global clock buffer problems

Dear all,

Here is my code for multiplexer. I used a secondary global clock buffer (BUFGS) in the code.

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library ieee;

use ieee.std_logic_1164.all;

entity clock_mux is

port (DATA, SEL : in std_logic;

SLOW_CLOCK, FAST_CLOCK : in std_logic;

DOUT : out std_logic);

end clock_mux;

architecture XILINX of clock_mux is

signal CLOCK : std_logic;

signal CLOCK_GBUF : std_logic;

component BUFGS

port (I : in std_logic;

O : out std_logic);

end component;

begin

Clock_MUX : process (SEL)

begin

if (SEL = '1') then

CLOCK<= FAST_CLOCK;

else

CLOCK<= SLOW_CLOCK;

end if;

end process;

GBUF_FOR_MUX_CLOCK : BUFGS

port map (I=> CLOCK, O=> CLOCK_GBUF);

Data_Path : process (CLOCK_GBUF, DATA)

begin

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

DOUT<= DATA;

end if;

end process;

end XILINX;

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But the error occurred after compile the code:

Error: Node instance "GBUF_FOR_MUX_CLOCK" instantiates undefined entity "BUFGS"

As i know, we need to instantiate the BUFGS by using the Insert Pads command. However, i do not clear about the Insert Pads command. Can anyone help me please.

Many thanks