74ls138实现四人表决器

74ls138可以实现四人表决器，具体实现方法如下：
1.将四个按键分别连接到74ls138的三个使能端（G1、G2A、G2B）和一个地址端（A0、A1、A2）上，将74ls138的输出端（Y0~Y7）连接到一个四输入与非门74ls20的输入端上。
2.将74ls20的输出端连接到指示灯F上。
3.当四个按键中有三个或以上按下时，74ls138的输出端会输出一个低电平，74ls20的输入端会接收到至少三个低电平，此时74ls20的输出端会输出一个高电平，指示灯F会亮起来。
4.当四个按键中有两个或以下按下时，74ls138的输出端会输出一个高电平，74ls20的输入端会接收到至少一个高电平，此时74ls20的输出端会输出一个低电平，指示灯F会熄灭。

-- 74ls138实现四人表决器
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

entity vote is
    Port ( A : in  STD_LOGIC_VECTOR (2 downto 0);
           G1 : in  STD_LOGIC;
           G2A : in  STD_LOGIC;
           G2B : in  STD_LOGIC;
           F : out  STD_LOGIC);
end vote;

architecture Behavioral of vote is

signal Y : std_logic_vector(7 downto 0);

begin

Y(0) <= not (A(2) and A(1) and A(0) and G1 and G2A and G2B);
Y(1) <= not (A(2) and A(1) and not A(0) and G1 and G2A and not G2B);
Y(2) <= not (A(2) and not A(1) and A(0) and G1 and not G2A and G2B);
Y(3) <= not (A(2) and not A(1) and not A(0) and G1 and not G2A and not G2B);
Y(4) <= not (not A(2) and A(1) and A(0) and not G1 and G2A and G2B);
Y(5) <= not (not A(2) and A(1) and not A(0) and not G1 and G2A and not G2B);
Y(6) <= not (not A(2) and not A(1) and A(0) and not G1 and not G2A and G2B);
Y(7) <= not (not A(2) and not A(1) and not A(0) and not G1 and not G2A and not G2B);

F <= not (Y(0) and Y(1) and Y(2) and Y(3) and Y(4) and Y(5) and Y(6) and Y(7));

end Behavioral;