Subversion Repositories mod_sim_exp

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----  x_shift_reg                                                 ---- 

----                                                              ---- 

----  This file is part of the                                    ----

----    Modular Simultaneous Exponentiation Core project          ---- 

----    http://www.opencores.org/cores/mod_sim_exp/               ---- 

----                                                              ---- 

----  Description                                                 ---- 

----    n bit shift register for the x operand of the multiplier  ----

----    with bit output                                           ----

----                                                              ---- 

----  Dependencies: none                                          ----

----                                                              ----

----  Authors:                                                    ----

----      - Geoffrey Ottoy, DraMCo research group                 ----

----      - Jonas De Craene, JonasDC@opencores.org                ---- 

----                                                              ---- 

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----                                                              ---- 

---- Copyright (C) 2011 DraMCo research group and OPENCORES.ORG   ---- 

----                                                              ---- 

---- This source file may be used and distributed without         ---- 

---- restriction provided that this copyright statement is not    ---- 

---- removed from the file and that any derivative work contains  ---- 

---- the original copyright notice and the associated disclaimer. ---- 

----                                                              ---- 

---- This source file is free software; you can redistribute it   ---- 

---- and/or modify it under the terms of the GNU Lesser General   ---- 

---- Public License as published by the Free Software Foundation; ---- 

---- either version 2.1 of the License, or (at your option) any   ---- 

---- later version.                                               ---- 

----                                                              ---- 

---- This source is distributed in the hope that it will be       ---- 

---- useful, but WITHOUT ANY WARRANTY; without even the implied   ---- 

---- warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR      ---- 

---- PURPOSE.  See the GNU Lesser General Public License for more ---- 

---- details.                                                     ---- 

----                                                              ---- 

---- You should have received a copy of the GNU Lesser General    ---- 

---- Public License along with this source; if not, download it   ---- 

---- from http://www.opencores.org/lgpl.shtml                     ---- 

----                                                              ---- 

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library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_arith.all;

use ieee.std_logic_unsigned.all;

-- shift register for the x operand of the multiplier

-- outputs the lsb of the register or bit at offset according to the

-- selected pipeline part 

entity x_shift_reg is

  generic(

    n  : integer := 1536; -- width of the operands (# bits)

    t  : integer := 48;   -- total number of stages

    tl : integer := 16    -- lower number of stages

  );

  port(

    -- clock input

    clk    : in  std_logic;

    -- x operand in (n-bit)

    x_in   : in  std_logic_vector((n-1) downto 0);

    -- control signals

    reset  : in  std_logic; -- reset, clears register

    load_x : in  std_logic; -- load operand into shift register   

    next_x : in  std_logic; -- next bit of x

    p_sel  : in  std_logic_vector(1 downto 0);  -- pipeline selection

    -- x operand bit out (serial)

    x_i    : out std_logic

  );

end x_shift_reg;

architecture Behavioral of x_shift_reg is

  signal x_reg  : std_logic_vector((n-1) downto 0); -- register

  constant s      : integer := n/t;   -- stage width

  constant offset : integer := s*tl;  -- calculate startbit pos of higher part of pipeline

begin

        REG_PROC: process(reset, clk)

        begin

                if reset = '1' then -- Reset, clear the register

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

                elsif rising_edge(clk) then

                        if load_x = '1' then -- Load_x, load the register with x_in

                                x_reg <= x_in;

                        elsif next_x = '1' then  -- next_x, shift to right. LSbit gets lost and zero's are shifted in

                                x_reg((n-2) downto 0) <= x_reg((n-1) downto 1);

                        else -- else remember state

                                x_reg <= x_reg;

                        end if;

                end if;

        end process;

        with p_sel select  -- pipeline select

                x_i <= x_reg(offset) when "10", -- use bit at offset for high part of pipeline

                                   x_reg(0) when others;    -- use LS bit for lower part of pipeline

end Behavioral;