Subversion Repositories funbase_ip_library

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-- Author               : Timo Alho

-- e-mail               : timo.a.alho@tut.fi

-- Date                 : 14.06.2004 14:11:03

-- File                 : Serial_subtractor.vhd

-- Design               : 

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-- Description  : Serial subtractor (in0 - in1). 

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LIBRARY ieee;

USE ieee.std_logic_1164.all;

USE ieee.std_logic_arith.all;

ENTITY Serial_subtractor IS

   PORT(

      clk     : IN     std_logic;

      in0     : IN     std_logic;   --serial data in 0

      in1     : IN     std_logic;   --serial data in 1

      rst_n   : IN     std_logic;

      start   : IN     std_logic;   --start (ignores borrowbit)

      sub_out : OUT    std_logic    --serial data out

   );

-- Declarations

END Serial_subtractor ;

--

ARCHITECTURE rtl OF Serial_subtractor IS

  SIGNAL borrow_bit_r : std_logic;      -- register for borrow bit

  SIGNAL borrow_bit   : std_logic;      -- internal signal for borrow bit

  SIGNAL sub : std_logic;               -- internal signal for result

BEGIN

  clocked : PROCESS (clk, rst_n)

  BEGIN  -- PROCESS clocked

    IF rst_n = '0' THEN                 -- asynchronous reset (active low)

      borrow_bit_r <= '0';

    ELSIF clk'event AND clk = '1' THEN  -- rising clock edge

      borrow_bit_r <= borrow_bit;

    END IF;

  END PROCESS clocked;

  calc             : PROCESS (borrow_bit_r, start, in0, in1)

    VARIABLE temp1 : std_logic_vector(1 DOWNTO 0);

    VARIABLE temp2 : std_logic_vector(2 DOWNTO 0);

  BEGIN  -- PROCESS calc

    IF (start = '1') THEN

      temp1 := in0 & in1;

      CASE temp1 IS

        WHEN "00"   =>

          sub      <= '0';

          borrow_bit <= '0';

        WHEN "01"   =>

          sub      <= '1';

          borrow_bit <= '1';

        WHEN "10"   =>

          sub      <= '1';

          borrow_bit <= '0';

        WHEN OTHERS =>

          sub      <= '0';

          borrow_bit <= '0';

      END CASE;

    ELSE

      temp2 := borrow_bit_r & in0 & in1;

      CASE temp2 IS

        WHEN "000"  =>

          sub      <= '0';

          borrow_bit <= '0';

        WHEN "001"  =>

          sub      <= '1';

          borrow_bit <= '1';

        WHEN "010"  =>

          sub      <= '1';

          borrow_bit <= '0';

        WHEN "011"  =>

          sub      <= '0';

          borrow_bit <= '0';

        WHEN "100"  =>

          sub      <= '1';

          borrow_bit <= '1';

        WHEN "101"  =>

          sub      <= '0';

          borrow_bit <= '1';

        WHEN "110"  =>

          sub      <= '0';

          borrow_bit <= '0';

        WHEN OTHERS =>

          sub      <= '1';

          borrow_bit <= '1';

      END CASE;

    END IF;

  END PROCESS calc;

  sub_out <= sub;

END rtl;