cnt25.vhd

假设某个十字路口是由一条主干道和一条次干道汇合而成

LIBRARY IEEE;
USE IEEE.STD_LOGIC_1164.ALL;
USE IEEE.STD_LOGIC_UNSIGNED.ALL;
ENTITY CNT25 IS
PORT( CLK:IN STD_LOGIC;
       EN:IN STD_LOGIC;
       S2:OUT STD_LOGIC;
     LOAD:IN BIT;
       Q2:OUT STD_LOGIC_VECTOR(7 DOWNTO 0));
END CNT25;
ARCHITECTURE BEHAVE OF  CNT25 IS
BEGIN
   PROCESS(CLK)
     VARIABLE CNT:STD_LOGIC_VECTOR(7 DOWNTO 0);
    BEGIN
       IF LOAD='1' THEN  CNT:="00011001";
       ELSIF (CLK'EVENT AND CLK='1')THEN
        IF EN='1' AND  CNT>"00000000" THEN
                   CNT:=CNT-1;
        ELSE CNT:="00011001";
        END IF;
      END IF;
    IF CNT="00000000"  THEN
       S2<='1';
       ELSE 
       S2<='0';
    END IF;
    Q2<=CNT; 
    END PROCESS;
  END BEHAVE;