Subversion Repositories open8_urisc

-- 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
-- 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_sys_timer
-- Description:  Provides an 8-bit microsecond resolution timer for generating
--            :   periodic interrupts for the Open8 CPU.
--
-- Notes      :  It is possible to set the value to zero, resulting in the
--            :   output staying high indefinitely. This may cause an issue if
--            :   the output is connected to an interrupt input.
--            :  Also provides uSec_Tick as an output
--
-- 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.
 
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 is
generic(
  Sys_Freq                   : real;
  Reset_Level                : std_logic;
  Address                    : ADDRESS_TYPE
);
port(
  Clock                      : in  std_logic;
  Reset                      : in  std_logic;
  uSec_Tick                  : out 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_sys_timer is
 
  constant User_Addr         : ADDRESS_TYPE := Address;
  alias  Comp_Addr           is Bus_Address(15 downto 0);
  signal Addr_Match          : std_logic := '0';
  signal Wr_En               : std_logic := '0';
  signal Wr_Data_q           : DATA_TYPE := OPEN8_NULLBUS;
  signal Rd_En               : 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";
 
  constant DLY_1USEC_VAL     : integer := integer(Sys_Freq / 1000000.0);
  constant DLY_1USEC_WDT     : integer := ceil_log2(DLY_1USEC_VAL - 1);
  constant DLY_1USEC         : std_logic_vector :=
                      conv_std_logic_vector(DLY_1USEC_VAL - 1, DLY_1USEC_WDT);
 
  signal uSec_Cntr           : std_logic_vector( DLY_1USEC_WDT - 1 downto 0 )
                               := (others => '0');
  signal uSec_Tick_i         : std_logic := '0';
begin
 
  uSec_Tick                  <= uSec_Tick_i;
  Addr_Match                 <= '1' when Comp_Addr = User_Addr else '0';
 
  io_reg: process( Clock, Reset )
  begin
    if( Reset = Reset_Level )then
      Wr_En                  <= '0';
      Wr_Data_q              <= x"00";
      Rd_En                  <= '0';
      Rd_Data                <= OPEN8_NULLBUS;
      Interval               <= x"00";
      Update_Interval        <= '0';
    elsif( rising_edge( Clock ) )then
      Wr_En                  <= Addr_Match and Wr_Enable;
      Wr_Data_q              <= Wr_Data;
      Update_Interval        <= '0';
      if( Wr_En = '1' )then
        Interval             <= Wr_Data_q;
        Update_Interval      <= '1';
      end if;
 
      Rd_Data                <= (others => '0');
      Rd_En                  <= Addr_Match and Rd_Enable;
      if( Rd_En = '1' )then
        Rd_Data              <= Interval;
      end if;
    end if;
  end process;
 
  uSec_Tick_i_proc: process( Clock, Reset )
  begin
    if( Reset = Reset_Level )then
      uSec_Cntr              <= (others => '0');
      uSec_Tick_i            <= '0';
    elsif( rising_edge( Clock ) )then
      uSec_Cntr              <= uSec_Cntr - 1;
      uSec_Tick_i            <= '0';
      if( uSec_Cntr = 0 )then
        uSec_Cntr            <= DLY_1USEC;
        uSec_Tick_i          <= '1';
      end if;
    end if;
  end process;
 
  Interval_proc: process( Clock, Reset )
  begin
    if( Reset = Reset_Level )then
      Timer_Cnt              <= x"00";
      Interrupt              <= '0';
    elsif( rising_edge(Clock) )then
      Interrupt              <= '0';
      Timer_Cnt              <= Timer_Cnt - uSec_Tick_i;
      if( Update_Interval = '1' )then
        Timer_Cnt            <= Interval;
      elsif( or_reduce(Timer_Cnt) = '0' )then
        Timer_Cnt            <= Interval;
        Interrupt            <= or_reduce(Interval); -- Only trigger on Int > 0
      end if;
    end if;
  end process;
 
end architecture;