-------------------------------------------------------------------------
-- Project: JCPU, a portable 8-bit RISC CPU written in VHDL
-- This file: The call/return/data stack

-- Copyright (C) 2007 J. Ahrensfeld

-- This library 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 library 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 library; if not, write to the Free Software
-- Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA

-- For questions and ideas, please contact the author at jens@jayfield.org

-----------------------------------------------------------------------
-- $Header: /tmp/cvsroot/VHDL/lib/FIFO/src/gray_counter.vhd,v 1.1 2008-08-23 08:20:28 Jens Exp $
-----------------------------------------------------------------------

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.numeric_std.ALL;

LIBRARY WORK;
USE WORK.FIFO_CTRL_PKG.ALL;

entity gray_counter is
	Generic (
		width : natural := 3;
		init_value : natural := 0
	);
	Port
	(
		rst			: in  STD_LOGIC;
		clk			: in  STD_LOGIC;
		ce			: in  STD_LOGIC;
		bcnt		: out unsigned (width-1 downto 0);
		bnxt		: out unsigned (width-1 downto 0);
		gcnt		: out unsigned (width-1 downto 0);
		gnxt		: out unsigned (width-1 downto 0)
	);
end gray_counter;

architecture Behavioral of gray_counter is

	signal cntb : unsigned (width-1 downto 0);
	signal nxtb : unsigned (width-1 downto 0);
	signal cntg : unsigned (width-1 downto 0);
	signal nxtg : unsigned (width-1 downto 0);

begin

	bnxt	<= nxtb;
	gnxt	<= nxtg;

	process(rst, clk, ce, cntb, nxtb)
	begin
		if rst = '1' then
			cntb	<= to_unsigned(init_value, width);
			nxtb	<= to_unsigned(init_value, width);
			cntg	<= to_unsigned(init_value, width);
			nxtg	<= to_unsigned(init_value, width);
			bcnt	<= to_unsigned(init_value, width);
			gcnt	<= to_unsigned(init_value, width);
		else
			if rising_edge(clk) then
				bcnt	<= nxtb;
				gcnt	<= nxtg;
				cntb	<= nxtb;
				cntg	<= nxtg;
			end if;	
			nxtg <= bin2gray(nxtb);
			nxtb <= cntb;
			if ce = '1' then
				nxtb <= cntb + 1;
			end if;	
		end if;	
	end process;

end Behavioral;