The ChipScope Pro Serial IO Tool Kit (also known as IBERT) has the ability to sweep through a variety of attribute settings for the GTP. When sweeping the data sample location through the eye, unexplainable errors have occurred in what would be the middle of the data eye. This Answer Record discusses the steps that can be followed to work around these errors.
The following ZIP file contains files that allow for the generation of an error-free IBERT design:
For Windows, extract the ZIP file into your ChipScope directory and regenerate the IBERT design with your desired parameters. Linux users will need to extract the "chipscope.jar" file to their "/ChipScope/bin/lin/" directory. This patch will only work with ISE and ChipScope 10.1 Service Pack 3.
This is resolved in ChipScope Pro 11.1.