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-- Copyright (c)2011, 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

-- ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT

-- (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_epoch_timer

-- Description:  Provides a 24-bit, 4uS resolution elapsed timer with

--            :   alarm and interrupt for the Open8 CPU.

--

-- Notes      :  Requires an externally provided uSec tick input - one clock

--            :   per microsecond.

--

-- Register Map:

-- Offset  Bitfield Description                        Read/Write

--   0x0   AAAAAAAA B0 of Buffered Setpoint (W) or Current Setpoint(R)

--   0x1   AAAAAAAA B1 of Buffered Setpoint (W) or Current Setpoint(R)

--   0x2   AAAAAAAA B2 of Buffered Setpoint (W) or Current Setpoint(R)

--   0x3   BA------ Status of buffer/alarm (1 = pending, 0 = current)

--                  A = Pending status (R)

--                  B = Alarm status (R)

--                  Note that any write will update the internal set point

--                  and clear the alarm

--   0x4   AAAAAAAA B0 of Current Epoch Time(RO)

--   0x5   AAAAAAAA B1 of Current Epoch Time(RO)

--   0x6   AAAAAAAA B2 of Current Epoch Time(RO)

--                  Note that any write to 0x04,0x05, or 0x06 will copy the

--                  current epoch time to a readable output buffer

--   0x7   -------- Epoch Time Latch/Clear Control Register

--                  Any write to 0x7 will clear/reset the all timer regs

--

-- Revision History

-- Author          Date     Change

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

-- Seth Henry      07/28/11 Design Start

-- Seth Henry      12/19/19 Renamed to "o8_epoch_timer" to fit "theme"

-- Seth Henry      04/10/20 Overhauled the register interface of the timer to

--                           make the interface more sensible to software.

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_epoch_timer is

generic(

  Reset_Level                : std_logic;

  Address                    : ADDRESS_TYPE

);

port(

  Clock                      : in  std_logic;

  Reset                      : in  std_logic;

  uSec_Tick                  : in  std_logic;

  --

  Bus_Address                : in  ADDRESS_TYPE;

  Wr_Enable                  : in  std_logic;

  Wr_Data                    : in  DATA_TYPE;

  Rd_Enable                  : in  std_logic;

  Rd_Data                    : out DATA_TYPE;

  Interrupt                  : out std_logic

);

end entity;

architecture behave of o8_epoch_timer is

  constant User_Addr         : std_logic_vector(15 downto 3)

                               := Address(15 downto 3);

  alias  Comp_Addr           is Bus_Address(15 downto 3);

  signal Addr_Match          : std_logic := '0';

  alias  Reg_Addr            is Bus_Address(2 downto 0);

  signal Reg_Addr_q          : std_logic_vector(2 downto 0) := (others => '0');

  signal Wr_En               : std_logic := '0';

  signal Wr_Data_q           : DATA_TYPE := x"00";

  signal Rd_En               : std_logic := '0';

  signal setpt_buffer        : std_logic_vector(23 downto 0) := (others => '0');

  alias  setpt_buffer_b0     is setpt_buffer(7 downto 0);

  alias  setpt_buffer_b1     is setpt_buffer(15 downto 8);

  alias  setpt_buffer_b2     is setpt_buffer(23 downto 16);

  signal epoch_buffer        : std_logic_vector(23 downto 0) := (others => '0');

  alias  epoch_buffer_b0     is epoch_buffer(7 downto 0);

  alias  epoch_buffer_b1     is epoch_buffer(15 downto 8);

  alias  epoch_buffer_b2     is epoch_buffer(23 downto 16);

  signal buffer_pending      : std_logic := '0';

  signal buffer_update       : std_logic := '0';

  signal timer_clear         : std_logic := '0';

  signal epoch_tmr           : std_logic_vector(25 downto 0) := (others => '0');

  alias  epoch_tmrcmp        is epoch_tmr(25 downto 2);

  signal epoch_setpt         : std_logic_vector(25 downto 0) := (others => '0');

  alias  epoch_setpt_b0      is epoch_setpt(7 downto 0);

  alias  epoch_setpt_b1      is epoch_setpt(15 downto 8);

  alias  epoch_setpt_b2      is epoch_setpt(23 downto 16);

  alias  epoch_setpt_u       is epoch_setpt(25 downto 2);

  alias  epoch_setpt_l       is epoch_setpt(1 downto 0);

  signal epoch_alarm         : std_logic := '0';

  signal epoch_alarm_q       : std_logic := '0';

begin

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

  io_reg: process( Clock, Reset )

  begin

    if( Reset = Reset_Level )then

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

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

      Wr_En                  <= '0';

      Rd_En                  <= '0';

      Rd_Data                <= OPEN8_NULLBUS;

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

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

      buffer_pending         <= '0';

      buffer_update          <= '0';

      timer_clear            <= '0';

    elsif( rising_edge( Clock ) )then

      Reg_Addr_q             <= Reg_Addr;

      Wr_En                  <= Addr_Match and Wr_Enable;

      Wr_Data_q              <= Wr_Data;

      buffer_update          <= '0';

      timer_clear            <= '0';

      if( Wr_En = '1' )then

        case( Reg_Addr_q )is

          when "000" =>

            setpt_buffer_b0  <= Wr_Data_q;

            buffer_pending   <= '1';

          when "001" =>

            setpt_buffer_b1  <= Wr_Data_q;

            buffer_pending   <= '1';

          when "010" =>

            setpt_buffer_b2  <= Wr_Data_q;

            buffer_pending   <= '1';

          when "011" =>

            buffer_update    <= '1';

            buffer_pending   <= '0';

          when "100" | "101" | "110" =>

            epoch_buffer     <= epoch_tmrcmp;

          when "111" =>

            timer_clear      <= '1';

          when others => null;

        end case;

      end if;

      Rd_Data                <= OPEN8_NULLBUS;

      Rd_En                  <= Addr_Match and Rd_Enable;

      if( Rd_En = '1' )then

        case( Reg_Addr_q )is

          when "000" =>

            Rd_Data          <= epoch_setpt_b0;

          when "001" =>

            Rd_Data          <= epoch_setpt_b1;

          when "010" =>

            Rd_Data          <= epoch_setpt_b2;

          when "011" =>

            Rd_Data          <= epoch_alarm & buffer_pending & "000000";

          when "100" =>

            Rd_Data          <= epoch_buffer_b0(7 downto 0);

          when "101" =>

            Rd_Data          <= epoch_buffer_b1(15 downto 8);

          when "110" =>

            Rd_Data          <= epoch_buffer_b2(23 downto 16);

          when others => null;

        end case;

      end if;

    end if;

  end process;

  timer_proc: process( Clock, Reset )

  begin

    if( Reset = Reset_Level )then

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

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

      epoch_alarm            <= '0';

      epoch_alarm_q          <= '0';

      Interrupt              <= '0';

    elsif( rising_edge(Clock) )then

      epoch_tmr              <= epoch_tmr + uSec_Tick;

      -- Set and hold on alarm condition

      if( epoch_tmr > epoch_setpt and epoch_setpt > 0 )then

        epoch_alarm          <= '1';

      end if;

      if( buffer_update = '1' )then

        epoch_setpt_u        <= setpt_buffer;

                  -- Force the lower bits of the setpoint to "11" so that the offset is

              --  reduced to 1uS (reproducing the original behavior). Software should

                    --  always subtract 4uS (-1) from the desired time to compensate

        epoch_setpt_l        <= (others => or_reduce(setpt_buffer));

        epoch_alarm          <= '0';

      end if;

      if( timer_clear = '1' )then

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

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

        epoch_alarm          <= '0';

      end if;

      epoch_alarm_q          <= epoch_alarm;

      -- Fire on rising edge of epoch_alarm

      Interrupt              <= epoch_alarm and not epoch_alarm_q;

    end if;

  end process;

end architecture;