Subversion Repositories open8_urisc

-- Copyright (c)2006, 2016, 2019 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_register
-- Description:  Provides a single addressible 8-bit register
--
-- Register Map:
-- Offset  Bitfield Description                        Read/Write
--   0x00  AAAAAAAA Registered Outputs                    (RW)
--
-- Revision History
-- Author          Date     Change
------------------ -------- ---------------------------------------------------
-- Seth Henry      12/20/19 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_register is
generic(
  Default_Value         : DATA_TYPE := x"00";
  Reset_Level           : std_logic;
  Address               : ADDRESS_TYPE
);
port(
  Clock                 : in  std_logic;
  Reset                 : 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;
  --
  Register_Out          : out DATA_TYPE
);
end entity;
 
architecture behave of o8_register is
 
  function ceil_log2 (x : in natural) return natural is
    variable retval     : natural;
  begin
    retval              := 1;
    while ((2**retval) - 1) < x loop
      retval            := retval + 1;
    end loop;
    return retval;
  end function;
 
  constant User_Addr    : std_logic_vector(15 downto 0)
                          := Address(15 downto 0);
  alias  Comp_Addr      is Bus_Address(15 downto 0);
  signal Addr_Match     : std_logic;
  signal Wr_En          : std_logic;
  signal Wr_Data_q      : DATA_TYPE;
  signal Reg_Out        : DATA_TYPE;
  signal Rd_En          : std_logic;
 
begin
 
  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         <= (others => '0');
      Reg_Out           <= Default_Value;
      Rd_En             <= '0';
      Rd_Data           <= x"00";
    elsif( rising_edge( Clock ) )then
      Wr_En             <= Addr_Match and Wr_Enable;
      Wr_Data_q         <= Wr_Data;
      if( Wr_En = '1' )then
        Reg_Out         <= Wr_Data_q;
      end if;
 
      Rd_Data           <= (others => '0');
      Rd_En             <= Addr_Match and Rd_Enable;
      if( Rd_En = '1' )then
        Rd_Data         <= Reg_Out;
      end if;
 
    end if;
  end process;
 
  Register_Out          <= Reg_Out;
 
end architecture;