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

-- Universal dongle board source code

-- 

-- Copyright (C) 2006 Artec Design <jyrit@artecdesign.ee>

-- 

-- This source code is free hardware; 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 code 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 St, Fifth Floor, Boston, MA  02110-1301  USA

-- 

-- 

-- The complete text of the GNU Lesser General Public License can be found in 

-- the file 'lesser.txt'.

library IEEE;

use IEEE.std_logic_1164.all;

use IEEE.std_logic_unsigned.all;

use IEEE.std_logic_arith.all;

entity flash_if is

  port (

    clk       : in  std_logic;

    reset_n   : in  std_logic;

    --flash Bus

    fl_addr   : out std_logic_vector(23 downto 0);

    fl_ce_n      : out std_logic;       --chip select   (timing is very chip dependent)

    fl_oe_n      : out std_logic;       --output enable for flash (timing is very chip dependent)

    fl_we_n      : out std_logic;       --write enable (timing is very chip dependent)

    fl_data      : inout std_logic_vector(15 downto 0);

    fl_rp_n      : out std_logic;       --reset signal

    fl_byte_n      : out std_logic;       --hold in byte mode

    fl_sts       : in std_logic;        --status signal

    -- mem Bus

    mem_addr  : in std_logic_vector(23 downto 0);

    mem_do    : out std_logic_vector(15 downto 0);

    mem_di    : in  std_logic_vector(15 downto 0);

    mem_wr    : in  std_logic;  --write not read signal

    mem_val   : in  std_logic;

    mem_ack   : out std_logic

    );

end flash_if;

architecture RTL of flash_if is

 type state_type is (RESETs,FLREADs,FLWRITEs,WAITs);

  signal CS : state_type;

  signal fl_cnt : std_logic_vector(3 downto 0);

  signal  fl_oe_nd      : std_logic;       --output enable for flash

begin

fl_rp_n <= reset_n;                     --make flash reset

fl_addr <= mem_addr(23 downto 0);

fl_byte_n <= '0';                       --all byte accesses

fl_oe_n<=fl_oe_nd;

fl_data <= mem_di when fl_oe_nd ='1' else

          (others =>'Z');

RD: process (clk, reset_n)

begin  -- process READ

  if reset_n='0' then

     fl_oe_nd <='1';

         CS <= RESETs;

         fl_cnt <= (others=>'0');

         mem_do <= (others=>'0');

         mem_ack <='0';

   elsif clk'event and clk = '1' then    -- rising clock edge

                case CS is

                        when RESETs =>

                                 mem_ack <='0';

                                 fl_ce_n <= (not mem_val);                 --chipselect 4 flash

                                 fl_we_n <= (not (mem_val and mem_wr));  --write enable 4 flash

                                 if mem_val='1' and mem_wr = '0' then --READ

                                        fl_oe_nd <='0';

                                        fl_cnt <= (others=>'0');

                                        CS <= FLREADs;

                                 elsif mem_val='1' and mem_wr = '1' then --WRITE

                                        fl_oe_nd <='1';

                                        fl_cnt <= (others=>'0');

                                        CS <= FLWRITEs;

                                 end if;   --elsif mem_cmd

                        when FLREADs =>

                                fl_cnt <= fl_cnt + 1;

                                if fl_cnt=x"3" then --3 cycles later

                                        mem_ack <='1';

                                        mem_do <= fl_data;      --registered is nicer

                                elsif fl_cnt=x"4" then --4 cycles later

                                        mem_ack <='0';

                                        fl_oe_nd <='1';

                                        CS <= WAITs;

                                end if;

                        when FLWRITEs =>

                                fl_cnt <= fl_cnt + 1;

                                if fl_cnt=x"3" then --3 cycles later

                                        mem_ack <='1';

                                elsif fl_cnt=x"4" then --4 cycles later

                                        mem_ack <='0';

                                        CS <= WAITs;

                                end if;

                        when WAITs =>

                                if      mem_val='0' then -- wait untill val is removed

                                        CS <= RESETs;

                                end if;

                end case;

  end if;                               --system

end process RD;

end RTL;