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。

以下是一个简单的4位抢答器的VHDL代码，其中使用了一个计数器来实现倒计时，并且使用了一个状态机控制抢答过程。其中使用了7个数码管来显示倒计时，4个LED灯来显示抢答结果。

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

entity response_timer is
    port(
        key1: in std_logic; -- 抢答键1
        key2: in std_logic; -- 抢答键2
        key3: in std_logic; -- 抢答键3
        key4: in std_logic; -- 抢答键4
        clk: in std_logic; -- 时钟
        reset: in std_logic; -- 复位
        seg: out std_logic_vector(6 downto 0); -- 数码管
        led: out std_logic_vector(3 downto 0) -- LED灯
    );
end response_timer;

architecture rtl of response_timer is
    signal count: unsigned(13 downto 0); -- 倒计时计数器
    signal state: integer range 0 to 5; -- 状态机状态
    signal response: std_logic_vector(3 downto 0); -- 抢答信号

begin

    -- 数码管显示模块
    seg_display: entity work.seven_segment_display
        port map(
            clk => clk,
            reset => reset,
            data => count(11 downto 8),
            seg => seg
        );

    -- 倒计时计数器
    count_process: process(clk, reset)
    begin
        if reset = '1' then
            count <= (others => '0');
        elsif rising_edge(clk) then
            if count = 0 then
                count <= std_logic_vector(to_unsigned(9999, count'length));
            else
                count <= count - 1;
            end if;
        end if;
    end process;

    -- 状态机
    state_process: process(clk, reset)
    begin
        if reset = '1' then
            state <= 0;
            response <= (others => '0');
            led <= (others => '0');
        elsif rising_edge(clk) then
            case state is
                when 0 => -- 显示倒计时
                    if count = 0 then
                        state <= 1; -- 进入抢答状态
                    end if;
                when 1 => -- 抢答状态
                    if key1 = '1' and response(0) = '0' then
                        response <= "0001";
                        state <= 2;
                    elsif key2 = '1' and response(1) = '0' then
                        response <= "0010";
                        state <= 2;
                    elsif key3 = '1' and response(2) = '0' then
                        response <= "0100";
                        state <= 2;
                    elsif key4 = '1' and response(3) = '0' then
                        response <= "1000";
                        state <= 2;
                    elsif count = 0 then
                        state <= 3; -- 进入结果显示状态
                    end if;
                when 2 => -- 抢答成功
                    state <= 3; -- 进入结果显示状态
                when 3 => -- 结果显示状态
                    if response = "0001" then
                        led <= "0001"; -- 选手1抢答成功
                    elsif response = "0010" then
                        led <= "0010"; -- 选手2抢答成功
                    elsif response = "0100" then
                        led <= "0100"; -- 选手3抢答成功
                    elsif response = "1000" then
                        led <= "1000"; -- 选手4抢答成功
                    else
                        led <= (others => '0'); -- 没有选手抢答成功
                    end if;
            end case;
        end if;
    end process;

end rtl;