Dear all:

我在ISE下模擬counter 開一個test bench waveform file

當我按下 behavior simulate model後

卻出現 警告 "warning:there is an 'U'|'x'|'W'|'Z'|'-' is an arithmetic operand, the result will be 'X'(es).

source code:

library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_arith.all;

use ieee.std_logic_unsigned.all;

entiey counter is

port( clk:in std_logic;

qut std_logic_vector(3 downto 0) );

end counter;

architecture a of counter is

signal qn:std_logic_vector(3 downto 0);

begin

process(clk)

begin

if(clk'event and clk='1') then

  qn<qn+1;

end if;

q<=qn;

end a;

WHY???