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-- # << NEORV32 - General Purpose FIFO Component >>                                                #

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-- # BSD 3-Clause License                                                                          #

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

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-- # 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;

entity neorv32_fifo is

  generic (

    FIFO_DEPTH : natural; -- number of fifo entries; has to be a power of two; min 1

    FIFO_WIDTH : natural; -- size of data elements in fifo

    FIFO_RSYNC : boolean; -- false = async read; true = sync read

    FIFO_SAFE  : boolean  -- true = allow read/write only if entry available

  );

  port (

    -- control --

    clk_i   : in  std_ulogic; -- clock, rising edge

    rstn_i  : in  std_ulogic; -- async reset, low-active

    clear_i : in  std_ulogic; -- sync reset, high-active

    level_o : out std_ulogic_vector(index_size_f(FIFO_DEPTH) downto 0); -- fill level

    half_o  : out std_ulogic; -- FIFO is at least half full

    -- write port --

    wdata_i : in  std_ulogic_vector(FIFO_WIDTH-1 downto 0); -- write data

    we_i    : in  std_ulogic; -- write enable

    free_o  : out std_ulogic; -- at least one entry is free when set

    -- read port --

    re_i    : in  std_ulogic; -- read enable

    rdata_o : out std_ulogic_vector(FIFO_WIDTH-1 downto 0); -- read data

    avail_o : out std_ulogic  -- data available when set

  );

end neorv32_fifo;

architecture neorv32_fifo_rtl of neorv32_fifo is

  -- FIFO --

  type fifo_data_t is array (0 to FIFO_DEPTH-1) of std_ulogic_vector(FIFO_WIDTH-1 downto 0);

  type fifo_t is record

    we    : std_ulogic; -- write enable

    re    : std_ulogic; -- read enable

    w_pnt : std_ulogic_vector(index_size_f(FIFO_DEPTH) downto 0); -- write pointer

    r_pnt : std_ulogic_vector(index_size_f(FIFO_DEPTH) downto 0); -- read pointer

    level : std_ulogic_vector(index_size_f(FIFO_DEPTH) downto 0); -- fill count

    data  : fifo_data_t; -- fifo memory

    datas : std_ulogic_vector(FIFO_WIDTH-1 downto 0);

    match : std_ulogic;

    empty : std_ulogic;

    full  : std_ulogic;

    free  : std_ulogic;

    avail : std_ulogic;

  end record;

  signal fifo : fifo_t;

  signal level_diff : std_ulogic_vector(index_size_f(FIFO_DEPTH) downto 0);

begin

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

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

  assert not (FIFO_DEPTH = 0) report "NEORV32 CONFIG ERROR: FIFO depth has to be > 0." severity error;

  assert not (is_power_of_two_f(FIFO_DEPTH) = false) report "NEORV32 CONFIG ERROR: FIFO depth has to be a power of two." severity error;

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

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

  fifo.re <= re_i when (FIFO_SAFE = false) else (re_i and fifo.avail); -- read only if data available

  fifo.we <= we_i when (FIFO_SAFE = false) else (we_i and fifo.free); -- write only if space left

  -- FIFO Control ---------------------------------------------------------------------------

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

  fifo_control: process(rstn_i, clk_i)

  begin

    if (rstn_i = '0') then

      fifo.w_pnt <= (others => '0');

      fifo.r_pnt <= (others => '0');

    elsif rising_edge(clk_i) then

      -- write port --

      if (clear_i = '1') then

        fifo.w_pnt <= (others => '0');

      elsif (fifo.we = '1') then

        fifo.w_pnt <= std_ulogic_vector(unsigned(fifo.w_pnt) + 1);

      end if;

      -- read port --

      if (clear_i = '1') then

        fifo.r_pnt <= (others => '0');

      elsif (fifo.re = '1') then

        fifo.r_pnt <= std_ulogic_vector(unsigned(fifo.r_pnt) + 1);

      end if;

    end if;

  end process fifo_control;

  -- status --

  fifo.match <= '1' when (fifo.r_pnt(fifo.r_pnt'left-1 downto 0) = fifo.w_pnt(fifo.w_pnt'left-1 downto 0)) or (FIFO_DEPTH = 1) else '0';

  fifo.full  <= '1' when (fifo.r_pnt(fifo.r_pnt'left) /= fifo.w_pnt(fifo.w_pnt'left)) and (fifo.match = '1') else '0';

  fifo.empty <= '1' when (fifo.r_pnt(fifo.r_pnt'left)  = fifo.w_pnt(fifo.w_pnt'left)) and (fifo.match = '1') else '0';

  fifo.free  <= not fifo.full;

  fifo.avail <= not fifo.empty;

  level_diff <= std_ulogic_vector(unsigned(fifo.w_pnt) - unsigned(fifo.r_pnt));

  fifo.level <= std_ulogic_vector(to_unsigned(FIFO_DEPTH, fifo.level'length)) when (fifo.full = '1') else level_diff;

  -- status output --

  level_o <= fifo.level;

  free_o  <= fifo.free;

  avail_o <= fifo.avail;

  fifo_half_level:

  if (FIFO_DEPTH > 1) generate

    half_o <= level_diff(level_diff'left-1) or fifo.full;

  end generate;

  fifo_half_level_simple:

  if (FIFO_DEPTH = 1) generate

    half_o <= fifo.full;

  end generate;

  -- FIFO Memory ----------------------------------------------------------------------------

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

  fifo_memory_write: process(clk_i)

  begin

    if rising_edge(clk_i) then

      if (fifo.we = '1') then

        if (FIFO_DEPTH = 1) then

          fifo.datas <= wdata_i;

        else

          fifo.data(to_integer(unsigned(fifo.w_pnt(fifo.w_pnt'left-1 downto 0)))) <= wdata_i;

        end if;

      end if;

    end if;

  end process fifo_memory_write;

  -- asynchronous read --

  fifo_read_async:

  if (FIFO_RSYNC = false) generate

    rdata_o <= fifo.datas when (FIFO_DEPTH = 1) else fifo.data(to_integer(unsigned(fifo.r_pnt(fifo.r_pnt'left-1 downto 0))));

  end generate;

  -- synchronous read --

  fifo_read_sync:

  if (FIFO_RSYNC = true) generate

    fifo_memory_read: process(clk_i)

    begin

      if rising_edge(clk_i) then

        if (FIFO_DEPTH = 1) then

          rdata_o <= fifo.datas;

        else

          rdata_o <= fifo.data(to_integer(unsigned(fifo.r_pnt(fifo.r_pnt'left-1 downto 0))));

        end if;

      end if;

    end process fifo_memory_read;

  end generate;

end neorv32_fifo_rtl;