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---------------------------------------------------------------------
----                                                             ----
----  OpenCores IDE Controller                                   ----
----  Linear Feedback Shift Register                             ----
----                                                             ----
----  Author: Richard Herveille                                  ----
----          richard@asics.ws                                   ----
----          www.asics.ws                                       ----
----                                                             ----
---------------------------------------------------------------------
----                                                             ----
---- Copyright (C) 2001, 2002 Richard Herveille                  ----
----                          richard@asics.ws                   ----
----                                                             ----
---- 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 SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY     ----
---- EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED   ----
---- TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS   ----
---- FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL THE AUTHOR      ----
---- 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.                                 ----
----                                                             ----
---------------------------------------------------------------------
 
-- rev.: 1.0 march 12th, 2001. Initial release
--
--  CVS Log
--
--  $Id: atahost_lfsr.vhd,v 1.1 2002-02-18 14:32:12 rherveille Exp $
--
--  $Date: 2002-02-18 14:32:12 $
--  $Revision: 1.1 $
--  $Author: rherveille $
--  $Locker:  $
--  $State: Exp $
--
-- Change History:
--               $Log: not supported by cvs2svn $
--
 
library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_arith.all;
 
entity atahost_lfsr is
	generic(
		TAPS : positive range 16 downto 3 :=8;
		OFFSET : natural := 0
	);
	port(
		clk : in std_logic;                 -- clock input
		ena : in std_logic;                 -- count enable
		nReset : in std_logic;              -- asynchronous active low reset
		rst : in std_logic;                 -- synchronous active high reset
 
		Q : out unsigned(TAPS downto 1);    -- count value
		Qprev : out unsigned(TAPS downto 1) -- previous count value
	);
end entity atahost_lfsr;
 
architecture dataflow of atahost_lfsr is
	function lsb(tap : positive range 16 downto 3; Q : unsigned(TAPS downto 1) ) return std_logic is
	begin
		case tap is
			when 3 =>
				return Q(3) xnor Q(2);
			when 4 =>
				return Q(4) xnor Q(3);
			when 5 =>
				return Q(5) xnor Q(3);
			when 6 =>
				return Q(6) xnor Q(5);
			when 7 =>
				return Q(7) xnor Q(6);
			when 8 =>
				return (Q(8) xnor Q(6)) xnor (Q(5) xnor Q(4));
			when 9 =>
				return Q(9) xnor Q(5);
			when 10 =>
				return Q(10) xnor Q(7);
			when 11 =>
				return Q(11) xnor Q(9);
			when 12 =>
				return (Q(12) xnor Q(6)) xnor (Q(4) xnor Q(1));
			when 13 =>
				return (Q(13) xnor Q(4)) xnor (Q(3) xnor Q(1));
			when 14 =>
				return (Q(14) xnor Q(5)) xnor (Q(3) xnor Q(1));
			when 15 =>
				return Q(15) xnor Q(14);
			when 16 =>
				return (Q(16) xnor Q(15)) xnor (Q(13) xnor Q(4));
		end case;
	end function lsb;
 
	signal msb : std_logic;
	signal iQ : unsigned(TAPS downto 1);
 
begin
	--
	--	generate register
	--
	gen_regs: process(clk, nReset)
		variable tmpQ : unsigned(TAPS downto 1);
		variable tmpmsb : std_logic;
	begin
		tmpQ := (others => '0');
		tmpmsb := '1';
 
		for n in 1 to offset loop
			tmpQ := (tmpQ(TAPS -1 downto 1) & lsb(TAPS, tmpQ) );
			tmpmsb := tmpQ(TAPS);
		end loop;
 
		if (nReset = '0') then
			iQ <= tmpQ;
			msb <= tmpmsb;
		elsif (clk'event and clk = '1') then
			if (rst = '1') then
				iQ <= tmpQ;
				msb <= tmpmsb;
			elsif (ena = '1') then
				iQ <= (iQ(TAPS -1 downto 1) & lsb(TAPS, iq) );
				msb <= iQ(TAPS);
			end if;
		end if;
	end process gen_regs;
 
	-- assign outputs
	Q <= iQ;
	Qprev <= (msb & iQ(TAPS downto 2));
end architecture dataflow;