用VHDL语言实现交通灯控制： 主干道和次干道上等待通过的车辆轮流通过，主、次干道上车流的通过时间分别为45秒和25秒，绿灯到红灯的切换过渡时间均为5秒（亮黄灯）。只有当次干道有车等待通过时才作上述切换，否则保持主干道通过（即主干道绿灯亮次干道红灯亮）。实验板上一个按钮开关模拟次干道是否有车等待，例如，按钮按下表示次干道有车等待通过，松开则表示无车等待通过。

好的，下面是一个基于 VHDL 的交通灯控制的代码示例：

library ieee;
use ieee.std_logic_1164.all;

entity traffic_control is
port(
    clk: in std_logic;
    reset: in std_logic;
    button: in std_logic;
    main_road_green: out std_logic;
    main_road_yellow: out std_logic;
    main_road_red: out std_logic;
    side_road_green: out std_logic;
    side_road_yellow: out std_logic;
    side_road_red: out std_logic
);
end entity traffic_control;

architecture fsm of traffic_control is
    type state_type is (main_road_green_state, main_road_yellow_state, side_road_green_state, side_road_yellow_state);
    signal state: state_type;
    signal main_road_timer: integer range 0 to 45;
    signal side_road_timer: integer range 0 to 25;

begin

    process(clk, reset)
    begin
        if reset = '1' then
            state <= main_road_green_state;
            main_road_timer <= 0;
            side_road_timer <= 0;
        elsif rising_edge(clk) then
            case state is
                when main_road_green_state =>
                    main_road_green <= '1';
                    main_road_yellow <= '0';
                    main_road_red <= '0';
                    side_road_green <= '0';
                    side_road_yellow <= '1' when (side_road_timer = 0 and button = '1') else '0';
                    side_road_red <= '1' when (side_road_timer = 0 and button = '1') else '0';
                    if main_road_timer = 45 then
                        state <= main_road_yellow_state;
                        main_road_timer <= 0;
                    else
                        main_road_timer <= main_road_timer + 1;
                    end if;
                when main_road_yellow_state =>
                    main_road_green <= '0';
                    main_road_yellow <= '1';
                    main_road_red <= '0';
                    side_road_green <= '0';
                    side_road_yellow <= '1' when (side_road_timer = 0 and button = '1') else '0';
                    side_road_red <= '1' when (side_road_timer = 0 and button = '1') else '0';
                    if main_road_timer = 5 then
                        if button = '1' then
                            state <= side_road_green_state;
                        else
                            state <= main_road_green_state;
                        end if;
                        main_road_timer <= 0;
                    else
                        main_road_timer <= main_road_timer + 1;
                    end if;
                when side_road_green_state =>
                    main_road_green <= '0';
                    main_road_yellow <= '1';
                    main_road_red <= '0';
                    side_road_green <= '1';
                    side_road_yellow <= '0';
                    side_road_red <= '0';
                    if side_road_timer = 25 then
                        state <= side_road_yellow_state;
                        side_road_timer <= 0;
                    else
                        side_road_timer <= side_road_timer + 1;
                    end if;
                when side_road_yellow_state =>
                    main_road_green <= '0';
                    main_road_yellow <= '1';
                    main_road_red <= '0';
                    side_road_green <= '0';
                    side_road_yellow <= '1';
                    side_road_red <= '0';
                    if side_road_timer = 5 then
                        state <= main_road_green_state;
                        side_road_timer <= 0;
                    else
                        side_road_timer <= side_road_timer + 1;
                    end if;
            end case;
        end if;
    end process;

end architecture fsm;

这个状态机包含四个状态：main_road_green_state、main_road_yellow_state、side_road_green_state和side_road_yellow_state，分别对应主干道绿灯、黄灯和次干道绿灯、黄灯。在每个状态下，输出的信号会设置为对应状态的灯亮起，而其他灯则熄灭。时钟信号 clk 用于驱动状态机的状态转换，而复位信号 reset 则用于将状态机重置为初始状态。按钮信号 button 用于模拟次干道是否有车等待通过，按下表示有车等待，松开表示无车等待。

输出信号 main_road_green、main_road_yellow 和 main_road_red 分别对应主干道的绿灯、黄灯和红灯，输出信号 side_road_green、side_road_yellow 和 side_road_red 分别对应次干道的绿灯、黄灯和红灯。在次干道有车等待时，次干道的黄灯和红灯会亮起，等待时间为5秒，然后次干道的绿灯亮起，通过时间为25秒，接着次干道的黄灯亮起，等待时间为5秒，然后主干道的绿灯亮起，通过时间为45秒。

希望这个代码示例能够对您有所帮助！如果您还有其他问题，可以继续向我提问。