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neorv32/rtl/core/neorv32_uart.vhd

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2024-02-24 08:25:27 +00:00
-- #################################################################################################
-- # << NEORV32 - Universal Asynchronous Receiver and Transmitter (UART) >> #
-- # ********************************************************************************************* #
-- # 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 #
-- # #
-- # 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 physical UART activity. Instead, the written #
-- # data is send to the simulation environment. The lowest 8 bits of the TX data are printed #
-- # as ASCII character to the simulator console. This character is also stored to the file #
-- # <SIM_LOG_FILE>. No interrupts are triggered when in SIMULATION MODE. #
-- # ********************************************************************************************* #
-- # BSD 3-Clause License #
-- # #
-- # The NEORV32 RISC-V Processor, https://github.com/stnolting/neorv32 #
-- # Copyright (c) 2024, 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. #
-- #################################################################################################
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 (
SIM_LOG_FILE : string; -- name of SIM mode log file
UART_RX_FIFO : natural range 1 to 2**15; -- RX fifo depth, has to be a power of two, min 1
UART_TX_FIFO : natural range 1 to 2**15 -- TX fifo depth, has to be a power of two, min 1
);
port (
clk_i : in std_ulogic; -- global clock line
rstn_i : in std_ulogic; -- global reset line, low-active, async
bus_req_i : in bus_req_t; -- bus request
bus_rsp_o : out bus_rsp_t; -- bus response
clkgen_en_o : out std_ulogic; -- enable clock generator
clkgen_i : in std_ulogic_vector(07 downto 0);
uart_txd_o : out std_ulogic; -- serial TX line
uart_rxd_i : in std_ulogic; -- serial RX line
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
irq_rx_o : out std_ulogic; -- RX interrupt
irq_tx_o : out std_ulogic -- TX interrupt
);
end neorv32_uart;
architecture neorv32_uart_rtl of neorv32_uart is
-- control register bits --
constant ctrl_en_c : natural := 0; -- r/w: UART enable
constant ctrl_sim_en_c : natural := 1; -- r/w: simulation-mode enable
constant ctrl_hwfc_en_c : natural := 2; -- r/w: enable RTS/CTS hardware flow-control
constant ctrl_prsc0_c : natural := 3; -- r/w: baud prescaler bit 0
constant ctrl_prsc1_c : natural := 4; -- r/w: baud prescaler bit 1
constant ctrl_prsc2_c : natural := 5; -- r/w: baud prescaler bit 2
constant ctrl_baud0_c : natural := 6; -- r/w: baud divisor bit 0
constant ctrl_baud1_c : natural := 7; -- r/w: baud divisor bit 1
constant ctrl_baud2_c : natural := 8; -- r/w: baud divisor bit 2
constant ctrl_baud3_c : natural := 9; -- r/w: baud divisor bit 3
constant ctrl_baud4_c : natural := 10; -- r/w: baud divisor bit 4
constant ctrl_baud5_c : natural := 11; -- r/w: baud divisor bit 5
constant ctrl_baud6_c : natural := 12; -- r/w: baud divisor bit 6
constant ctrl_baud7_c : natural := 13; -- r/w: baud divisor bit 7
constant ctrl_baud8_c : natural := 14; -- r/w: baud divisor bit 8
constant ctrl_baud9_c : natural := 15; -- r/w: baud divisor bit 9
--
constant ctrl_rx_nempty_c : natural := 16; -- r/-: RX FIFO not empty
constant ctrl_rx_half_c : natural := 17; -- r/-: RX FIFO at least half-full
constant ctrl_rx_full_c : natural := 18; -- r/-: RX FIFO full
constant ctrl_tx_empty_c : natural := 19; -- r/-: TX FIFO empty
constant ctrl_tx_nhalf_c : natural := 20; -- r/-: TX FIFO not at least half-full
constant ctrl_tx_full_c : natural := 21; -- r/-: TX FIFO full
constant ctrl_irq_rx_nempty_c : natural := 22; -- r/w: RX FIFO not empty
constant ctrl_irq_rx_half_c : natural := 23; -- r/w: RX FIFO at least half-full
constant ctrl_irq_rx_full_c : natural := 24; -- r/w: RX FIFO full
constant ctrl_irq_tx_empty_c : natural := 25; -- r/w: TX FIFO empty
constant ctrl_irq_tx_nhalf_c : natural := 26; -- r/w: TX FIFO not at least half-full
--
constant ctrl_rx_over_c : natural := 30; -- r/-: RX FIFO overflow
constant ctrl_tx_busy_c : natural := 31; -- r/-: UART transmitter is busy and TX FIFO not empty
-- data register bits --
constant data_rtx_lsb_c : natural := 0; -- r/w: RX/TX data LSB
constant data_rtx_msb_c : natural := 7; -- r/w: RX/TX data MSB
constant data_rx_fifo_size_lsb : natural := 8; -- r/-: log2(RX fifo size) LSB
constant data_rx_fifo_size_msb : natural := 11; -- r/-: log2(RX fifo size) MSB
constant data_tx_fifo_size_lsb : natural := 12; -- r/-: log2(TX fifo size) LSB
constant data_tx_fifo_size_msb : natural := 15; -- r/-: log2(TX fifo size) MSB
-- clock generator --
signal uart_clk : std_ulogic;
-- control register --
type ctrl_t is record
enable : std_ulogic;
sim_mode : std_ulogic;
hwfc_en : std_ulogic;
prsc : std_ulogic_vector(2 downto 0);
baud : std_ulogic_vector(9 downto 0);
irq_rx_nempty : std_ulogic;
irq_rx_half : std_ulogic;
irq_rx_full : std_ulogic;
irq_tx_empty : std_ulogic;
irq_tx_nhalf : std_ulogic;
end record;
signal ctrl : ctrl_t;
-- UART transmitter --
type tx_engine_t is record
state : std_ulogic_vector(2 downto 0);
sreg : std_ulogic_vector(8 downto 0);
bitcnt : std_ulogic_vector(3 downto 0);
baudcnt : std_ulogic_vector(9 downto 0);
done : std_ulogic;
busy : std_ulogic;
cts_sync : std_ulogic_vector(1 downto 0);
txd : std_ulogic;
end record;
signal tx_engine : tx_engine_t;
-- UART receiver --
type rx_engine_t is record
state : std_ulogic_vector(1 downto 0);
sreg : std_ulogic_vector(9 downto 0);
bitcnt : std_ulogic_vector(3 downto 0);
baudcnt : std_ulogic_vector(9 downto 0);
done : std_ulogic;
sync : std_ulogic_vector(2 downto 0);
over : std_ulogic;
end record;
signal rx_engine : rx_engine_t;
-- FIFO interface --
type fifo_t is record
clear : std_ulogic; -- sync reset, high-active
we : std_ulogic; -- write enable
re : std_ulogic; -- read enable
wdata : std_ulogic_vector(7 downto 0); -- write data
rdata : std_ulogic_vector(7 downto 0); -- read data
free : std_ulogic; -- free entry available?
avail : std_ulogic; -- data available?
half : std_ulogic; -- at least half full
end record;
signal rx_fifo, tx_fifo : fifo_t;
begin
-- Bus Access -----------------------------------------------------------------------------
-- -------------------------------------------------------------------------------------------
bus_access: process(rstn_i, clk_i)
begin
if (rstn_i = '0') then
bus_rsp_o.ack <= '0';
bus_rsp_o.err <= '0';
bus_rsp_o.data <= (others => '0');
ctrl.enable <= '0';
ctrl.sim_mode <= '0';
ctrl.hwfc_en <= '0';
ctrl.prsc <= (others => '0');
ctrl.baud <= (others => '0');
ctrl.irq_rx_nempty <= '0';
ctrl.irq_rx_half <= '0';
ctrl.irq_rx_full <= '0';
ctrl.irq_tx_empty <= '0';
ctrl.irq_tx_nhalf <= '0';
elsif rising_edge(clk_i) then
-- bus handshake --
bus_rsp_o.ack <= bus_req_i.stb;
bus_rsp_o.err <= '0';
bus_rsp_o.data <= (others => '0');
if (bus_req_i.stb = '1') then
-- write access --
if (bus_req_i.rw = '1') then
if (bus_req_i.addr(2) = '0') then -- control register
ctrl.enable <= bus_req_i.data(ctrl_en_c);
ctrl.sim_mode <= bus_req_i.data(ctrl_sim_en_c);
ctrl.hwfc_en <= bus_req_i.data(ctrl_hwfc_en_c);
ctrl.prsc <= bus_req_i.data(ctrl_prsc2_c downto ctrl_prsc0_c);
ctrl.baud <= bus_req_i.data(ctrl_baud9_c downto ctrl_baud0_c);
--
ctrl.irq_rx_nempty <= bus_req_i.data(ctrl_irq_rx_nempty_c);
ctrl.irq_rx_half <= bus_req_i.data(ctrl_irq_rx_half_c);
ctrl.irq_rx_full <= bus_req_i.data(ctrl_irq_rx_full_c);
ctrl.irq_tx_empty <= bus_req_i.data(ctrl_irq_tx_empty_c);
ctrl.irq_tx_nhalf <= bus_req_i.data(ctrl_irq_tx_nhalf_c);
end if;
-- read access --
else
if (bus_req_i.addr(2) = '0') then -- control register
bus_rsp_o.data(ctrl_en_c) <= ctrl.enable;
bus_rsp_o.data(ctrl_sim_en_c) <= ctrl.sim_mode;
bus_rsp_o.data(ctrl_hwfc_en_c) <= ctrl.hwfc_en;
bus_rsp_o.data(ctrl_prsc2_c downto ctrl_prsc0_c) <= ctrl.prsc;
bus_rsp_o.data(ctrl_baud9_c downto ctrl_baud0_c) <= ctrl.baud;
--
bus_rsp_o.data(ctrl_rx_nempty_c) <= rx_fifo.avail;
bus_rsp_o.data(ctrl_rx_half_c) <= rx_fifo.half;
bus_rsp_o.data(ctrl_rx_full_c) <= not rx_fifo.free;
bus_rsp_o.data(ctrl_tx_empty_c) <= not tx_fifo.avail;
bus_rsp_o.data(ctrl_tx_nhalf_c) <= not tx_fifo.half;
bus_rsp_o.data(ctrl_tx_full_c) <= not tx_fifo.free;
--
bus_rsp_o.data(ctrl_irq_rx_nempty_c) <= ctrl.irq_rx_nempty;
bus_rsp_o.data(ctrl_irq_rx_half_c) <= ctrl.irq_rx_half;
bus_rsp_o.data(ctrl_irq_rx_full_c) <= ctrl.irq_rx_full;
bus_rsp_o.data(ctrl_irq_tx_empty_c) <= ctrl.irq_tx_empty;
bus_rsp_o.data(ctrl_irq_tx_nhalf_c) <= ctrl.irq_tx_nhalf;
--
bus_rsp_o.data(ctrl_rx_over_c) <= rx_engine.over;
bus_rsp_o.data(ctrl_tx_busy_c) <= tx_engine.busy or tx_fifo.avail;
else -- data register
bus_rsp_o.data(data_rtx_msb_c downto data_rtx_lsb_c) <= rx_fifo.rdata;
bus_rsp_o.data(data_rx_fifo_size_msb downto data_rx_fifo_size_lsb) <= std_ulogic_vector(to_unsigned(index_size_f(UART_RX_FIFO), 4));
bus_rsp_o.data(data_tx_fifo_size_msb downto data_tx_fifo_size_lsb) <= std_ulogic_vector(to_unsigned(index_size_f(UART_TX_FIFO), 4));
end if;
end if;
end if;
end if;
end process bus_access;
-- UART clock enable --
clkgen_en_o <= ctrl.enable;
uart_clk <= clkgen_i(to_integer(unsigned(ctrl.prsc)));
-- TX FIFO --------------------------------------------------------------------------------
-- -------------------------------------------------------------------------------------------
tx_engine_fifo_inst: entity neorv32.neorv32_fifo
generic map (
FIFO_DEPTH => UART_TX_FIFO,
FIFO_WIDTH => 8,
FIFO_RSYNC => true,
FIFO_SAFE => true,
FULL_RESET => false
)
port map (
-- control --
clk_i => clk_i,
rstn_i => rstn_i,
clear_i => tx_fifo.clear,
half_o => tx_fifo.half,
wdata_i => tx_fifo.wdata,
we_i => tx_fifo.we,
free_o => tx_fifo.free,
re_i => tx_fifo.re,
rdata_o => tx_fifo.rdata,
avail_o => tx_fifo.avail
);
tx_fifo.clear <= '1' when (ctrl.enable = '0') or (ctrl.sim_mode = '1') else '0';
tx_fifo.wdata <= bus_req_i.data(data_rtx_msb_c downto data_rtx_lsb_c);
tx_fifo.we <= '1' when (bus_req_i.stb = '1') and (bus_req_i.rw = '1') and (bus_req_i.addr(2) = '1') else '0';
tx_fifo.re <= '1' when (tx_engine.state = "100") else '0';
-- TX interrupt generator --
tx_irq_generator: process(rstn_i, clk_i)
begin
if (rstn_i = '0') then
irq_tx_o <= '0';
elsif rising_edge(clk_i) then
irq_tx_o <= ctrl.enable and (
(ctrl.irq_tx_empty and (not tx_fifo.avail)) or -- fire IRQ if TX FIFO empty
(ctrl.irq_tx_nhalf and (not tx_fifo.half))); -- fire IRQ if TX FIFO not at least half full
end if;
end process tx_irq_generator;
-- RX FIFO --------------------------------------------------------------------------------
-- -------------------------------------------------------------------------------------------
rx_engine_fifo_inst: entity neorv32.neorv32_fifo
generic map (
FIFO_DEPTH => UART_RX_FIFO,
FIFO_WIDTH => 8,
FIFO_RSYNC => true,
FIFO_SAFE => true,
FULL_RESET => false
)
port map (
clk_i => clk_i,
rstn_i => rstn_i,
clear_i => rx_fifo.clear,
half_o => rx_fifo.half,
wdata_i => rx_fifo.wdata,
we_i => rx_fifo.we,
free_o => rx_fifo.free,
re_i => rx_fifo.re,
rdata_o => rx_fifo.rdata,
avail_o => rx_fifo.avail
);
rx_fifo.clear <= '1' when (ctrl.enable = '0') or (ctrl.sim_mode = '1') else '0';
rx_fifo.wdata <= rx_engine.sreg(8 downto 1);
rx_fifo.we <= rx_engine.done;
rx_fifo.re <= '1' when (bus_req_i.stb = '1') and (bus_req_i.rw = '0') and (bus_req_i.addr(2) = '1') else '0';
-- RX interrupt generator --
rx_irq_generator: process(rstn_i, clk_i)
begin
if (rstn_i = '0') then
irq_rx_o <= '0';
elsif rising_edge(clk_i) then
irq_rx_o <= ctrl.enable and (
(ctrl.irq_rx_nempty and rx_fifo.avail) or -- fire IRQ if RX FIFO not empty
(ctrl.irq_rx_half and rx_fifo.half) or -- fire IRQ if RX FIFO at least half full
(ctrl.irq_rx_full and (not rx_fifo.free))); -- fire IRQ if RX FIFO full
end if;
end process rx_irq_generator;
-- Transmit Engine ------------------------------------------------------------------------
-- -------------------------------------------------------------------------------------------
transmitter: process(rstn_i, clk_i)
begin
if (rstn_i = '0') then
tx_engine.cts_sync <= (others => '0');
tx_engine.done <= '0';
tx_engine.state <= (others => '0');
tx_engine.baudcnt <= (others => '0');
tx_engine.bitcnt <= (others => '0');
tx_engine.sreg <= (others => '0');
tx_engine.txd <= '1';
elsif rising_edge(clk_i) then
-- synchronize clear-to-send --
tx_engine.cts_sync <= tx_engine.cts_sync(0) & uart_cts_i;
-- defaults --
tx_engine.done <= '0';
tx_engine.txd <= '1';
-- FSM --
tx_engine.state(2) <= ctrl.enable;
case tx_engine.state is
when "100" => -- IDLE: wait for new data to send
-- ------------------------------------------------------------
tx_engine.baudcnt <= ctrl.baud;
tx_engine.bitcnt <= "1011"; -- 1 start-bit + 8 data-bits + 1 stop-bit + 1 pause-bit
tx_engine.sreg <= tx_fifo.rdata & '0'; -- data & start-bit
if (tx_fifo.avail = '1') then
tx_engine.state(1 downto 0) <= "01";
end if;
when "101" => -- WAIT: check if we are allowed to start sending
-- ------------------------------------------------------------
if (tx_engine.cts_sync(1) = '0') or (ctrl.hwfc_en = '0') then -- allowed to send OR flow-control disabled
tx_engine.state(1 downto 0) <= "11";
end if;
when "111" => -- SEND: transmit data
-- ------------------------------------------------------------
tx_engine.txd <= tx_engine.sreg(0);
if (uart_clk = '1') then
if (tx_engine.baudcnt = "0000000000") then -- bit done?
tx_engine.baudcnt <= ctrl.baud;
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.baudcnt <= std_ulogic_vector(unsigned(tx_engine.baudcnt) - 1);
end if;
end if;
if (tx_engine.bitcnt = "0000") then -- all bits send?
tx_engine.done <= '1';
tx_engine.state(1 downto 0) <= "00";
end if;
when others => -- "0--": disabled
-- ------------------------------------------------------------
tx_engine.state(1 downto 0) <= "00";
end case;
end if;
end process transmitter;
-- transmitter busy --
tx_engine.busy <= '0' when (tx_engine.state(1 downto 0) = "00") else '1';
-- serial data output --
uart_txd_o <= tx_engine.txd;
-- Receive Engine -------------------------------------------------------------------------
-- -------------------------------------------------------------------------------------------
receiver: process(rstn_i, clk_i)
begin
if (rstn_i = '0') then
rx_engine.sync <= (others => '0');
rx_engine.done <= '0';
rx_engine.state <= (others => '0');
rx_engine.baudcnt <= (others => '0');
rx_engine.bitcnt <= (others => '0');
rx_engine.sreg <= (others => '0');
elsif rising_edge(clk_i) then
-- input synchronizer --
rx_engine.sync(2) <= uart_rxd_i;
if (uart_clk = '1') then
rx_engine.sync(1) <= rx_engine.sync(2);
rx_engine.sync(0) <= rx_engine.sync(1);
end if;
-- defaults --
rx_engine.done <= '0';
-- FSM --
rx_engine.state(1) <= ctrl.enable;
case rx_engine.state is
when "10" => -- IDLE: wait for incoming transmission
-- ------------------------------------------------------------
rx_engine.baudcnt <= '0' & ctrl.baud(9 downto 1); -- half baud delay at the beginning to sample in the middle of each bit
rx_engine.bitcnt <= "1010"; -- 1 start-bit + 8 data-bits + 1 stop-bit
if (rx_engine.sync(1 downto 0) = "01") then -- start bit detected (falling edge)?
rx_engine.state(0) <= '1';
end if;
when "11" => -- RECEIVE: sample receive data
-- ------------------------------------------------------------
if (uart_clk = '1') then
if (rx_engine.baudcnt = "0000000000") then -- bit done
rx_engine.baudcnt <= ctrl.baud;
rx_engine.bitcnt <= std_ulogic_vector(unsigned(rx_engine.bitcnt) - 1);
rx_engine.sreg <= rx_engine.sync(2) & rx_engine.sreg(rx_engine.sreg'left downto 1);
else
rx_engine.baudcnt <= std_ulogic_vector(unsigned(rx_engine.baudcnt) - 1);
end if;
end if;
if (rx_engine.bitcnt = "0000") then -- all bits received?
rx_engine.done <= '1'; -- receiving done
rx_engine.state(0) <= '0';
end if;
when others => -- "0-": disabled
-- ------------------------------------------------------------
rx_engine.state(0) <= '0';
end case;
end if;
end process receiver;
-- RX overrun flag --
fifo_overrun: process(rstn_i, clk_i)
begin
if (rstn_i = '0') then
rx_engine.over <= '0';
elsif rising_edge(clk_i) then
if (ctrl.enable = '0') then -- clear when disabled
rx_engine.over <= '0';
elsif (rx_fifo.we = '1') and (rx_fifo.free = '0') then -- writing to full FIFO
rx_engine.over <= '1';
end if;
end if;
end process fifo_overrun;
-- HW flow-control: ready to receive? --
rtr_control: process(rstn_i, clk_i)
begin
if (rstn_i = '0') then
uart_rts_o <= '0';
elsif rising_edge(clk_i) then
if (ctrl.hwfc_en = '1') then
if (ctrl.enable = '0') or -- UART disabled
(rx_fifo.half = '1') then -- RX FIFO at least half-full: no "safe space" left in RX FIFO
uart_rts_o <= '1'; -- NOT allowed to send
else
uart_rts_o <= '0'; -- ready to receive
end if;
else
uart_rts_o <= '0'; -- always ready to receive when HW flow-control is disabled
end if;
end if;
end process rtr_control;
-- SIMULATION Transmitter -----------------------------------------------------------------
-- -------------------------------------------------------------------------------------------
simulation_transmitter:
if is_simulation_c generate -- for simulation only!
sim_tx: process(clk_i)
file file_out : text open write_mode is SIM_LOG_FILE;
variable char_v : integer;
variable line_screen_v : line;
variable line_file_v : line;
begin
if rising_edge(clk_i) then -- no reset required
if (ctrl.enable = '1') and (ctrl.sim_mode = '1') and (bus_req_i.stb = '1') and (bus_req_i.rw = '1') and (bus_req_i.addr(2) = '1') then
-- convert lowest byte to ASCII char --
char_v := to_integer(unsigned(bus_req_i.data(7 downto 0)));
if (char_v >= 128) then -- out of printable range?
char_v := 0;
end if;
-- ASCII output --
if (char_v /= 10) and (char_v /= 13) then -- skip line breaks - they are issued via "writeline"
write(line_screen_v, character'val(char_v)); -- console
write(line_file_v, character'val(char_v)); -- log file
elsif (char_v = 10) then -- line break: write to screen and text file
writeline(output, line_screen_v); -- console
writeline(file_out, line_file_v); -- log file
end if;
end if;
end if;
end process sim_tx;
end generate;
end neorv32_uart_rtl;