clkmux.vhd

free hardware ip core about sparcv8,a soc cpu in vhdl

------------------------------------------------------------------------------
--  This file is a part of the GRLIB VHDL IP LIBRARY
--  Copyright (C) 2003, Gaisler Research
--
--  This program is free software; you can redistribute it and/or modify
--  it under the terms of the GNU General Public License as published by
--  the Free Software Foundation; either version 2 of the License, or
--  (at your option) any later version.
--
--  This program 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 General Public License for more details.
--
--  You should have received a copy of the GNU General Public License
--  along with this program; if not, write to the Free Software
--  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA 
-----------------------------------------------------------------------------
-- Entity: 	clkmux
-- File:	clkmux.vhd
-- Author:	Edvin Catovic - Gaisler Research
-- Description:	Glitch-free clock multiplexer
------------------------------------------------------------------------------

library ieee;
use ieee.std_logic_1164.all;
use work.gencomp.all;
use work.libclk.all;

entity clkmux is
  generic( tech : integer := 0);
  port(
    i0, i1  :  in  std_ulogic;
    sel     :  in  std_ulogic;
    o       :  out std_ulogic
  );
end entity;

architecture rtl of clkmux is
  signal sel0, sel1, cg0, cg1 : std_ulogic;  
begin
  xil : if (tech = virtex2) or (tech = spartan3) or (tech = spartan3e)
          or (tech = virtex4) or (tech = virtex5) generate
    buf : clkmux_unisim port map(sel => sel, I0 => i0, I1 => i1, O => o);
  end generate;

  gen : if has_clkmux(tech) = 0 generate

    p0 : process(i0)
    begin
      if falling_edge(i0) then
        sel0 <= (not sel) and (not sel1);
      end if;
    end process;
    
    p1 : process(i1)
    begin
      if falling_edge(i1) then
        sel1 <= sel and (not sel0);
      end if;
    end process;

    cg0 <= i0 and sel0;
    cg1 <= i1 and sel1;
    o   <= cg0 or cg1;    
    
  end generate;
end architecture;