大家知道这个程序是做什么用的吗

library ieee;
use ieee.std_logic_1164.all        ;
use ieee.std_logic_arith.all;
use ieee.std_logic_unsigned.all;

entity twice is
port (clk,rst:in std_logic;
       clk_temp:inout std_logic;
       d_out:inout std_logic;
       d_outn:inout std_logic;
       clk_outut std_logic);
end twice;

architecture rtl of twice is
begin
process(clk_temp,rst)
begin
        if(rst='1') then
                d_out<='0';
        else
                if(clk_temp'event and clk_temp='1') then
                        d_out<=d_outn;
                end if;
        end if;
end process;

clk_temp<=clk xor d_out;

clk_out<=clk_temp;

d_outn<=not d_out;
end rtl;

D触发器哈！