VHDL PROGRAM OF A4006B_vhd

-- ============================================================================
-- 4006B 18-Stage Static Shift Register
--
-- Copyright (C),2004 Andrew John Jacobs.
--
-- This program is provided free of charge for educational purposes
--
-- Redistribution and use in binary form without modification, is permitted
-- provided that the above copyright notice, this list of conditions and the
-- following disclaimer in the documentation and/or other materials provided
-- with the distribution.
--
-- THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS '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 THE AUTHOR OR CONTRIBUTORS 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.
-- ============================================================================

library ieee;
use ieee.std_logic_1164.all;

entity A4006B is
 generic (
  tplh  : time := 0 ns;
  tphl  : time := 0 ns);
 port (
  cp_n  : in  std_logic;
  da   : in  std_logic;
  db   : in  std_logic;
  dc   : in  std_logic;
  dd   : in  std_logic;
  o3a   : out  std_logic;
  o3b   : out  std_logic;
  o3c   : out  std_logic;
  o4c   : out  std_logic;
  o3d   : out  std_logic;
  o4d   : out  std_logic);
end entity;

-- ============================================================================

library ieee;
use ieee.std_logic_1164.all;

architecture behaviour of A4006B is
 -- Shift registers
 signal sa  : std_logic_vector (1 to 4) := "UUUU";
 signal sb  : std_logic_vector (1 to 4) := "UUUU";
 signal sc  : std_logic_vector (1 to 5) := "UUUUU";
 signal sd  : std_logic_vector (1 to 5) := "UUUUU";
begin
 -- Shift the contents on falling edge of clock
 process (cp_n)
 begin
  if falling_edge (cp_n) then
   sa <= da & sa(1 to 3);
   sb <= db & sb(1 to 3);
   sc <= dc & sc(1 to 4);
   sd <= dd & sd(1 to 4);
  end if;
 end process;

 -- Extract outputs from shift registers
 o3a <= '1' after tplh when sa(4) = '1' else
     '0' after tphl;

 o3b <= '1' after tplh when sb(4) = '1' else
     '0' after tphl;

 o3c <= '1' after tplh when sc(4) = '1' else
     '0' after tphl;

 o4c <= '1' after tplh when sc(5) = '1' else
     '0' after tphl;

 o3d <= '1' after tplh when sd(4) = '1' else
     '0' after tphl;

 o4d <= '1' after tplh when sd(5) = '1' else
     '0' after tphl;
end architecture;