1．系統設計要求
?????用6個發光管模擬6個汽車尾燈（左右各3個），用4個開關作為汽車控制信號,分別為：左拐、右拐、故障和剎車。
????車勻速行駛時，6個汽車尾燈全滅；右拐時，車右邊3個尾燈從左至右順序亮滅；左拐時，車左邊3個尾燈從右至左順序亮滅；故障時車6個尾燈一起明滅閃爍；剎車時，6個尾燈全亮
2．系統設計方案
???根據系統設計要求，采用自頂向下設計方法，頂層設計采用原理圖設計，它由主控模塊、左邊燈控制模塊和右邊燈控制模塊三部分組成。
3參考VHDL源程序
（1） 主控制模塊
?????說明：此程序為系統主控制模塊。當左轉時，lft信號有效；右轉時，rit信號有效；當左右信號都有效的時，lr有效。
library?ieee;
use?ieee.std_logic_1164.all;
entity?kz?is
???port(left,right:in?std_logic;
????????lft,rit,lr:out?std_logic);
end?kz;
architecture?kz_arc?of?kz?is
begin
???process(left,right)
???variable?a:std_logic_vector(1?downto?0);
???begin
?????a:=left&right;
?????case?a?is?
????????when"00"=>lft<='0';
??????????????????rit<='0';
??????????????????lr<='0';
????????when"10"=>lft<='1';
??????????????????rit<='0';
??????????????????lr<='0';
????????when"01"=>rit<='1';
??????????????????lft<='0';
??????????????????lr<='0';
????????when?others=>rit<='1';
?????????????????????lft<='1';
?????????????????????lr<='1';
?????end?case;
???end?process;
end?kz_arc;
????????
(2)左邊燈控制模塊
說明：此模塊的功能是當左轉時控制左邊的3個燈，當左右信號都有效時，輸出為全“1”。
library?ieee;
use?ieee.std_logic_1164.all;
entity?lfta?is
???port(en,clk,lr:in?std_logic;
????????l2,l1,l0:out?std_logic);
end?lfta;
architecture?lft_arc?of?lfta?is
begin
???process(clk,en,lr)
???variable?tmp:std_logic_vector(2?downto?0);
???begin
??????if?lr='1'?then
?????????tmp:="111";
??????elsif?en='0'?then
?????????tmp:="000";
??????elsif?clk'event?and?clk='1'?then
?????????if?tmp="000"?then
????????????tmp:="001";
?????????else
????????????tmp:=tmp(1?downto?0)&'0';
?????????end?if;
??????end?if;
??????l2<=tmp(2);
??????l1<=tmp(1);
??????l0<=tmp(0);
???end?process;
end?lft_arc;
?
（2） 右邊燈控制模塊
說明：此模塊的功能是控制右邊的3個燈，與上面模塊相似。
library?ieee;
use?ieee.std_logic_1164.all;
entity?rita?is
????port(en,clk,lr:in?std_logic;
?????????r2,r1,r0:out?std_logic);
end?rita;
architecture?rit_arc?of?rita?is
begin
???process(clk,en,lr)
???variable?tmp:std_logic_vector(2?downto?0);
???begin
????if?lr='1'?then
???????tmp:="111";
????elsif?en='0'?then
???????tmp:="000";
????elsif?clk'event?and?clk='1'?then
???????if?tmp="000"?then
??????????tmp:="100";
???????else
?????????tmp:='0'&tmp(2?downto?1);
???????end?if;
????end?if;
???????r2<=tmp(2);
???????r1<=tmp(1);
???????r0<=tmp(0);
????
???end?process;
end?rit_arc;