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1.WITH-SELECT语句
1.WITH-SELECT语句
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 CNTCIS
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；
