用VHDL代码写一个交通灯控制系统

好的，以下是一个简单的交通灯控制系统的VHDL代码：

entity traffic_light is
  port (
    clk     : in  std_logic;
    reset   : in  std_logic;
    green   : out std_logic;
    yellow  : out std_logic;
    red     : out std_logic
  );
end traffic_light;

architecture behavior of traffic_light is
  type state_type is (stop, ready, go, slow);
  signal state : state_type := stop;
  signal timer : integer range 0 to 10000000 := 0;

  constant green_time  : integer := 5000000;
  constant yellow_time : integer := 1000000;
  constant red_time    : integer := 5000000;

begin
  process (clk, reset)
  begin
    if reset = '1' then
      state <= stop;
      timer <= 0;
      green <= '0';
      yellow <= '0';
      red <= '1';
    elsif rising_edge(clk) then
      case state is
        when stop =>
          if timer = red_time then
            state <= ready;
            timer <= 0;
            green <= '1';
            yellow <= '0';
            red <= '0';
          else
            timer <= timer + 1;
          end if;
        when ready =>
          if timer = yellow_time then
            state <= go;
            timer <= 0;
            green <= '0';
            yellow <= '1';
            red <= '0';
          else
            timer <= timer + 1;
          end if;
        when go =>
          if timer = green_time then
            state <= slow;
            timer <= 0;
            green <= '0';
            yellow <= '1';
            red <= '0';
          else
            timer <= timer + 1;
          end if;
        when slow =>
          if timer = yellow_time then
            state <= stop;
            timer <= 0;
            green <= '0';
            yellow <= '0';
            red <= '1';
          else
            timer <= timer + 1;
          end if;
      end case;
    end if;
  end process;
end behavior;

以上代码模拟一个简单的红绿灯交替控制系统，当reset信号为‘1’时，系统会重置为初始状态，即红灯亮；当clk上升沿到来时，根据当前状态来调整信号，使得红绿灯交替闪烁，形成交通流畅的状态。