Unused LUTs and carry-chains were instantiated in the same SLICEs used for the read capture RPMs in the Virtex-5 DDR2 design. This was done to prevent unrelated logic from being packed in these slices. When this happens, there is potential for the unrelated logic to cause the DIRT strings associated with the read capture RPMs to be unroutable.
This error has occurred in the 9.x and might also occur in 10.1 design tools. This issue has been fixed in version 10.1. To work around this, the RPMs can be grouped into an area group. Then, attaching a CLOSE attribute to AREA_GROUP will prevent the implementation tools from packing logic that are not part of the area group with the RPMs. Edits to the phy_dq_iob.v/vhd file are necessary to implement this fix and are provided in the following link:
Replace the current calibration file with the one linked above.
The UCF (User Constraints File) must also be edited. Add the line below to attach the CLOSE attribute to the RPM area groups.
AREA_GROUP "DDR_CAPTURE_FFS" GROUP = CLOSED;
The V5 DDR2 design will implement the above fix in MIG 2.2.