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;
q ut 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???
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