用vhdl语言设计实现一个8421码转换为格雷码的代码转换器代码

下面是一个用VHDL语言设计实现8421码转换为格雷码的代码转换器的示例代码：

library ieee;
use ieee.std_logic_1164.all;

entity gray_to_binary is
  port (
    binary: in std_logic_vector(3 downto 0);
    gray: out std_logic_vector(3 downto 0)
  );
end gray_to_binary;

architecture behavioral of gray_to_binary is
begin
  process (binary)
  begin
    case binary is
      when "0000" =>
        gray <= "0000";
      when "0001" =>
        gray <= "0001";
      when "0010" =>
        gray <= "0011";
      when "0011" =>
        gray <= "0010";
      when "0100" =>
        gray <= "0110";
      when "0101" =>
        gray <= "0111";
      when "0110" =>
        gray <= "0101";
      when "0111" =>
        gray <= "0100";
      when "1000" =>
        gray <= "1100";
      when "1001" =>
        gray <= "1101";
      when "1010" =>
        gray <= "1111";
      when "1011" =>
        gray <= "1110";
      when "1100" =>
        gray <= "1010";
      when "1101" =>
        gray <= "1011";
      when "1110" =>
        gray <= "1001";
      when "1111" =>
        gray <= "1000";
      when others =>
        gray <= "XXXX"; -- 错误输入
    end case;
  end process;
end behavioral;

上面的代码定义了一个名为`gray_to_binary`的模块，它有一个`binary`输入端口和一个`gray`输出端口。`binary`是四位的8421码输入，`gray`是四位的格雷码输出。

在`process`进程中，使用`case`语句根据`binary`的取值转换为对应的格雷码，并将结果赋值给`gray`端口。如果输入的`binary`不在0到15的范围内，则`gray`的值被设置为`XXXX`，表示错误的输入。

这个代码转换器可以用于将任意的8421码输入转换为对应的格雷码输出。