--　　二進制碼->格雷碼（編碼）：
--    從最右邊一位起，依次將每一位與左邊一位異或(XOR)，
--    作為對應格雷碼該位的值，最左邊一位不變.
library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_arith.all;
use ieee.std_logic_signed.all;

entity bin_to_gray is
generic (n : integer := 3);
port (
      reset : in  std_logic;
	  bin   : in  std_logic_vector(n downto 0);
	  gray  : out std_logic_vector(n downto 0) 
	 );
end bin_to_gray;

architecture behav of bin_to_gray is
  signal temp1,temp2 : std_logic_vector(n downto 0);
begin
  temp1 <= bin;
  process(reset,temp1)
    variable i : integer range 0 to n-1 := 0;
  begin
    if reset = '1' then
      temp2 <= (others => '0');
	else
      for i in 0 to n-1 loop
        temp2(i) <= temp1(i+1) xor temp1(i);     
      end loop;
      temp2(n) <= temp1(n);
    end if;
  end process;
  gray <= temp2;
end behav;