When I simulate a design that has bi-directional ports coming out of an OBUFTDS bi-directional primitive, the output is always an unknown force ('X'). Inputs to the OBUFTDS appear valid.
How can I resolve this issue?
This is a known issue. It has been fixed in ISE Design Suite 11 Update 3 (11.3). Please download and install the latest ISE Design Suite update from the Download Center at: