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-- Copyright (c)2006, 2016, 2019, 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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-- THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

--

-- VHDL Units :  o8_sys_timer

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

--            :   periodic interrupts for the Open8 CPU.

--

-- Register Map:

-- Offset  Bitfield Description                        Read/Write

--   0x00  AAAAAAAA Req Interval Byte 0                   (RW)

--   0x01  AAAAAAAA Req Interval Byte 1                   (RW)

--   0x02  AAAAAAAA Req Interval Byte 2                   (RW)

--   0x03  BA------ Control/Status Register               (RW)

--                   A: Update timer (WR) or pending (RD) (RW)

--                   B: Output Enable

--

-- Notes      :  Setting the output to 0x000000 will disable the timer

--            :  Update pending is true if bit A is 1, otherwise false

--

-- Revision History

-- Author          Date     Change

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

-- Seth Henry      07/28/11 Design Start

-- Seth Henry      12/19/19 Renamed Tmr_Out to Interrupt

-- Seth Henry      04/09/20 Modified timer update logic to reset the timer on

--                           interval write.

-- Seth Henry      04/16/20 Modified to use Open8 bus record

-- Seth Henry      04/17/20 Altered interval to be a 24-bit counter

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_sys_timer_ii is

generic(

  Address                    : ADDRESS_TYPE

);

port(

  Open8_Bus                  : in  OPEN8_BUS_TYPE;

  Rd_Data                    : out DATA_TYPE;

  Interrupt                  : out std_logic

);

end entity;

architecture behave of o8_sys_timer_ii is

  alias Clock                is Open8_Bus.Clock;

  alias Reset                is Open8_Bus.Reset;

  alias uSec_Tick            is Open8_Bus.uSec_Tick;

  constant User_Addr         : std_logic_vector(15 downto 2) :=

                                Address(15 downto 2);

  alias  Comp_Addr           is Open8_Bus.Address(15 downto 2);

  alias  Reg_Addr            is Open8_Bus.Address(1 downto 0);

  signal Addr_Match          : std_logic := '0';

  signal Reg_Sel             : std_logic_vector(1 downto 0) := "00";

  signal Wr_En               : std_logic := '0';

  signal Wr_Data_q           : DATA_TYPE := x"00";

  signal Rd_En               : std_logic := '0';

  signal Rd_En_q             : std_logic := '0';

  signal Req_Interval        : std_logic_vector(23 downto 0) := x"000000";

  alias  Req_Interval_B0     is Req_Interval( 7 downto  0);

  alias  Req_Interval_B1     is Req_Interval(15 downto  8);

  alias  Req_Interval_B2     is Req_Interval(23 downto 16);

  signal Int_Interval        : std_logic_vector(23 downto 0) := x"000000";

  signal Update_Interval     : std_logic := '0';

  signal Update_Pending      : std_logic := '0';

  signal Output_Enable       : std_logic := '0';

  signal Timer_Cnt           : std_logic_vector(23 downto 0) := x"000000";

begin

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

  io_reg: process( Clock, Reset )

  begin

    if( Reset = Reset_Level )then

      Reg_Sel                <= "00";

      Wr_En                  <= '0';

      Wr_Data_q              <= x"00";

      Rd_En                  <= '0';

      Rd_Data                <= OPEN8_NULLBUS;

      Req_Interval           <= x"000000";

      Update_Interval        <= '0';

      Update_Pending         <= '0';

      Output_Enable          <= '0';

    elsif( rising_edge( Clock ) )then

      Reg_Sel                <= Reg_Addr;

      Wr_En                  <= Addr_Match and Open8_Bus.Wr_En;

      Wr_Data_q              <= Open8_Bus.Wr_Data;

      Update_Interval        <= '0';

      if( Wr_En = '1' )then

        case( Reg_Sel )is

          when "00" =>

            Req_Interval_B0  <= Wr_Data_q;

            Update_Pending   <= '1';

          when "01" =>

            Req_Interval_B1  <= Wr_Data_q;

            Update_Pending   <= '1';

          when "10" =>

            Req_Interval_B2  <= Wr_Data_q;

            Update_Pending   <= '1';

          when "11" =>

            Output_Enable    <= Wr_Data_q(7);

            Update_Interval  <= Wr_Data_q(6);

          when others => null;

        end case;

      end if;

      if( Update_Interval = '1' )then

        Update_Pending       <= '0';

      end if;

      Rd_Data                <= OPEN8_NULLBUS;

      Rd_En                  <= Addr_Match and Open8_Bus.Rd_En;

      if( Rd_En = '1' )then

        case( Reg_Sel )is

          when "00" =>

            Rd_Data          <= Req_Interval_B0;

          when "01" =>

            Rd_Data          <= Req_Interval_B1;

          when "10" =>

            Rd_Data          <= Req_Interval_B2;

          when "11" =>

            Rd_Data          <= Output_Enable & Update_Pending & "000000";

          when others => null;

        end case;

      end if;

    end if;

  end process;

  Interval_proc: process( Clock, Reset )

  begin

    if( Reset = Reset_Level )then

      Int_Interval           <= x"000000";

      Timer_Cnt              <= x"000000";

      Interrupt              <= '0';

    elsif( rising_edge(Clock) )then

      Interrupt              <= '0';

      Timer_Cnt              <= Timer_Cnt - uSec_Tick;

      if( Update_Interval = '1' )then

        Int_Interval         <= Req_Interval;

        Timer_Cnt            <= Req_Interval;

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

        Timer_Cnt            <= Int_Interval;

        Interrupt            <= Output_Enable;

      end if;

    end if;

  end process;

end architecture;