Subversion Repositories astron_fifo

-------------------------------------------------------------------------------
--
-- Copyright 2020
-- ASTRON (Netherlands Institute for Radio Astronomy) <http://www.astron.nl/>
-- P.O.Box 2, 7990 AA Dwingeloo, The Netherlands
-- 
-- Licensed under the Apache License, Version 2.0 (the "License");
-- you may not use this file except in compliance with the License.
-- You may obtain a copy of the License at
-- 
--     http://www.apache.org/licenses/LICENSE-2.0
-- 
-- Unless required by applicable law or agreed to in writing, software
-- distributed under the License is distributed on an "AS IS" BASIS,
-- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-- See the License for the specific language governing permissions and
-- limitations under the License.
--
-------------------------------------------------------------------------------
 
-- Purpose: Dual clock FIFO
 
LIBRARY IEEE, common_pkg_lib, common_components_lib, technology_lib, tech_fifo_lib;
USE IEEE.STD_LOGIC_1164.ALL;
USE common_pkg_lib.common_pkg.ALL;
USE technology_lib.technology_select_pkg.ALL;
 
ENTITY common_fifo_dc IS
  GENERIC (
    g_technology  : NATURAL := c_tech_select_default;
    g_note_is_ful : BOOLEAN := TRUE;   -- when TRUE report NOTE when FIFO goes full, fifo overflow is always reported as FAILURE
    g_fail_rd_emp : BOOLEAN := FALSE;  -- when TRUE report FAILURE when read from an empty FIFO
    g_dat_w       : NATURAL := 36;
    g_nof_words   : NATURAL := 256   -- 36 * 256 = 1 M9K
  );
  PORT (
    rst     : IN  STD_LOGIC;
    wr_clk  : IN  STD_LOGIC;
    wr_dat  : IN  STD_LOGIC_VECTOR(g_dat_w-1 DOWNTO 0);
    wr_req  : IN  STD_LOGIC;
    wr_ful  : OUT STD_LOGIC;
    wrusedw : OUT STD_LOGIC_VECTOR(ceil_log2(g_nof_words)-1 DOWNTO 0);
    rd_clk  : IN  STD_LOGIC;
    rd_dat  : OUT STD_LOGIC_VECTOR(g_dat_w-1 DOWNTO 0);
    rd_req  : IN  STD_LOGIC;
    rd_emp  : OUT STD_LOGIC;
    rdusedw : OUT STD_LOGIC_VECTOR(ceil_log2(g_nof_words)-1 DOWNTO 0);
    rd_val  : OUT STD_LOGIC := '0'    
  );
END common_fifo_dc;
 
 
ARCHITECTURE str of common_fifo_dc IS
 
  CONSTANT c_nof_words  : NATURAL := 2**ceil_log2(g_nof_words);  -- ensure size is power of 2 for dual clock FIFO
 
  SIGNAL wr_rst  : STD_LOGIC;
  SIGNAL wr_init : STD_LOGIC;
  SIGNAL wr_en   : STD_LOGIC;
  SIGNAL rd_en   : STD_LOGIC;
  SIGNAL ful     : STD_LOGIC; 
  SIGNAL emp     : STD_LOGIC;
 
  SIGNAL nxt_rd_val : STD_LOGIC;
 
BEGIN
 
  -- Control logic copied from LOFAR common_fifo_dc(virtex4).vhd
 
  -- Need to make sure the reset lasts at least 3 cycles (see fifo_generator_ug175.pdf)
  -- Wait at least 4 cycles after reset release before allowing FIFO wr_en (see fifo_generator_ug175.pdf)
 
  -- Use common_areset to:
  -- . asynchronously detect rst even when the wr_clk is stopped
  -- . synchronize release of rst to wr_clk domain
  -- Using common_areset is equivalent to using common_async with same signal applied to rst and din.
  u_wr_rst : ENTITY common_components_lib.common_areset
  GENERIC MAP (
    g_rst_level => '1',
    g_delay_len => 3
  )
  PORT MAP (
    in_rst    => rst,
    clk       => wr_clk,
    out_rst   => wr_rst
  );
 
  -- Delay wr_init to ensure that FIFO ful has gone low after reset release
  u_wr_init : ENTITY common_components_lib.common_areset
  GENERIC MAP (
    g_rst_level => '1',
    g_delay_len => 4
  )
  PORT MAP (
    in_rst  => wr_rst,
    clk     => wr_clk,
    out_rst => wr_init   -- assume init has finished g_delay_len cycles after release of wr_rst
  );
 
  -- The FIFO under read and over write protection are kept enabled in the MegaWizard
  wr_en <= wr_req AND NOT wr_init;  -- check on NOT ful is not necessary when overflow_checking="ON" (Altera) or according to fifo_generator_ug175.pdf (Xilinx)
  rd_en <= rd_req;                  -- check on NOT emp is not necessary when underflow_checking="ON" (Altera)
 
  nxt_rd_val <= rd_req AND NOT emp;  -- check on NOT emp is necessary for rd_val
 
  wr_ful <= ful WHEN wr_init='0' ELSE '0';
 
  rd_emp <= emp;
 
  p_rd_clk : PROCESS(rd_clk)
  BEGIN
    IF rising_edge(rd_clk) THEN
      rd_val <= nxt_rd_val;
    END IF;
  END PROCESS;
 
  u_fifo : ENTITY tech_fifo_lib.tech_fifo_dc
  GENERIC MAP (
    g_technology => g_technology,
    g_dat_w      => g_dat_w,
    g_nof_words  => c_nof_words
  )
  PORT MAP (
    aclr    => wr_rst,   -- MegaWizard fifo_dc seems to use aclr synchronous with wr_clk
    data    => wr_dat,
    rdclk   => rd_clk,
    rdreq   => rd_en,
    wrclk   => wr_clk,
    wrreq   => wr_en,
    q       => rd_dat,
    rdempty => emp,
    rdusedw => rdusedw,
    wrfull  => ful,
    wrusedw => wrusedw
  );
 
  proc_common_fifo_asserts("common_fifo_dc", g_note_is_ful, g_fail_rd_emp, wr_rst, wr_clk, ful, wr_en, rd_clk, emp, rd_en);
 
END str;