对于vhdl中的常量接口，没有实例值

-- 定义一个包含常量接口的包
package constant_pkg is
    constant COUNTER_SIZE : integer := 8;
end package constant_pkg;

-- 使用常量接口的实体
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use work.constant_pkg.all; -- 引用包含常量接口的包

entity counter is
    Port ( clk : in STD_LOGIC;
           reset : in STD_LOGIC;
           count : out STD_LOGIC_VECTOR (COUNTER_SIZE-1 downto 0) );
end counter;

-- 实现计数器
architecture Behavioral of counter is
begin
    process(clk, reset)
    variable count_var : integer := 0;
    begin
        if reset = '1' then
            count_var := 0;
        elsif rising_edge(clk) then
            count_var := count_var + 1;
        end if;
        count <= STD_LOGIC_VECTOR(to_unsigned(count_var, COUNTER_SIZE));
    end process;
end Behavioral;