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--===========================================================================--

--                                                                           --

--                       Peripheral Bus Interface                            --

--                                                                           --

--===========================================================================--

--

--  File name      : peripheral_bus.vhd

--

--  Entity name    : peripheral_bus

--

--  Purpose        : Implements a 16 bit peripheral bus interface 

--                   On the XESS XST-3.0 carrier board it is shared 

--                   by an IDE interface, and Ethernet MAC

--                   and two 16 bit expansion slots.

--                   The same bus structure is used on the 

--                   BurchED B3 and B5-X300 Spartan 2 boards

--                   to implement an IDE Compact Flash interface.

--

--                   The 16 bit data bus is accessed by two

--                   consecutive byte wide read or write cycles.

--

--                   On an even byte read a read strobe is generated

--                   on the peripheral bus and the high bits of the

--                   peripheral data bus are output to the CPU

--                   data bus and the lower 8 bits latched.

--                   A bus hold cycle is generated to allow time

--                   for the peripheral data bus to settle.

--                   On the odd byte read, the latched lower data 

--                   bits of the peripheral bus are output on the

--                   CPU data bus.

--

--                   Conversely, on an even byte write the CPU data

--                   bus value is latched. On the odd byte write, the

--                   latched value is output to the high 8 bits of

--                   the peripheral bus, and the CPU data is output

--                   to the lower 8 bits of the peripheral bus and

--                   a peripheral write strobe is generated.

--                   A hold signal is geneated back to the CPU to

--                   allow the peripheral bus to settle.

--                  

--  Dependencies   : ieee.std_logic_1164

--                   ieee.numeric_std

--                   unisim.vcomponents

--

--  Author         : John E. Kent

--

--  Email          : dilbert57@opencores.org      

--

--  Web            : http://opencores.org/project,system09

--

--  Memory Map     :

--

--  IO address + $00 IDE Compact Flash interface

--  IO address + $40 Ethernet MAC interface (XESS XST-3.0)

--  IO address + $80 Expansion Slot 0       (XESS XST-3.0)

--  IO address + $C0 Expansion Slot 1       (XESS XST-3.0)

--

--

--  Copyright (C) 2010 John Kent

--

--  This program is free software: you can redistribute it and/or modify

--  it under the terms of the GNU General Public License as published by

--  the Free Software Foundation, either version 3 of the License, or

--  (at your option) any later version.

--

--  This program 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 General Public License for more details.

--

--  You should have received a copy of the GNU General Public License

--  along with this program.  If not, see <http://www.gnu.org/licenses/>.

--

--===========================================================================--

--                                                                           --

--                              Revision  History                            --

--                                                                           --

--===========================================================================--

--

-- Version Author       Date         Changes

--

-- 0.1     John Kent    2010-08-28   New model

--

library ieee;

  use ieee.std_logic_1164.all;

  use ieee.numeric_std.all;

  use ieee.std_logic_unsigned.all;

library unisim;

  use unisim.vcomponents.all;

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

--                 Entity for peripheral bus                         --

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

entity peripheral_bus is

  port (

    --

    -- CPU Interface signals

    --

    clk      : in  std_logic;                     -- System Clock

    rst      : in  std_logic;                     -- Reset input (active high)

    cs       : in  std_logic;                     -- Peripheral Bus Chip Select

    addr     : in  std_logic_vector(7 downto 0);  -- Register Select

    rw       : in  std_logic;                     -- Read / Not Write

    data_in  : in  std_logic_vector(7 downto 0);  -- Data Bus In 

    data_out : out std_logic_vector(7 downto 0);  -- Data Bus Out

    hold     : out std_logic;                     -- Hold bus cycle output

    --

    -- Peripheral Bus Interface Signals

    -- IO + ($00 - $FF) 

    -- (for compatibility with XSA-3S1000 / XST 3.0)

    --

    pb_rd_n  : out   std_logic; -- ide pin 25

    pb_wr_n  : out   std_logic; -- ide pin 23

    pb_addr  : out   std_logic_vector( 4 downto 0);

    pb_data  : inout std_logic_vector(15 downto 0);

    -- Peripheral chip selects on Peripheral Bus 

    ide_cs   : out  std_logic;  -- IDE / CF interface ($00 - $3F)

    eth_cs   : out  std_logic;  -- Ethernet interface ($40 - $7F)

    sl1_cs   : out  std_logic;  -- Expansion slot 1   ($80 - $BF)

    sl2_cs   : out  std_logic   -- Expansion slot 2   ($C0 - $FF)

    );

end peripheral_bus;

--================== End of entity ==============================--

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

-- Architecture for peripheral bus interface

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

architecture rtl of peripheral_bus is

  type hold_state_type is ( hold_release_state, hold_request_state );

  signal pb_hold_state : hold_state_type := hold_release_state;

  signal pb_wru        : std_logic;       -- upper byte write strobe

  signal pb_wrl        : std_logic;       -- lower byte write strobe

  signal pb_rdu        : std_logic;       -- upper byte read strobe

  signal pb_rdl        : std_logic;       -- lower byte read strobe

  signal pb_hold       : std_logic := '0';         -- hold peripheral bus access

  signal pb_count      : std_logic_vector(3 downto 0) := (others=>'0'); -- hold counter

  signal pb_wreg       : std_logic_vector(7 downto 0) := (others=>'0'); -- lower byte write register

  signal pb_rreg       : std_logic_vector(7 downto 0) := (others=>'0'); -- lower byte read register

begin

peripheral_bus_decode : process( addr, cs )

begin

  ide_cs <= '0';

  eth_cs <= '0';

  sl1_cs <= '0';

  sl2_cs <= '0';

  case addr(7 downto 6) is

  --

  -- IDE Interface $E100 to $E13F

  --

  when "00" =>

    ide_cs <= cs;

  --

  -- Ethernet Interface $E140 to $E17F

  --

  when "01" =>

    eth_cs <= cs;

  --

  -- Slot 1 Interface $E180 to $E1BF

  --

  when "10" =>

    sl1_cs <= cs;

  --

  -- Slot 2 Interface $E1C0 to $E1FF

  --

  when "11" =>

    sl2_cs <= cs;

  --

  -- Nothing else

  --

  when others =>

    null;

  end case;

end process;

--

-- 16-bit Peripheral Bus

-- 6809 Big endian

-- ISA bus little endian

-- Not sure about IDE interface

--

peripheral_bus_control: process( clk, rst, cs, addr, rw, data_in,

                                 pb_hold, pb_wreg, pb_rreg,

                                 pb_wru, pb_wrl, pb_rdu, pb_rdl, pb_data )

begin

   pb_addr <= addr(5 downto 1);

   --

   -- internal read/write strobes

   --

   pb_wru  <= cs and (not rw) and (not addr(0));

   pb_wrl  <= cs and (not rw) and      addr(0) ;

   pb_rdu  <= cs and      rw  and (not addr(0));

   pb_rdl  <= cs and      rw  and      addr(0) ;

   pb_wr_n  <= not pb_wrl;

   pb_rd_n  <= not pb_rdu;

   --

   -- The peripheral bus will be an output 

   -- the registered even byte on data(15 downto 8)

   -- and the CPU odd bytes on data(7 downto 0)

   -- on odd byte writes

   --

   if pb_wrl = '1' then

     pb_data <= pb_wreg & data_in;

   else

     pb_data <= (others => 'Z');

   end if;

   --

   -- On even byte reads,

   -- the CPU reads the low (even) byte of the peripheral bus

   -- On odd byte reads,

   -- the CPU reads the registered (odd byte) input from the peripheral bus

   --

   if pb_rdu = '1' then

      data_out <= pb_data(15 downto 8);

   elsif pb_rdl = '1' then

      data_out <= pb_rreg;

   else

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

   end if;

   --

   -- Register upper byte from CPU on first CPU write

   -- and lower byte from the peripheral bus on first CPU read

   --

   if falling_edge(clk) then

     if rst = '1' then

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

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

     else

       if pb_wru = '1' then

         pb_wreg <= data_in;

       end if;

       if pb_rdu = '1' then

         pb_rreg <= pb_data(7 downto 0);

       end if;

     end if;

   end if;

end process;

--

-- Hold Peripheral bus accesses for a few cycles

--

peripheral_bus_hold: process( clk, rst, cs,

                              pb_hold_state, pb_hold,

                              pb_rdu, pb_wrl )

begin

  if rising_edge( clk ) then

    if rst = '1' then

                 pb_hold       <= '0';

                 pb_count      <= "0000";

            pb_hold_state <= hold_release_state;

         else

      --

      -- The perpheral bus hold signal should be generated on 

      -- 16 bit bus even upper byte read or 

      -- 16 bit bus odd lower byte write.

      -- 

      case pb_hold_state is

                when hold_release_state =>

                  if (pb_rdu = '1') or (pb_wrl = '1') then

          pb_count      <= "0011";

          pb_hold       <= '1';

          pb_hold_state <= hold_request_state;

        else

          pb_hold       <= '0';

          pb_hold_state <= hold_release_state;

        end if;

                 when hold_request_state =>

         if pb_count = "0000" then

           pb_hold       <= '0';

           pb_hold_state <= hold_release_state;

         else

                     pb_count <= pb_count - "0001";

                        end if;

       when others =>

                    null;

       end case;

         end if;

  end if;

  hold <= cs and pb_hold;

end process;

end rtl;