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3-5用VHDLSHEJIYIGE3-8译码器，要求分别用赋值语句,case语句,if-else语句或移位操作符来完成。比较这4种方式中，那一种更节省逻辑资源。

本题用的是74LS138真值表：

一 顺序赋值语句1 CASE语句,2 IF-ELSE语句

1.case语句

LIBRARY IEEE;

USE IEEE.STD_LOGIC_1164.ALL;

ENTITY decoder3to8 IS

 PORT(A0,A1, A2 ,S0,S1,S2 : IN STD_LOGIC;

               Y:OUT STD_LOGIC_VECTOR(7 DOWNTO 0)

）;

END ENTITY decoder3to8;

ARCHITECTURE BEH38 OF decoder3to8 IS

SIGNAL DATA: STD_LOGIC_VECTOR (2 DOWNTO 0);

BEGIN

  DATA<= A0&A1&A2;

 PROCESS(DATA,S0,S1,S2,)

  BEGIN

    IF S0='1' and S1='0' and S2='0') THEN

       CASE(DATA) IS

         WHEN "000"  => y <="11111110";

         WHEN "001"  => y <="11111101";

         WHEN "010"  => y <="11111011";

         WHEN "011"  => y <="11110111";

         WHEN "100"  => y <="11101111";

         WHEN "101"  => y <="11011111";

         WHEN "110"  => y <="10111111";

         WHEN "111"  => y <="01111111";

         WHEN OTHERS => y <="11111111";

      END CASE;

    END IF;

  END PROCESS;

END ARCHITECTURE BEH38;

2.If-else语句

LIBRARY IEEE;

USE IEEE.STD_LOGIC_1164.ALL;

ENTITY decoder3to8 IS

 PORT(A0,A1, A2 ,S0,S1,S2 : IN STD_LOGIC;

        Y: OUT STD_LOGIC_VECTOR (7 DOWNTO 0)

）;

END ENTITY decoder3to8;

ARCHITECTURE BEH38 OF decoder3to8 IS

SIGNAL DATA: STD_LOGIC_VECTOR (2 DOWNTO 0);

BEGIN

  DATA<= A0&A1&A2;

 PROCESS(DATA,S0,S1,S2,)

  BEGIN

  IF (S0='1' and S1='0' and S2='0') THEN

        IF (DATA= "000") THEN  y<="11111110";

     ELSIF (DATA= "001") THEN  y<="11111101";

     ELSIF (DATA= "010") THEN  y<="11111011";

     ELSIF (DATA= "011" )THEN  y<="11110111";

     ELSIF (DATA= "100" )THEN  y<="11101111";

     ELSIF (DATA= "101" ) THEN y<="11011111";

     ELSIF (DATA= "110" ) THEN y<="10111111";

     ELSIF (DATA= "111" ) THEN y<="01111111";

  ELSE   y<="11111111";

 END IF;

END PROCESS;

END ARCHITECTURE BEH38;

二：并行赋值语句1WHEN-ELSE语句2.WITH-SELECT语句

1．WHEN-ELSE语句

LIBRARY IEEE;

USE IEEE.STD_LOGIC_1164.ALL;

ENTITY decoder3to8 IS

 PORT(A0,A1, A2 ,S0,S1,S2 : IN STD_LOGIC;

               Y:OUT STD_LOGIC_VECTOR(7 DOWNTO 0)

）;

END ENTITY decoder3to8;

ARCHITECTURE BEH38 OF decoder3to8 IS

SIGNAL DATA: STD_LOGIC_VECTOR (2 DOWNTO 0);

BEGIN

  DATA<= A0&A1&A2;

    IF （S0='1' and S1='0' and S2='0') THEN

    y <="11111110"  WHEN DATA ="000"  ELSE

        "11111101"  WHEN DATA="001"   ELSE       

        "11111011"  WHEN DATA="010"   ELSE      

        "11110111"  WHEN DATA="011"   ELSE     

        "11101111"  WHEN DATA="100"   ELSE

"11011111"  WHEN DATA="101"   ELSE

"10111111"  WHEN DATA="110"   ELSE

"01111111"  WHEN DATA="111"   ELSE

    y <="11111111" ;

END IF

END ARCHITECTURE BEH38;

2.WITH-SELECT语句

LIBRARY IEEE;

USE IEEE.STD_LOGIC_1164.ALL;

ENTITY decoder3to8 IS

 PORT(A0,A1, A2 ,S0,S1,S2 : IN STD_LOGIC;

               Y:OUT STD_LOGIC_VECTOR(7 DOWNTO 0)

）;

END ENTITY decoder3to8;

ARCHITECTURE BEH38 OF decoder3to8 IS

SIGNAL DATA: STD_LOGIC_VECTOR (2 DOWNTO 0);

BEGIN

  DATA<= A0&A1&A2;

WITH DATA SELECT

    IF （S0='1' and S1='0' and S2='0') THEN

y <="11111110"; WHEN  "000" 

        "11111101"  WHEN  "001"          

        "11111011"  WHEN  "010"       

        "11110111"  WHEN  "011"       

        "11101111"  WHEN  "100"  

"11011111"  WHEN  "101" 

 "10111111"  WHEN  "110" 

 "01111111"  WHEN  "111"

         ELSE

   y <= "11111111" ;

END IF

END ARCHITECTURE BEH38;

三.移位操作符

LIBRARY IEEE;

USE IEEE.STD_LOGIC_1164.ALL;

USE IEEE.STD_LOGIC_UNSIGNED.ALL;

ENEITY decoder3to8 IS

 PORT (DIN : IN STD_LOGIC_VECTOR (2 DOWNTO 0);

         DOUT:OUT BIT_ VECTOR( 7 DOWNTO 0)

);

END decoder3to8 ;

ARCHITECTURE BEH38 OF decoder3to8 IS

BEGIN

  DOUT<=“11111110” SLL CONV_ INTEGER(DIN);

END  ARCHITECTURE BEH38 ;

所以四种综合看移位操作符比较节省逻辑资源。

3-14 用循环语句设计一个7人投票表决器。

LIBRARY IEEE;

USE IEEE.STD_LOGIC_1164.ALL;

USE IEEE.STD_LOGIC_UNSIGNED.ALL;

ENEITY CNTC IS;

PORT (DIN : IN STD_LOGIC_VECTOR (6 DOWNTO 0);

     DOUT: OUT BIT_ VECTOR( 2 DOWNTO 0)

 Y:OUT STD_LOGIC;

);

END ENEITY CNTC;

ARCHITECTURE ONE OF CNTC IS

BEGIN

PROCESS (DIN)

VARIABLE Q : STD_LOGIC_VECTO(2 DOWNTO 0);

BEGIN

  Q :=”000”;

FOR n in 0 to 6 LOOP

IF (DIN(n)=’1’) THEN Q:=Q+1;

END IF;

END LOOP;

CNTC<=Q;

IF（CNTC >=‘100’）THEN Y=’1’

ELSE Y=’0’;

END PROCESS;

END ARCHITECTURE ONE；