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---------------------------------------------------------------------------- ---- ---- ---- ---- ---- This file is part of the srl_fifo project ---- ---- http://www.opencores.org/cores/srl_fifo ---- ---- ---- ---- Description ---- ---- Implementation of srl_fifo IP core according to ---- ---- srl_fifo IP core specification document. ---- ---- ---- ---- To Do: ---- ---- NA ---- ---- ---- ---- Author(s): ---- ---- Andrew Mulcock, amulcock@opencores.org ---- ---- ---- ---------------------------------------------------------------------------- ---- ---- ---- Copyright (C) 2008 Authors and OPENCORES.ORG ---- ---- ---- ---- This source file may be used and distributed without ---- ---- restriction provided that this copyright statement is not ---- ---- removed from the file and that any derivative work contains ---- ---- the original copyright notice and the associated disclaimer. ---- ---- ---- ---- This source file 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 2.1 of the License, or (at your option) any ---- ---- later version. ---- ---- ---- ---- This source 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 source; if not, download it ---- ---- from http://www.opencores.org/lgpl.shtml ---- ---- ---- ---------------------------------------------------------------------------- ---- ---- -- CVS Revision History ---- ---- ---- -- $Log: not supported by cvs2svn $ ---- ---- ---- ---- ---- -- quick description -- -- Based upon the using a shift register as a fifo which has been -- around for years ( decades ), but really came of use to VHDL -- when the Xilinx FPGA's started having SRL's. -- -- In my view, the definitive article on shift register logic fifo's -- comes from Mr Chapman at Xilinx, in the form of his BBFIFO -- tecXeclusive article, which as at early 2008, Xilinx have -- removed. -- -- -- using Xilinx ISE 10.1 and later, the tools are getting real clever. -- In previous version of ISE, SRL inferance was not this clever. -- now if one infers a 32 bit srl in a device that has inherantly 16 -- bit srls, then an srl and a series of registers was created. -- In 10,1 and later, if you infer a 32 bit srl for a device with -- 16 bit srls in, then you end up with the cascaded srls as expected. -- -- Well done Xilinx.. -- library IEEE; use IEEE.STD_LOGIC_1164.ALL; use ieee.NUMERIC_STD.all; entity srl_fifo_32 is generic ( width : integer := 8 ); -- set to how wide fifo is to be port( data_in : in std_logic_vector (width -1 downto 0); data_out : out std_logic_vector (width -1 downto 0); reset : in std_logic; write : in std_logic; read : in std_logic; full : out std_logic; half_full : out std_logic; data_present : out std_logic; clk : in std_logic ); -- Declarations end srl_fifo_32 ; -- ------------------------------------------------------------------------------------ -- architecture rtl of srl_fifo_32 is -- ------------------------------------------------------------------------------------ -- ------------------------------------------------------------------------------------ -- ------------------------------------------------------------------------------------ -- constant srl_length : integer := 32; -- set to srl 'type' 16 or 32 bit length constant pointer_vec : integer := 5; -- set to number of bits needed to store pointer = log2(srl_length) type srl_array is array ( srl_length - 1 downto 0 ) of STD_LOGIC_VECTOR ( WIDTH - 1 downto 0 ); signal fifo_store : srl_array; signal pointer : integer range 0 to srl_length - 1; signal pointer_zero : std_logic; signal pointer_full : std_logic; signal valid_write : std_logic; signal half_full_int : std_logic_vector( pointer_vec - 1 downto 0); signal empty : std_logic := '1'; signal valid_count : std_logic ; ------------------------------------------------------------------------------------ -- ------------------------------------------------------------------------------------ -- begin -- Valid write, high when valid to write data to the store. valid_write <= '1' when ( read = '1' and write = '1' ) or ( write = '1' and pointer_full = '0' ) else '0'; -- data store SRL's data_srl :process( clk ) begin if rising_edge( clk ) then if valid_write = '1' then fifo_store <= fifo_store( fifo_store'left - 1 downto 0) & data_in; end if; end if; end process; data_out <= fifo_store( pointer ); process( clk) begin if rising_edge( clk ) then if reset = '1' then empty <= '1'; elsif empty = '1' and write = '1' then empty <= '0'; elsif pointer_zero = '1' and read = '1' and write = '0' then empty <= '1'; end if; end if; end process; -- W R Action -- 0 0 pointer <= pointer -- 0 1 pointer <= pointer - 1 Read, but no write, so less data in counter -- 1 0 pointer <= pointer + 1 Write, but no read, so more data in fifo -- 1 1 pointer <= pointer Read and write, so same number of words in fifo -- valid_count <= '1' when ( (write = '1' and read = '0' and pointer_full = '0' and empty = '0' ) or (write = '0' and read = '1' and pointer_zero = '0' ) ) else '0'; process( clk ) begin if rising_edge( clk ) then if valid_count = '1' then if write = '1' then pointer <= pointer + 1; else pointer <= pointer - 1; end if; end if; end if; end process; -- Detect when pointer is zero and maximum pointer_zero <= '1' when pointer = 0 else '0'; pointer_full <= '1' when pointer = srl_length - 1 else '0'; -- assign internal signals to outputs full <= pointer_full; half_full_int <= std_logic_vector(to_unsigned(pointer, pointer_vec)); half_full <= half_full_int(half_full_int'left); data_present <= not( empty ); end rtl; ------------------------------------------------------------------------------------ -- ------------------------------------------------------------------------------------