实验内容

  1. 设计一个8-3线优先编码器（74LS148）；
  2. 设计一个3-8线译码器（74LS138）。

1. 设计一个8-3线优先编码器（74LS148）

实验原理：

74LS148真值表

image-20220211134604812

74LS148引脚图：

img

当使能端EI=0时，74LS148正常编码。位数越高优先级越高，输入输出均为正常逻辑的反向。输出为位值为‘0’的最高位所处位对应三位二进制数的取反。

实验模块代码：

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 LIBRARY IEEE;
USE IEEE.STD_LOGIC_1164.ALL;
USE IEEE.STD_LOGIC_UNSIGNED.ALL;

ENTITY CD8_3 IS
    PORT(INPUT: IN STD_LOGIC_VECTOR(7 DOWNTO 0);
            EN: IN STD_LOGIC;
            OUTPUT: OUT STD_LOGIC_VECTOR(2 DOWNTO 0);
            GS: OUT STD_LOGIC;
            E0: OUT STD_LOGIC
    );
END ENTITY;

ARCHITECTURE BEHAV OF CD8_3 IS
BEGIN
    PROCESS(INPUT,EN)
    BEGIN 
        IF(EN='0') THEN 
            IF (INPUT(7) = '0') THEN OUTPUT<="000"; GS<='0';E0<='1';
            ELSIF   (INPUT(6) = '0') THEN OUTPUT<="001"; GS<='0';E0<='1';
            ELSIF(INPUT(5) = '0') THEN OUTPUT<="010"; GS<='0';E0<='1';
            ELSIF(INPUT(4) = '0') THEN OUTPUT<="011"; GS<='0';E0<='1';
            ELSIF (INPUT(3) = '0') THEN OUTPUT<="100"; GS<='0';E0<='1';
            ELSIF (INPUT(2) = '0') THEN OUTPUT<="101"; GS<='0';E0<='1';
            ELSIF (INPUT(1) = '0') THEN OUTPUT<="110"; GS<='0';E0<='1';
            ELSIF (INPUT(0) = '0') THEN OUTPUT<="111"; GS<='0';E0<='1';
            ELSE  OUTPUT<="111"; GS<='1';E0<='0';
            END IF;
        ELSE OUTPUT<="111"; GS<='1';E0<='1';
    END IF;
    END PROCESS;
END ARCHITECTURE;

TestBench:

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LIBRARY ieee;           
USE ieee.std_logic_1164.all;          
USE IEEE.STD_LOGIC_UNSIGNED.ALL;

ENTITY CD8_3_vhd_tst IS
END CD8_3_vhd_tst;
ARCHITECTURE CD8_3_arch OF CD8_3_vhd_tst IS
                                               
SIGNAL E0 : STD_LOGIC;
SIGNAL EN : STD_LOGIC :='0';
SIGNAL GS : STD_LOGIC;
SIGNAL INPUT : STD_LOGIC_VECTOR(7 DOWNTO 0) :="00000000";
SIGNAL OUTPUT : STD_LOGIC_VECTOR(2 DOWNTO 0);
COMPONENT CD8_3
    PORT (
    E0 : OUT STD_LOGIC;
    EN : IN STD_LOGIC;
    GS : OUT STD_LOGIC;
    INPUT : IN STD_LOGIC_VECTOR(7 DOWNTO 0);
    OUTPUT : OUT STD_LOGIC_VECTOR(2 DOWNTO 0)
    );
END COMPONENT;
BEGIN
    U1 : CD8_3
    PORT MAP (
-- list connections between master ports and signals
    E0 => E0,
    EN => EN,
    GS => GS,
    INPUT => INPUT,
    OUTPUT => OUTPUT
    );

EN<= '0', '1' AFTER 200 NS;
INPUT <="10000000", "00010000" AFTER 100 NS, "11111111" AFTER 150 NS, "00000011" AFTER 200 NS; 

END CD8_3_arch;

仿真波形图：

img

实验结果：按照74LS148引脚图，实验模块设计中输入输出均为取反的情况，对照真值表得到仿真结果正确。

2. 设计一个3-8线译码器（74LS138）

实验原理：

74LS138真值表

image-20220211134817862

74LS138引脚图

img

当G1=1,/G2=0,/G3=0时，译码器正常译码，按照输入的3位二进制数，使输入对应数值的输出端口输出‘0’，其余输出‘1’（输出为正常逻辑的反向）。

实验模块代码：

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LIBRARY IEEE;
USE IEEE.STD_LOGIC_1164.ALL;
USE IEEE.STD_LOGIC_UNSIGNED.ALL;

ENTITY CD3_8 IS
    PORT(INPUT: IN STD_LOGIC_VECTOR(2 DOWNTO 0);
            G1: IN STD_LOGIC;
            G2_FAN,G3_FAN: IN STD_LOGIC;
            OUTPUT_FAN: OUT STD_LOGIC_VECTOR(7 DOWNTO 0)
    );
END ENTITY;

ARCHITECTURE BEHAV OF CD3_8 IS
BEGIN

    PROCESS(INPUT,G1,G2_FAN,G3_FAN)
    BEGIN 

        IF((G1='1') AND (G2_FAN='0') AND (G3_FAN='0')) THEN 
        CASE INPUT IS 
            WHEN "000" =>OUTPUT_FAN<="11111110";
            WHEN "001" =>OUTPUT_FAN<="11111101";
            WHEN "010" =>OUTPUT_FAN<="11111011";
            WHEN "011" =>OUTPUT_FAN<="11110111";
            WHEN "100" =>OUTPUT_FAN<="11101111";
            WHEN "101" =>OUTPUT_FAN<="11011111";
            WHEN "110" =>OUTPUT_FAN<="10111111";
            WHEN "111" =>OUTPUT_FAN<="01111111";
            WHEN OTHERS=>OUTPUT_FAN<="11111111";
        END CASE;
        ELSE OUTPUT_FAN<="11111111";
    END IF;
    END PROCESS;
END ARCHITECTURE;

Testbench:

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LIBRARY ieee;                                               
USE ieee.std_logic_1164.all;                                

ENTITY CD3_8_vhd_tst IS
END CD3_8_vhd_tst;
ARCHITECTURE CD3_8_arch OF CD3_8_vhd_tst IS
                                                
SIGNAL G1 : STD_LOGIC:='1';
SIGNAL G2_FAN : STD_LOGIC:='0';
SIGNAL G3_FAN : STD_LOGIC:='0';
SIGNAL INPUT : STD_LOGIC_VECTOR(2 DOWNTO 0):="000";
SIGNAL OUTPUT_FAN : STD_LOGIC_VECTOR(7 DOWNTO 0);
COMPONENT CD3_8
    PORT (
    G1 : IN STD_LOGIC;
    G2_FAN : IN STD_LOGIC;
    G3_FAN : IN STD_LOGIC;
    INPUT : IN STD_LOGIC_VECTOR(2 DOWNTO 0);
    OUTPUT_FAN : OUT STD_LOGIC_VECTOR(7 DOWNTO 0)
    );
END COMPONENT;
BEGIN
    u1 : CD3_8
    PORT MAP (
-- list connections between master ports and signals
    G1 => G1,
    G2_FAN => G2_FAN,
    G3_FAN => G3_FAN,
    INPUT => INPUT,
    OUTPUT_FAN => OUTPUT_FAN
    );
     
G1<= '1', '0' AFTER 200 NS,'1' AFTER 250 NS;
G2_FAN<='0', '1' AFTER 200 NS,'0' AFTER 250 NS;
G3_FAN<='0';
INPUT <="010", "101" AFTER 100 NS, "110" AFTER 150 NS, "111" AFTER 200 NS; 
                                          
END CD3_8_arch;

仿真波形：

img

实验结果：当G1=1,/G2=0,/G3=0时，译码正常；否则输出二进制码位全为‘1’。