counter24.vhd

这个是数字电子时钟

library IEEE;
      use IEEE.STD_LOGIC_1164.ALL;
      use IEEE.STD_LOGIC_ARITH.ALL;
      use IEEE.STD_LOGIC_UNSIGNED.ALL;
      entity counter24 is
      Port ( clk : in std_logic;
      reset : in std_logic;
      din : in std_logic_vector(5 downto 0);
      dout : out std_logic_vector(5 downto 0));
      end counter24;
      architecture Behavioral of counter24 is
      signal count : std_logic_vector(5 downto 0);
      begin
      dout <= count;
      process(clk,reset,din)
      begin
      if reset= '0' then
      count <= din;
      elsif rising_edge(clk) then 
      if count(3 downto 0)="1001" then
      count(3 downto 0)<="0000";
      count(5 downto 4)<=count(5 downto 4) +1;
      else
      count(3 downto 0)<=count(3 downto 0)+1;
      end if;
      if count="100011" then
      count<="000000";
      end if;
      end if;
      end process;
      end Behavioral;