zer0_JuLz:
how to do that enabling the pull up resistor?
If the grounds are not connected then enabling the internal pull ups is not going to help.
You only need to enable the internal pull up if you have an open collector output on your other device.
This is written for power supplies but it applies equally to signals:-
http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html