1．            系统功能及要求：
汽车上有一转弯控制杆,此杆有三种状态:中间位置时,汽车不转弯;向上时,汽车左转;
向下时汽车右转。汽车转弯时相应尾灯相应头灯均闪烁,当应急开关合上时,头灯尾
灯均闪烁。汽车刹车时,2个尾灯发出一直亮的信号。如果汽车刹车时正在转弯，则
相应的转弯闪烁信号不受影响。
2．    逻缉抽象：
由要求转换成真值表
          输入信号                          输出信号
         刹车  应急  左转  右转         左头灯 右头灯  左尾灯  右尾灯
         开关  开关  开关  开关          状态   状态    状态    状态
        （A）  (B)   (C)    (D)            (E)    (F)      (G)     (H)
          0     0     0      0             断     断      断      断   
          0     0     0      1             断     闪      断      闪  
          0     0     1      0             闪     断      闪      断  
          0     0     1      1             断     断      断      断
          0     1     0      0             闪     闪      闪      闪
          0     1     0      1             闪     闪      闪      闪
          0     1     1      0             闪     闪      闪      闪
          0     1     1      1             断     断      断      断
          1     0     0      0             断     断      通      通
          1     0     0      1             断     闪      通      闪
          1     0     1      0             闪     断      闪      通
          1     0     1      1             断     断      断      断
          1     1     0      0             闪     闪      通      通
          1     1     0      1             闪     闪      通      闪
          1     1     1      0             闪     闪      闪      通
          1     1     1      1             断     断      断      断
     注：断——灯不亮    闪——灯闪烁    通——灯一直亮
3．    由真值表得出逻缉表达式：
         E(断)=(C AND D) OR ((NOT B) AND (NOT C));
         E(闪)=(B AND (NOT C) ) OR (C AND (NOY(D));
         E(通)=0;
         F(断)=(C AND D) OR ((NOT B) AND (NOT D));
         F(闪)=((NOT C) AND D) OR (B AND (NOT(D));
         F(通)=0;
         G(断)=(C AND D) OR ((NOT A) AND (NOT B) AND (NOT C));
         G(闪)=(C AND (NOT D)) OR ((NOT A) AND B AND (NOT C));
         G(通)=A AND (NOT C);
         H(断)=(C AND D) OR ((NOT A) AND (NOT B) AND (NOT D));
         H(闪)=((NOT C) AND D) OR ((NOT A) AND B AND (NOT D));
         H(通)=A AND (NOT D);
4．    VHDL语言实现（源程序注释）：
   library IEEE;                  ——定义IEEE库，使用std_logic_1164   
   use IEEE.std_logic_1164.all;         std_logic_unsigned 包集合
   use IEEE.std_logic_unsigned.all;

   entity autolight is               ——定义实体autolight    
port (                            A,B,C,D对应上述四个开关，信号为输入
        A: in STD_LOGIC;             E,F,G,H 对应上述前后车灯，信号为输出
        C: in STD_LOGIC;
        B: in STD_LOGIC;
        D: in STD_LOGIC;
        E: out STD_LOGIC;
        F: out STD_LOGIC;
        G: out STD_LOGIC;
        H: out STD_LOGIC
    );
end autolight;

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