请好心人帮忙解答高分求助

hellomcu

请好心人帮忙解答高分求助 [复制链接]

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entity frequencydivider is
Port ( clk : in  STD_LOGIC;
         clk_div2 : out  STD_LOGIC;
         clk_div4 : out  STD_LOGIC;
         clk_div8 : out  STD_LOGIC);
end frequencydivider;

architecture Behavioral of frequencydivider is
signal count : std_logic_vector(3 downto 0) ;--= "0000";

begin

   process(clk)
begin
      if(clk'event and clk ='1')then
      if(count = "1111") then
      count <= (others => '0');
   else
      count <= count + 1;
end if;
   end if;
end process;
clk_div2 <= count(0)after 20ns;
clk_div4 <= count(1) after 20ns;
clk_div8 <= count(2) after 20ns;

end Behavioral;

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请问在用波形测试的时候为什么结果为U 和X 呢
warning信息:Warning: There is an 'U'|'X'|'W'|'Z'|'-'
in an arithmetic operand, the result will be 'X'(es).

谢谢了~