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-- Copyright (c)2020 Jeremy Seth Henry

-- All rights reserved.

--

-- Redistribution and use in source and binary forms, with or without

-- modification, are permitted provided that the following conditions are met:

--     * Redistributions of source code must retain the above copyright

--       notice, this list of conditions and the following disclaimer.

--     * Redistributions in binary form must reproduce the above copyright

--       notice, this list of conditions and the following disclaimer in the

--       documentation and/or other materials provided with the distribution,

--       where applicable (as part of a user interface, debugging port, etc.)

--

-- THIS SOFTWARE IS PROVIDED BY JEREMY SETH HENRY ``AS IS'' AND ANY

-- EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED

-- WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE

-- DISCLAIMED. IN NO EVENT SHALL JEREMY SETH HENRY 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

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-- (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF

-- THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

--

-- VHDL Units :  o8_int_mgr16

-- Description:  Provides an 8-bit microsecond resolution timer for generating

--            :   periodic interrupts for the Open8 CPU as well as providing a

--            :   second level interrupt manager for I/O interrupts

--

-- Register Map:

-- Offset  Bitfield Description                        Read/Write

--   0x00  AAAAAAAA PIT Timer Interval (0 = disabled)    (RW)

--   0x01  -------- (unused)

--   0x02  AAAAAAAA External Interrupt Mask (lower)      (RW)

--   0x03  AAAAAAAA External Interrupt Mask (upper)      (RW)

--   0x04  AAAAAAAA Pending External Ints* (lower)       (RW)

--   0x05  AAAAAAAA Pending External Ints* (upper)       (RW)

--   0x06  -------- (unused)

--   0x07  A------- Interrupt Requested (write to clear) (RW)

--

-- Note: Each bit in the pending register is individually clearable by writing

--        a '1' to it, allowing interrupts to be cleared individually

--

-- Revision History

-- Author          Date     Change

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

-- Seth Henry      05/21/20 Design Start

library ieee;

use ieee.std_logic_1164.all;

  use ieee.std_logic_unsigned.all;

  use ieee.std_logic_arith.all;

  use ieee.std_logic_misc.all;

library work;

  use work.open8_pkg.all;

entity o8_int_mgr16 is

generic(

  Default_Int_Mask           : ADDRESS_TYPE := x"0000";

  Address                    : ADDRESS_TYPE

);

port(

  Open8_Bus                  : in  OPEN8_BUS_TYPE;

  Interrupts                 : in  ADDRESS_TYPE := x"0000";

  Rd_Data                    : out DATA_TYPE;

  PIT_Interrupt              : out std_logic;

  EXT_Interrupt              : out std_logic

);

end entity;

architecture behave of o8_int_mgr16 is

  alias  Clock               is Open8_Bus.Clock;

  alias  Reset               is Open8_Bus.Reset;

  alias  uSec_Tick           is Open8_Bus.uSec_Tick;

  alias  CPU_ISR_En          is Open8_Bus.GP_Flags(EXT_ISR);

  alias  CPU_Wr_En           is Open8_Bus.Wr_En;

  alias  CPU_Rd_En           is Open8_Bus.Rd_En;

  constant User_Addr         : std_logic_vector(15 downto 3) :=

                                Address(15 downto 3);

  alias  Comp_Addr           is Open8_Bus.Address(15 downto 3);

  signal Addr_Match          : std_logic := '0';

  alias  Reg_Sel_d           is Open8_Bus.Address(2 downto 0);

  signal Reg_Sel_q           : std_logic_vector(2 downto 0);

  signal Wr_En_d             : std_logic;

  signal Wr_En_q             : std_logic := '0';

  alias  Wr_Data_d           is Open8_Bus.Wr_Data;

  signal Wr_Data_q           : DATA_TYPE := x"00";

  signal Rd_En_d               : std_logic := '0';

  signal Rd_En_q             : std_logic := '0';

  signal Interval            : DATA_TYPE := x"00";

  signal Update_Interval     : std_logic;

  signal Timer_Cnt           : DATA_TYPE := x"00";

  signal Int_Mask            : ADDRESS_TYPE := x"0000";

  alias  Int_Mask_l          is Int_Mask(7 downto 0);

  alias  Int_Mask_h          is Int_Mask(15 downto 8);

  signal Clear_Pending       : ADDRESS_TYPE := x"0000";

  alias  Clear_Pending_l     is Clear_Pending(7 downto 0);

  alias  Clear_Pending_h     is Clear_Pending(15 downto 8);

  signal Ack_IO_Ints         : std_logic;

  signal Pending             : ADDRESS_TYPE := x"0000";

  alias  Pending_l           is Pending(7 downto 0);

  alias  Pending_h           is Pending(15 downto 8);

  signal Pending_q           : ADDRESS_TYPE := x"0000";

  signal Pending_RE          : ADDRESS_TYPE := x"0000";

  signal IO_Int_Pending      : std_logic;

begin

  Addr_Match                 <= '1' when Comp_Addr = User_Addr else '0';

  Wr_En_d                    <= Addr_Match and CPU_Wr_En and CPU_ISR_En;

  Rd_En_d                    <= Addr_Match and CPU_Rd_En;

  io_reg: process( Clock, Reset )

  begin

    if( Reset = Reset_Level )then

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

      Wr_En_q                <= '0';

      Wr_Data_q              <= x"00";

      Rd_En_q                <= '0';

      Rd_Data                <= OPEN8_NULLBUS;

      Interval               <= x"00";

      Update_Interval        <= '0';

      Int_Mask               <= Default_Int_Mask;

      Clear_Pending          <= x"0000";

      Ack_IO_Ints            <= '0';

    elsif( rising_edge( Clock ) )then

      Reg_Sel_q              <= Reg_Sel_d;

      Wr_En_q                <= Wr_En_d;

      Wr_Data_q              <= Wr_Data_d;

      Update_Interval        <= Wr_En_q;

      Clear_Pending          <= x"0000";

      Ack_IO_Ints            <= '0';

      if( Wr_En_q = '1' )then

        case( Reg_Sel_q )is

          when "000" =>

            Interval         <= Wr_Data_q;

          when "010" =>

            Int_Mask_l       <= Wr_Data_q;

          when "011" =>

            Int_Mask_h       <= Wr_Data_q;

          when "100" =>

            Clear_Pending_l  <= Wr_Data_q;

          when "101" =>

            Clear_Pending_h  <= Wr_Data_q;

          when "111" =>

            Ack_IO_Ints      <= '1';

          when others =>

            null;

        end case;

      end if;

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

      Rd_En_q                <= Rd_En_d;

      if( Rd_En_q = '1' )then

        case( Reg_Sel_q )is

          when "000" =>

            Rd_Data          <= Interval;

          when "010" =>

            Rd_Data          <= Int_Mask_l;

          when "011" =>

            Rd_Data          <= Int_Mask_h;

          when "100" =>

            Rd_Data          <= Pending_l;

          when "101" =>

            Rd_Data          <= Pending_h;

          when "111" =>

            Rd_Data          <= IO_Int_Pending & "0000000";

          when others =>

            null;

        end case;

      end if;

    end if;

  end process;

  Interval_proc: process( Clock, Reset )

  begin

    if( Reset = Reset_Level )then

      Timer_Cnt              <= x"00";

      PIT_Interrupt          <= '0';

    elsif( rising_edge(Clock) )then

      PIT_Interrupt              <= '0';

      Timer_Cnt              <= Timer_Cnt - uSec_Tick;

      if( Update_Interval = '1' )then

        Timer_Cnt            <= Interval;

      elsif( or_reduce(Timer_Cnt) = '0' )then

        Timer_Cnt            <= Interval;

        PIT_Interrupt        <= or_reduce(Interval); -- Only trigger on Int > 0

      end if;

    end if;

  end process;

  Interrupt_proc: process( Clock, Reset )

    variable i               : integer := 0;

  begin

    if( Reset = Reset_Level )then

      Pending                <= x"0000";

      Pending_q              <= x"0000";

      Pending_RE             <= x"0000";

      IO_Int_Pending         <= '0';

      EXT_Interrupt          <= '0';

    elsif( rising_edge(Clock) )then

      for i in 0 to 15 loop

        if( Interrupts(i) = '1' and Int_Mask(i) = '1' )then

          Pending(i)         <= '1';

        elsif( Clear_Pending(i) = '1' )then

          Pending(i)         <= '0';

        end if;

        Pending_q(i)         <= Pending(i);

        Pending_RE(i)        <= Pending(i) and not Pending_q(i);

      end loop;

      EXT_Interrupt          <= '0';

      if( or_reduce(Pending_RE) = '1' and IO_Int_Pending = '0' )then

        IO_Int_Pending       <= '1';

        EXT_Interrupt        <= '1';

      elsif( Ack_IO_Ints = '1' )then

        IO_Int_Pending       <= '0';

      end if;

    end if;

  end process;

end architecture;