8421转换余3码的vhdl

8421码转换为余3码的VHDL代码如下：

library ieee;
use ieee.std_logic_1164.all;

entity bcd_to_excess_3 is
    port (bcd_input: in std_logic_vector(3 downto 0);
          excess_3_output: out std_logic_vector(3 downto 0));
end bcd_to_excess_3;

architecture Behavioral of bcd_to_excess_3 is
begin
    process (bcd_input)
    begin
        case bcd_input is
            when "0000" => excess_3_output <= "0011";
            when "0001" => excess_3_output <= "0100";
            when "0010" => excess_3_output <= "0101";
            when "0011" => excess_3_output <= "0110";
            when "0100" => excess_3_output <= "0111";
            when "0101" => excess_3_output <= "1000";
            when "0110" => excess_3_output <= "1001";
            when "0111" => excess_3_output <= "1010";
            when "1000" => excess_3_output <= "1011";
            when "1001" => excess_3_output <= "1100";
            when "1010" => excess_3_output <= "1101";
            when "1011" => excess_3_output <= "1110";
            when "1100" => excess_3_output <= "1111";
            when others => excess_3_output <= "0000"; -- 若输入不在0-9范围内，则输出全0
        end case;
    end process;
end Behavioral;

这个VHDL代码定义了一个名为"bcd_to_excess_3"的模块，包含一个4位的BCD输入信号（bcd_input）和一个4位的余3码输出信号（excess_3_output）。

在架构部分的进程中，使用了一个case语句根据BCD输入的不同情况进行判断和处理。根据输入的BCD码值，赋值给相应的余3码值。当输入不在0-9的范围时，设置输出为全0。

这样，当输入为8421码时，即可通过该VHDL代码将其转换为余3码，并输出到excess_3_output信号。