hi! Understanding a coding (counter), help

HI!! help please...Why it gives me this error? I want to put a function of k n

library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_arith.all;

use ieee.std_logic_signed.all;

use ieee.numeric_std.all;

entity contador is

generic (

k : natural:= 8;

n : natural:= LOG2(k+1)--> error (10482): vhdl error at contador.vhd(12): object "log2" is used but not declared

);

port

(

clock: in std_logic;

reset_n: in std_logic;

Q : out std_logic_vector (n-1 downto 0)

);

end contador;

architecture rtl of contador is

signal clock,reset_n: std_logic;

signal Q_int: std_logic_vector(n-1 downto 0);

begin

PROCESS(clock, reset_n)

begin

if (reset_n = '0') then

Q_int<= (others => '0');

elsif ((clock'event) and (clock = '1')) then

if (Q_int<k) then

Q_int <= Q_int + 1;

else

Q_int<= (others => '0');

end if;

end if;

end process;

Q <= Q_int;

end rtl;