;

  signal tx_engine : tx_engine_t;

  -- UART receiver --

  type rx_state_t is (S_RX_IDLE, S_RX_RECEIVE);

  type rx_engine_t is record

    state    : rx_state_t;

    sync     : std_ulogic_vector(04 downto 0);

    bitcnt   : std_ulogic_vector(03 downto 0);

    sreg     : std_ulogic_vector(09 downto 0);

    baud_cnt : std_ulogic_vector(11 downto 0);

    overr    : std_ulogic;

    rtr      : std_ulogic; -- ready to receive when 1

  end record;

  signal rx_engine : rx_engine_t;

  -- TX FIFO --

  type tx_buffer_t is record

    we    : std_ulogic; -- write enable

    re    : std_ulogic; -- read enable

    clear : std_ulogic; -- sync reset, high-active

    wdata : std_ulogic_vector(31 downto 0); -- write data

    rdata : std_ulogic_vector(31 downto 0); -- read data

    avail : std_ulogic; -- data available?

    free  : std_ulogic; -- free entry available?

    half  : std_ulogic; -- half full

  end record;

  signal tx_buffer : tx_buffer_t;

  -- RX FIFO --

  type rx_buffer_t is record

    we    : std_ulogic; -- write enable

    re    : std_ulogic; -- read enable

    clear : std_ulogic; -- sync reset, high-active

    wdata : std_ulogic_vector(9 downto 0); -- write data

    rdata : std_ulogic_vector(9 downto 0); -- read data

    avail : std_ulogic; -- data available?

    free  : std_ulogic; -- free entry available?

    half  : std_ulogic; -- half full

  end record;

  signal rx_buffer : rx_buffer_t;

begin

  -- Sanity Checks --------------------------------------------------------------------------

  -- -------------------------------------------------------------------------------------------

  assert not (is_power_of_two_f(UART_RX_FIFO) = false) report "NEORV32 PROCESSOR CONFIG ERROR: UART" &

  cond_sel_string_f(UART_PRIMARY, "0", "1") & " <UART_RX_FIFO> has to be a power of two." severity error;

  assert not (is_power_of_two_f(UART_TX_FIFO) = false) report "NEORV32 PROCESSOR CONFIG ERROR: UART" &

  cond_sel_string_f(UART_PRIMARY, "0", "1") & " <UART_TX_FIFO> has to be a power of two." severity error;

  -- Access Control -------------------------------------------------------------------------

  -- -------------------------------------------------------------------------------------------

  acc_en <= '1' when (addr_i(hi_abb_c downto lo_abb_c) = uart_id_base_c(hi_abb_c downto lo_abb_c)) else '0';

  addr   <= uart_id_base_c(31 downto lo_abb_c) & addr_i(lo_abb_c-1 downto 2) & "00"; -- word aligned

  wr_en  <= acc_en and wren_i;

  rd_en  <= acc_en and rden_i;

  -- Read/Write Access ----------------------------------------------------------------------

  -- -------------------------------------------------------------------------------------------

  rw_access: process(clk_i)

  begin

    if rising_edge(clk_i) then

      -- bus access acknowledge --

      ack_o <= wr_en or rd_en;

      -- write access --

      if (wr_en = '1') then

        if (addr = uart_id_ctrl_addr_c) then

          ctrl <= (others => '0');

          ctrl(ctrl_baud11_c downto ctrl_baud00_c) <= data_i(ctrl_baud11_c downto ctrl_baud00_c);

          ctrl(ctrl_sim_en_c)                      <= data_i(ctrl_sim_en_c);

          ctrl(ctrl_pmode1_c downto ctrl_pmode0_c) <= data_i(ctrl_pmode1_c downto ctrl_pmode0_c);

          ctrl(ctrl_prsc2_c  downto ctrl_prsc0_c)  <= data_i(ctrl_prsc2_c  downto ctrl_prsc0_c);

          ctrl(ctrl_rts_en_c)                      <= data_i(ctrl_rts_en_c);

          ctrl(ctrl_cts_en_c)                      <= data_i(ctrl_cts_en_c);

          ctrl(ctrl_rx_irq_c)                      <= data_i(ctrl_rx_irq_c);

          ctrl(ctrl_tx_irq_c)                      <= data_i(ctrl_tx_irq_c);

          ctrl(ctrl_en_c)                          <= data_i(ctrl_en_c);

        end if;

      end if;

      -- read access --

      data_o <= (others => '0');

      if (rd_en = '1') then

        if (addr = uart_id_ctrl_addr_c) then

          data_o(ctrl_baud11_c downto ctrl_baud00_c) <= ctrl(ctrl_baud11_c downto ctrl_baud00_c);

          data_o(ctrl_sim_en_c)                      <= ctrl(ctrl_sim_en_c);

          data_o(ctrl_pmode1_c downto ctrl_pmode0_c) <= ctrl(ctrl_pmode1_c downto ctrl_pmode0_c);

          data_o(ctrl_prsc2_c  downto ctrl_prsc0_c)  <= ctrl(ctrl_prsc2_c  downto ctrl_prsc0_c);

          data_o(ctrl_rts_en_c)                      <= ctrl(ctrl_rts_en_c);

          data_o(ctrl_cts_en_c)                      <= ctrl(ctrl_cts_en_c);

          data_o(ctrl_rx_empty_c)                    <= not rx_buffer.avail;

          data_o(ctrl_rx_half_c)                     <= rx_buffer.half;

          data_o(ctrl_rx_full_c)                     <= not rx_buffer.free;

          data_o(ctrl_tx_empty_c)                    <= not tx_buffer.avail;

          data_o(ctrl_tx_half_c)                     <= tx_buffer.half;

          data_o(ctrl_tx_full_c)                     <= not tx_buffer.free;

          data_o(ctrl_en_c)                          <= ctrl(ctrl_en_c);

          data_o(ctrl_rx_irq_c)                      <= ctrl(ctrl_rx_irq_c) and bool_to_ulogic_f(boolean(UART_RX_FIFO > 1)); -- tie to zero if UART_RX_FIFO = 1

          data_o(ctrl_tx_irq_c)                      <= ctrl(ctrl_tx_irq_c) and bool_to_ulogic_f(boolean(UART_TX_FIFO > 1)); -- tie to zero if UART_TX_FIFO = 1

          data_o(ctrl_tx_busy_c)                     <= tx_engine.busy;

          data_o(ctrl_cts_c)                         <= uart_cts_ff(1);

        else -- uart_id_rtx_addr_c

          data_o(data_msb_c downto data_lsb_c) <= rx_buffer.rdata(7 downto 0);

          data_o(data_rx_perr_c)               <= rx_buffer.rdata(8);

          data_o(data_rx_ferr_c)               <= rx_buffer.rdata(9);

          data_o(data_rx_overr_c)              <= rx_engine.overr;

          data_o(data_rx_avail_c)              <= rx_buffer.avail; -- data available (valid?)

        end if;

      end if;

    end if;

  end process rw_access;

  -- number of bits to be sampled --

  -- if parity flag is ENABLED:  11 bit -> "1011" (1 start bit + 8 data bits + 1 parity bit + 1 stop bit)

  -- if parity flag is DISABLED: 10 bit -> "1010" (1 start bit + 8 data bits + 1 stop bit)

  num_bits <= "1011" when (ctrl(ctrl_pmode1_c) = '1') else "1010";

  -- Clock Selection ------------------------------------------------------------------------

  -- -------------------------------------------------------------------------------------------

  -- clock enable --

  clkgen_en_o <= ctrl(ctrl_en_c);

  -- uart clock select --

  uart_clk <= clkgen_i(to_integer(unsigned(ctrl(ctrl_prsc2_c downto ctrl_prsc0_c))));

  -- TX FIFO --------------------------------------------------------------------------------

  -- -------------------------------------------------------------------------------------------

  tx_engine_fifo_inst: neorv32_fifo

  generic map (

    FIFO_DEPTH => UART_TX_FIFO, -- number of fifo entries; has to be a power of two; min 1

    FIFO_WIDTH => 32,           -- size of data elements in fifo (32-bit only for simulation)

    FIFO_RSYNC => false,        -- async read

    FIFO_SAFE  => true          -- safe access

  )

  port map (

    -- control --

    clk_i   => clk_i,           -- clock, rising edge

    rstn_i  => '1',             -- async reset, low-active

    clear_i => tx_buffer.clear, -- sync reset, high-active

    level_o => open,

    half_o  => tx_buffer.half,  -- FIFO at least half-full

    -- write port --

    wdata_i => tx_buffer.wdata, -- write data

    we_i    => tx_buffer.we,    -- write enable

    free_o  => tx_buffer.free,  -- at least one entry is free when set

    -- read port --

    re_i    => tx_buffer.re,    -- read enable

    rdata_o => tx_buffer.rdata, -- read data

    avail_o => tx_buffer.avail  -- data available when set

  );

  -- control --

  tx_buffer.clear <= not ctrl(ctrl_en_c);

  -- write access --

  tx_buffer.we    <= '1' when (wr_en = '1') and (addr = uart_id_rtx_addr_c) else '0';

  tx_buffer.wdata <= data_i;

  -- UART Transmitter Engine ----------------------------------------------------------------

  -- -------------------------------------------------------------------------------------------

  uart_tx_engine: process(clk_i)

  begin

    if rising_edge(clk_i) then

      -- defaults --

      uart_txd_o   <= '1'; -- keep TX line idle (=high) if waiting for permission to start sending (->CTS)

      tx_buffer.re <= '0';

      -- FSM --

      if (ctrl(ctrl_en_c) = '0') then -- disabled

        tx_engine.state <= S_TX_IDLE;

      else

        case tx_engine.state is

          when S_TX_IDLE => -- wait for new data to send

          -- ------------------------------------------------------------

            if (tx_buffer.avail = '1') then -- new data available

              if (ctrl(ctrl_sim_en_c) = '0') then -- normal mode

                tx_engine.state <= S_TX_GET;

              else -- simulation mode

                tx_engine.state <= S_TX_SIM;

              end if;

              tx_buffer.re <= '1';

            end if;

          when S_TX_GET => -- get new data from buffer and prepare transmission

          -- ------------------------------------------------------------

            tx_engine.baud_cnt <= ctrl(ctrl_baud11_c downto ctrl_baud00_c);

            tx_engine.bitcnt   <= num_bits;

            if (ctrl(ctrl_pmode1_c) = '1') then -- add parity flag

              -- stop bit & parity bit & data (8-bit) & start bit

              tx_engine.sreg <= '1' & (xor_reduce_f(tx_buffer.rdata(7 downto 0)) xor ctrl(ctrl_pmode0_c)) & tx_buffer.rdata(7 downto 0) & '0';

            else

              -- (dummy fill-bit &) stop bit & data (8-bit) & start bit

              tx_engine.sreg <= '1' & '1' & tx_buffer.rdata(7 downto 0) & '0';

            end if;

            tx_engine.state <= S_TX_CHECK;

          when S_TX_CHECK => -- check if allowed to send

          -- ------------------------------------------------------------

            if (tx_engine.cts = '1') then -- clear to send

              tx_engine.state <= S_TX_TRANSMIT;

            end if;

          when S_TX_TRANSMIT => -- transmit data

          -- ------------------------------------------------------------

            if (uart_clk = '1') then

              if (tx_engine.baud_cnt = x"000") then

                tx_engine.baud_cnt <= ctrl(ctrl_baud11_c downto ctrl_baud00_c);

                tx_engine.bitcnt   <= std_ulogic_vector(unsigned(tx_engine.bitcnt) - 1);

                tx_engine.sreg     <= '1' & tx_engine.sreg(tx_engine.sreg'left downto 1);

              else

                tx_engine.baud_cnt <= std_ulogic_vector(unsigned(tx_engine.baud_cnt) - 1);

              end if;

            end if;

            uart_txd_o <= tx_engine.sreg(0);

            if (tx_engine.bitcnt = "0000") then -- all bits send?

              tx_engine.state <= S_TX_IDLE;

            end if;

          when S_TX_SIM => -- simulation mode output

          -- ------------------------------------------------------------

            tx_engine.state <= S_TX_IDLE;

          when others => -- undefined

          -- ------------------------------------------------------------

            tx_engine.state <= S_TX_IDLE;

        end case;

      end if;

    end if;

  end process uart_tx_engine;

  -- transmitter busy --

  tx_engine.busy <= '0' when (tx_engine.state = S_TX_IDLE) else '1';

  -- UART Receiver Engine -------------------------------------------------------------------

  -- -------------------------------------------------------------------------------------------

  uart_rx_engine: process(clk_i)

  begin

    if rising_edge(clk_i) then

      -- input synchronizer --

      rx_engine.sync <= uart_rxd_i & rx_engine.sync(4 downto 1);

      -- FSM --

      if (ctrl(ctrl_en_c) = '0') then -- disabled

        rx_engine.overr <= '0';

        rx_engine.state <= S_RX_IDLE;

      else

        case rx_engine.state is

          when S_RX_IDLE => -- idle; prepare receive

          -- ------------------------------------------------------------

            rx_engine.baud_cnt <= '0' & ctrl(ctrl_baud11_c downto ctrl_baud01_c); -- half baud delay at the beginning to sample in the middle of each bit

            rx_engine.bitcnt   <= num_bits;

            if (rx_engine.sync(2 downto 0) = "001") then -- start bit? (falling edge)

              rx_engine.state <= S_RX_RECEIVE;

            end if;

          when S_RX_RECEIVE => -- receive data

          -- ------------------------------------------------------------

            if (uart_clk = '1') then

              if (rx_engine.baud_cnt = x"000") then

                rx_engine.baud_cnt <= ctrl(ctrl_baud11_c downto ctrl_baud00_c);

                rx_engine.bitcnt   <= std_ulogic_vector(unsigned(rx_engine.bitcnt) - 1);

                rx_engine.sreg     <= rx_engine.sync(0) & rx_engine.sreg(rx_engine.sreg'left downto 1);

              else

                rx_engine.baud_cnt <= std_ulogic_vector(unsigned(rx_engine.baud_cnt) - 1);

              end if;

            end if;

            if (rx_engine.bitcnt = "0000") then -- all bits received?

              rx_engine.state <= S_RX_IDLE;

            end if;

          when others => -- undefined

          -- ------------------------------------------------------------

            rx_engine.state <= S_RX_IDLE;

        end case;

        -- overrun flag --

        if (rd_en = '1') and (addr = uart_id_rtx_addr_c) then -- clear when reading data register

          rx_engine.overr <= '1';

        elsif (rx_buffer.we = '1') and (rx_buffer.free = '0') then -- write to full FIFO

          rx_engine.overr <= '0';

        end if;

      end if;

    end if;

  end process uart_rx_engine;

  -- RX engine ready for a new char? --

  rx_engine.rtr <= '1' when (rx_engine.state = S_RX_IDLE) and (ctrl(ctrl_en_c) = '1') else '0';

  -- RX FIFO --------------------------------------------------------------------------------

  -- -------------------------------------------------------------------------------------------

  rx_engine_fifo_inst: neorv32_fifo

  generic map (

    FIFO_DEPTH => UART_RX_FIFO, -- number of fifo entries; has to be a power of two; min 1

    FIFO_WIDTH => 10,           -- size of data elements in fifo

    FIFO_RSYNC => false,        -- async read

    FIFO_SAFE  => true          -- safe access

  )

  port map (

    -- control --

    clk_i   => clk_i,           -- clock, rising edge

    rstn_i  => '1',             -- async reset, low-active

    clear_i => rx_buffer.clear, -- sync reset, high-active

    level_o => open,

    half_o  => rx_buffer.half,  -- FIFO at least half-full

    -- write port --

    wdata_i => rx_buffer.wdata, -- write data

    we_i    => rx_buffer.we,    -- write enable

    free_o  => rx_buffer.free,  -- at least one entry is free when set

    -- read port --

    re_i    => rx_buffer.re,    -- read enable

    rdata_o => rx_buffer.rdata, -- read data

    avail_o => rx_buffer.avail  -- data available when set

  );

  -- control --

  rx_buffer.clear <= not ctrl(ctrl_en_c);

  -- read/write access --

  rx_buffer.wdata(7 downto 0) <= rx_engine.sreg(7 downto 0) when (ctrl(ctrl_pmode1_c) = '1') else rx_engine.sreg(8 downto 1); -- RX data

  rx_buffer.wdata(8) <= ctrl(ctrl_pmode1_c) and (xor_reduce_f(rx_engine.sreg(8 downto 0)) xor ctrl(ctrl_pmode0_c)); -- parity error flag

  rx_buffer.wdata(9) <= not rx_engine.sreg(9); -- frame error flag: check stop bit (error if not set)

  rx_buffer.we <= '1' when (rx_engine.bitcnt = "0000") and (rx_engine.state = S_RX_RECEIVE) else '0'; -- RX complete

  rx_buffer.re <= '1' when (rd_en = '1') and (addr = uart_id_rtx_addr_c) else '0';

  -- Hardware Flow Control ------------------------------------------------------------------

  -- -------------------------------------------------------------------------------------------

  tx_engine.cts <= (not uart_cts_ff(1)) when (ctrl(ctrl_cts_en_c) = '1') else '1'; -- input is low-active, internal signal is high-active

  uart_rts      <= (not rx_engine.rtr)  when (ctrl(ctrl_rts_en_c) = '1') else '0'; -- output is low-active

  -- flow-control input/output synchronizer --

  flow_control_buffer: process(clk_i)

  begin

    if rising_edge(clk_i) then -- should be mapped to IOBs

      uart_cts_ff <= uart_cts_ff(0) & uart_cts_i;

      uart_rts_o  <= uart_rts;

    end if;

  end process flow_control_buffer;

  -- Interrupts -----------------------------------------------------------------------------

  -- -------------------------------------------------------------------------------------------

  irq_generator: process(clk_i)

  begin

    if rising_edge(clk_i) then

      if (ctrl(ctrl_en_c) = '0') then -- no interrupts when disabled

        irq_txd_o <= '0';

        irq_rxd_o <= '0';

      else

        -- TX interrupt --

        if (UART_TX_FIFO = 1) then

          irq_txd_o <= tx_buffer.free; -- fire IRQ if FIFO is not full

        else

          if (ctrl(ctrl_tx_irq_c) = '1') then

            irq_txd_o <= not tx_buffer.half; -- fire IRQ if FIFO is less than half-full

          else

            irq_txd_o <= tx_buffer.free; -- fire IRQ if FIFO is not full

          end if;

        end if;

        -- RX interrupt --

        if (UART_RX_FIFO = 1) then

          irq_rxd_o <= rx_buffer.avail; -- fire IRQ if FIFO is not empty

        else

          if (ctrl(ctrl_rx_irq_c) = '1') then

            irq_rxd_o <= rx_buffer.half; -- fire IRQ if FIFO is at least half-full

          else

            irq_rxd_o <= rx_buffer.avail; -- fire IRQ is FIFO is not empty

          end if;

        end if;

      end if;

    end if;

  end process irq_generator;

  -- SIMULATION Transmitter -----------------------------------------------------------------

  -- -------------------------------------------------------------------------------------------

-- pragma translate_off

-- synthesis translate_off

-- RTL_SYNTHESIS OFF

  sim_output: process(clk_i) -- for SIMULATION ONLY!

    file file_uart_text_out : text open write_mode is sim_uart_text_file_c;

    file file_uart_data_out : text open write_mode is sim_uart_data_file_c;

    variable char_v         : integer;

    variable line_screen_v  : line; -- we need several line variables here since "writeline" seems to flush the source variable

    variable line_text_v    : line;

    variable line_data_v    : line;

  begin

    if rising_edge(clk_i) then

      if (tx_engine.state = S_TX_SIM) then -- UART simulation mode

        -- print lowest byte as ASCII char --

        char_v := to_integer(unsigned(tx_buffer.rdata(7 downto 0)));

        if (char_v >= 128) then -- out of range?

          char_v := 0;

        end if;

        if (char_v /= 10) and (char_v /= 13) then -- skip line breaks - they are issued via "writeline"

          if (sim_screen_output_en_c = true) then

            write(line_screen_v, character'val(char_v));

          end if;

          if (sim_text_output_en_c = true) then

            write(line_text_v, character'val(char_v));

          end if;

        end if;

        if (char_v = 10) then -- line break: write to screen and text file

          if (sim_screen_output_en_c = true) then