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-- Copyright (C) 2010 Arif Endro Nugroho

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library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_unsigned.all;

entity c4b is

  port (

  cnt : out bit_vector (  3 downto 0);

  clk : in  bit;

  rst : in  bit

  );

end c4b;

architecture phy of c4b is

  signal sum : bit_vector (  3 downto 0); -- sum

  signal cr  : bit_vector (  3 downto 0); -- carry

  begin

    cr(0)            <= '0'; -- LSB always zero

    cr(3 downto 1) <= ( ((sum(2 downto 0) and B"001") or (sum(2 downto 0) and cr(2 downto 0))) or (B"001" and cr(2 downto 0)) );

    process (clk)

    begin

      if (clk = '1' and clk'event) then

        if (rst = '1') then

          sum <= B"0000";

        else

          sum <= ((sum xor B"0001") xor cr); -- sum = ((addend xor augend) xor carry)

        end if;

      end if;

    end process;

    cnt <= sum;

end phy;