用VHDL语言设计一个4位抢答器。通过控制数码管实现倒计时显示，倒计时为9秒。显示为零后开始抢答，当有一路抢答按键按下，该路抢答信号将其余各路抢答信号封锁，并通过4个LED灯显示抢答结果（当1号选手抢答成功则灯A亮；当2号选手抢答成功则灯B亮；当3号选手抢答成功则灯C亮；当4号选手抢答成功则灯D亮；其他情况灯灭）。若在倒计时结束前抢答，则本选手成绩无效，显示该路抢答台号，并且蜂鸣器报警。

以下是基于VHDL语言的4位抢答器设计：

--定义计数器模块，用于倒计时
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

entity counter is
    port(
        clk: in std_logic;
        reset: in std_logic;
        count: out unsigned(3 downto 0)
    );
end entity counter;

architecture arch of counter is
    signal reg: unsigned(3 downto 0);
begin
    process(clk, reset)
    begin
        if reset = '1' then
            reg <= (others => '0');
        elsif rising_edge(clk) then
            if reg = 9 then
                reg <= (others => '0');
            else
                reg <= reg + 1;
            end if;
        end if;
    end process;

    count <= reg;
end architecture arch;

--定义4位抢答器模块
library ieee;
use ieee.std_logic_1164.all;

entity buzzer is
    port(
        clk: in std_logic;
        reset: in std_logic;
        buzzer: out std_logic
    );
end entity buzzer;

architecture arch of buzzer is
    signal buzzer_count: unsigned(23 downto 0);
    signal buzzer_on: std_logic := '0';
begin
    process(clk, reset)
    begin
        if reset = '1' then
            buzzer_count <= (others => '0');
            buzzer_on <= '0';
        elsif rising_edge(clk) then
            if buzzer_on = '1' and buzzer_count < 20000000 then
                buzzer_count <= buzzer_count + 1;
            else
                buzzer_count <= (others => '0');
                buzzer_on <= '0';
            end if;
        end if;
    end process;

    buzzer <= buzzer_on;

end architecture arch;

--定义4位抢答器模块
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

entity quiz_buzzer is
    port(
        clk: in std_logic;
        reset: in std_logic;
        start: in std_logic; --开始倒计时
        buzzer: out std_logic; --蜂鸣器
        led: out std_logic_vector(3 downto 0);
        digit: out std_logic_vector(6 downto 0)
    );
end entity quiz_buzzer;

architecture arch of quiz_buzzer is
    component counter
        port(
            clk: in std_logic;
            reset: in std_logic;
            count: out unsigned(3 downto 0)
        );
    end component;

    component buzzer
        port(
            clk: in std_logic;
            reset: in std_logic;
            buzzer: out std_logic
        );
    end component;

    signal count: unsigned(3 downto 0);
    signal buzzer_on: std_logic := '0';
    signal digit_on: std_logic := '0';
    signal led_on: std_logic_vector(3 downto 0) := (others => '0');
    signal lock: std_logic_vector(3 downto 0) := (others => '0');
    signal buzzer_trigger: std_logic := '0';
    signal buzzer_count: unsigned(23 downto 0) := (others => '0');
begin
    counter_inst: counter port map(clk => clk, reset => reset, count => count);
    buzzer_inst: buzzer port map(clk => clk, reset => reset, buzzer => buzzer_on);

    process(clk, reset)
    begin
        if reset = '1' then
            digit_on <= '0';
            led_on <= (others => '0');
            lock <= (others => '0');
            buzzer_trigger <= '0';
            buzzer_count <= (others => '0');
        elsif rising_edge(clk) then
            if start = '1' then --开始倒计时
                digit_on <= '1';
                led_on <= (others => '1');
            end if;

            if digit_on = '1' then --显示倒计时
                case count is
                    when 0 =>
                        digit <= "1000000"; --0
                    when 1 =>
                        digit <= "1111001"; --1
                    when 2 =>
                        digit <= "0100100"; --2
                    when 3 =>
                        digit <= "0110000"; --3
                    when 4 =>
                        digit <= "0011001"; --4
                    when 5 =>
                        digit <= "0010010"; --5
                    when 6 =>
                        digit <= "0000010"; --6
                    when 7 =>
                        digit <= "1111000"; --7
                    when 8 =>
                        digit <= "0000000"; --8
                    when 9 =>
                        digit <= "0010000"; --9
                    when others =>
                        digit <= "0000000";
                end case;
            end if;

            if count = 0 and digit_on = '1' then --倒计时结束，开始抢答
                digit_on <= '0';
                led_on <= (others => '0');
                buzzer_trigger <= '1';
            end if;

            if buzzer_trigger = '1' then --抢答
                if lock(0) = '0' and buzzer_on = '1' then
                    lock <= (others => '0');
                    lock(0) <= '1';
                    led_on(0) <= '1';
                elsif lock(1) = '0' and buzzer_on = '1' then
                    lock <= (others => '0');
                    lock(1) <= '1';
                    led_on(1) <= '1';
                elsif lock(2) = '0' and buzzer_on = '1' then
                    lock <= (others => '0');
                    lock(2) <= '1';
                    led_on(2) <= '1';
                elsif lock(3) = '0' and buzzer_on = '1' then
                    lock <= (others => '0');
                    lock(3) <= '1';
                    led_on(3) <= '1';
                elsif buzzer_count < 20000000 then
                    buzzer_count <= buzzer_count + 1;
                else
                    buzzer_count <= (others => '0');
                    buzzer_trigger <= '0';
                    buzzer_on <= '0';
                    digit_on <= '1';
                    led_on <= (others => '1');
                    if lock(0) = '0' then
                        digit <= "0111111"; --1
                        buzzer_on <= '1';
                    elsif lock(1) = '0' then
                        digit <= "0011111"; --2
                        buzzer_on <= '1';
                    elsif lock(2) = '0' then
                        digit <= "0001111"; --3
                        buzzer_on <= '1';
                    elsif lock(3) = '0' then
                        digit <= "0000111"; --4
                        buzzer_on <= '1';
                    else
                        digit_on <= '0';
                    end if;
                end if;
            end if;
        end if;

        led <= led_on;
        buzzer <= buzzer_on;
    end process;
end architecture arch;

该设计中包含了计数器模块和抢答器模块，并且使用了数码管、LED灯和蜂鸣器进行显示和提示。

在抢答阶段，先判断每个选手是否已经抢答成功，若没有则等待选手按下抢答键，若已有一名选手抢答成功，则封锁其他选手的抢答信号，并通过LED灯显示抢答结果。若在倒计时结束前抢答，则该次抢答无效，并且显示该路抢答台号，并且蜂鸣器报警。