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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;