写一个VHDL代码实现多功能数字钟，具备下列功能： 1、数字钟：能计时，实现小时、分钟、秒的显示； 2、数字跑表：精度至0.01秒 比如显示12.97秒； 3、闹钟: 可以设定闹钟，用试验箱上的蜂鸣器作为闹铃； 4、调时：可以对时间进行设定； 5、日期设定：能设定日期并显示当前日期； 6、除调时状态，其他状态均不应影响系统计时。

以下是一个简单的VHDL代码实现多功能数字钟：

entity Clock is
    Port ( clk: in std_logic;
           reset: in std_logic;
           btn1: in std_logic;
           btn2: in std_logic;
           btn3: in std_logic;
           buzzer: out std_logic_vector(1 downto 0);
           seg: out std_logic_vector(6 downto 0);
           an: out std_logic_vector(3 downto 0)
           );
end Clock;

architecture Behavioral of Clock is
    signal count: integer range 0 to 9999 := 0;
    signal sec: integer range 0 to 59 := 0;
    signal min: integer range 0 to 59 := 0;
    signal hr: integer range 0 to 23 := 0;
    signal timer: integer range 0 to 9999 := 0;
    signal timer_sec: integer range 0 to 99 := 0;
    signal alarm_hr: integer range 0 to 23 := 0;
    signal alarm_min: integer range 0 to 59 := 0;
    signal alarm_set: boolean := false;
    signal time_set: boolean := false;
    signal date_set: boolean := false;
    signal seg_data: std_logic_vector(6 downto 0);
    signal an_data: std_logic_vector(3 downto 0);
    signal buzzer_on: boolean := false;
    signal buzzer_count: integer range 0 to 10 := 0;
    signal date: std_logic_vector(5 downto 0) := "000000";
begin
    -- 时钟计数器
    process(clk, reset)
    begin
        if reset = '1' then
            count <= 0;
            sec <= 0;
            min <= 0;
            hr <= 0;
            timer <= 0;
            timer_sec <= 0;
            alarm_hr <= 0;
            alarm_min <= 0;
            alarm_set <= false;
            time_set <= false;
            date_set <= false;
            date <= "000000";
        elsif rising_edge(clk) then
            if count = 9999 then
                count <= 0;
                sec <= sec + 1;
                if sec = 60 then
                    sec <= 0;
                    min <= min + 1;
                    if min = 60 then
                        min <= 0;
                        hr <= hr + 1;
                        if hr = 24 then
                            hr <= 0;
                        end if;
                    end if;
                end if;
                if alarm_set and alarm_hr = hr and alarm_min = min then
                    buzzer_on <= true;
                else
                    buzzer_on <= false;
                end if;
            else
                count <= count + 1;
            end if;
            if timer_sec = 99 then
                timer_sec <= 0;
                timer <= timer + 1;
            else
                timer_sec <= timer_sec + 1;
            end if;
        end if;
    end process;

    -- 闹钟设置
    process(btn1, btn2, btn3)
    begin
        if btn1 = '1' then
            if not alarm_set then
                alarm_set <= true;
            else
                alarm_set <= false;
                buzzer_on <= false;
            end if;
        end if;
        if btn2 = '1' and alarm_set then
            alarm_hr <= alarm_hr + 1;
            if alarm_hr = 24 then
                alarm_hr <= 0;
            end if;
        end if;
        if btn3 = '1' and alarm_set then
            alarm_min <= alarm_min + 1;
            if alarm_min = 60 then
                alarm_min <= 0;
            end if;
        end if;
    end process;

    -- 时间设置
    process(btn1, btn2, btn3)
    begin
        if btn1 = '1' then
            time_set <= true;
        end if;
        if time_set then
            if btn2 = '1' then
                hr <= hr + 1;
                if hr = 24 then
                    hr <= 0;
                end if;
            end if;
            if btn3 = '1' then
                min <= min + 1;
                if min = 60 then
                    min <= 0;
                end if;
            end if;
        end if;
    end process;

    -- 日期设置
    process(btn1, btn2, btn3)
    begin
        if btn1 = '1' then
            date_set <= true;
        end if;
        if date_set then
            if btn2 = '1' then
                date(5 downto 4) <= date(5 downto 4) + 1;
                if date(5 downto 4) = "11" then
                    date(5 downto 4) <= "01";
                end if;
            end if;
            if btn3 = '1' then
                date(3 downto 2) <= date(3 downto 2) + 1;
                if date(3 downto 2) = "10" then
                    date(3 downto 2) <= "01";
                end if;
            end if;
        end if;
    end process;

    -- 数码管控制
    process(count, sec, min, hr, timer, timer_sec, alarm_set, time_set, date_set, date, buzzer_on, buzzer_count)
    begin
        -- 闹钟设置状态
        if alarm_set then
            seg_data <= "0111111";
            an_data <= "1110";
        -- 时间设置状态
        elsif time_set then
            seg_data <= "0111111";
            an_data <= "1101";
        -- 日期设置状态
        elsif date_set then
            case date(5 downto 4) is
                when "01" =>
                    seg_data <= "0111111";
                when "10" =>
                    seg_data <= "0000110";
                when "11" =>
                    seg_data <= "0101101";
            end case;
            case date(3 downto 2) is
                when "01" =>
                    an_data <= "0111";
                when "10" =>
                    an_data <= "1011";
                when "11" =>
                    an_data <= "1101";
            end case;
        -- 数字钟状态
        else
            case sec mod 2 is
                when 0 =>
                    seg_data <= "0111111";
                when 1 =>
                    seg_data <= "1111111";
            end case;
            case timer_sec is
                when 0 =>
                    case timer mod 10 is
                        when 0 =>
                            seg_data <= "0000001";
                            an_data <= "0111";
                        when 1 =>
                            seg_data <= "1001111";
                            an_data <= "0111";
                        when 2 =>
                            seg_data <= "0010010";
                            an_data <= "0111";
                        when 3 =>
                            seg_data <= "0000110";
                            an_data <= "0111";
                        when 4 =>
                            seg_data <= "1001100";
                            an_data <= "0111";
                        when 5 =>
                            seg_data <= "0100100";
                            an_data <= "0111";
                        when 6 =>
                            seg_data <= "0100000";
                            an_data <= "0111";
                        when 7 =>
                            seg_data <= "0001111";
                            an_data <= "0111";
                        when 8 =>
                            seg_data <= "0000000";
                            an_data <= "0111";
                        when 9 =>
                            seg_data <= "0000100";
                            an_data <= "0111";
                    end case;
                else
                    case sec is
                        when 0 =>
                            case hr is
                                when 0 =>
                                    seg_data <= "0000000";
                                    an_data <= "1111";
                                when 1 =>
                                    seg_data <= "1000000";
                                    an_data <= "1111";
                                when 2 =>
                                    seg_data <= "0100000";
                                    an_data <= "1111";
                                when 3 =>
                                    seg_data <= "1100000";
                                    an_data <= "1111";
                                when 4 =>
                                    seg_data <= "0010000";
                                    an_data <= "1111";
                                when 5 =>
                                    seg_data <= "1010000";
                                    an_data <= "1111";
                                when 6 =>
                                    seg_data <= "0110000";
                                    an_data <= "1111";
                                when 7 =>
                                    seg_data <= "1110000";
                                    an_data <= "1111";
                                when 8 =>
                                    seg_data <= "0001000";
                                    an_data <= "1111";
                                when 9 =>
                                    seg_data <= "1001000";
                                    an_data <= "1111";
                                when 10 =>
                                    seg_data <= "0101000";
                                    an_data <= "1111";
                                when 11 =>
                                    seg_data <= "1101000";
                                    an_data <= "1111";
                                when 12 =>
                                    seg_data <= "0011000";
                                    an_data <= "1111";
                                when 13 =>
                                    seg_data <= "1011000";
                                    an_data <= "1111";
                                when 14 =>
                                    seg_data <= "0111000";
                                    an_data <= "1111";
                                when 15 =>
                                    seg_data <= "1111000";
                                    an_data <= "1111";
                                when 16 =>
                                    seg_data <= "0000100";
                                    an_data <= "1111";
                                when 17 =>
                                    seg_data <= "1000100";
                                    an_data <= "1111";
                                when 18 =>
                                    seg_data <= "0100100";
                                    an_data <= "1111";
                                when 19 =>
                                    seg_data <= "1100100";
                                    an_data <= "1111";
                                when 20 =>
                                    seg_data <= "0010100";
                                    an_data <= "1111";
                                when 21 =>
                                    seg_data <= "1010100";
                                    an_data <= "1111";
                                when 22 =>
                                    seg_data <= "0110100";
                                    an_data <= "1111";
                                when 23 =>
                                    seg_data <= "1110100";
                                    an_data <= "1111";
                                when others =>
                                    seg_data <= "0111111";
                                    an_data <= "1110";
                            end case;
                        when 1 =>
                            seg_data <= "1001111";
                            an_data <= "1011";
                        when 2 =>
                            seg_data <= "0010010";
                            an_data <= "1011";
                        when 3 =>
                            seg_data <= "0000110";
                            an_data <= "1011";
                        when 4 =>
                            seg_data <= "1001100";
                            an_data <= "1011";
                        when 5 =>
                            seg_data <= "0100100";
                            an_data <= "1011";
                        when 6 =>
                            seg_data <= "0100000";
                            an_data <= "1011";
                        when 7 =>
                            seg_data <= "0001111";
                            an_data <= "1011";
                        when 8 =>
                            seg_data <= "0000000";
                            an_data <= "1011";
                        when 9 =>
                            seg_data <= "0000100";
                            an_data <= "1011";
                        when others =>
                            seg_data <= "0111111";
                            an_data <= "1110";
                    end case;
            end case;
        end if;
        if buzzer_on and buzzer_count < 5 then
            buzzer <= "11";
            buzzer_count <= buzzer_count + 1;
        else
            buzzer <= "00";
            buzzer_count <= 0;
        end if;
        seg <= seg_data;
        an <= an_data;
    end process;
end Behavioral;

在这个代码中，我们用到了许多信号和过程。时钟计数器用于计时并更新小时、分钟和秒，闹钟设置用于设置闹钟并控制蜂鸣器，时间设置和日期设置用于设置时间和日期，并且数字钟状态用于控制数字显示。我们还有一个用于控制蜂鸣器的信号，它会在闹钟响铃时使用，并且在五秒钟内间隔地开关蜂鸣器。

当然，这只是一个简单的实现，您可以根据您的需求和资源进行适当的修改。