A delay of 1ps or greater is required. A work-around is included in <component name>/source/<component name>.v. This file is generated as part of the files delivered by CORE Generator.
For other Spartan-6 FPGA Integrated Block Wrapper v1.1 for PCI Express Known Issues and Release Notes, see
(Xilinx Answer 32743).
This issue is resolved in ISE 11.3 and Spartan-6 FPGA Integrated Block Wrapper v1.2.
Revision History 09/16/2009 - Added fix version.
06/24/2009 - Initial Release