-- ############################################################################
-- # Project    :    Leonardo CBT-Kernel                                      #
-- #                                                                          #
-- # Filename   :    shifters.vhd                                             #
-- #                                                                          #
-- # Component  :    siso : Serial in - Serial Out shift register with        #
-- #                        direction control.                                #
-- #                                                                          #
-- # Model      :    rtl                                                      #
-- #                                                                          #
-- # Designer   :    S. Theoharis,N. Zervas                                   #
-- # Institute  :    VLSI Design Lab., University of Patras                   #
-- # Date       :    01.05.1999                                               #
-- ############################################################################

library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.std_logic_arith.all;

-- siso Entity Description
ENTITY siso IS
   GENERIC(x : INTEGER := 8);
   PORt(
      CLK,R,DIR_R,SE,SI: in std_ulogic;
      SO: out std_ulogic
   );
END siso;

-- siso Architecture Description
ARCHITECTURE rtl OF siso IS
   SIGNAL pre_Q : std_ulogic_vector((x-1) DOWNTO 0) := (OTHERS => 'X');
begin
   SHIFT_REGISTER_Process: process(CLK,R)
   BEGIN
      IF (R = '1') THEN
         pre_Q <= (OTHERS => '0');
      ELSIF (CLK'event AND (CLK = '1') AND (CLK'last_value = '0')) THEN
         IF (SE = '1') AND (DIR_R = '1') THEN
            pre_Q(x-1) <= SI;
            pre_Q((x-2) DOWNTO 0) <= pre_Q((x-1) DOWNTO 1);
	 ELSIF (SE = '1') AND (DIR_R = '0') THEN
         
	    pre_Q((x-1) DOWNTO 1) <= pre_Q((x-2) DOWNTO 0);
            pre_Q(0) <= SI;
	 END IF;
      END IF;
   END process SHIFT_REGISTER_Process;
   SO <= pre_Q(0) WHEN DIR_R = '1' ELSE
         pre_Q(x-1) WHEN DIR_R = '0' ELSE
         'X';
END rtl;