The example design has low device utilization and a low utilization window is created, but sometime timing cannot be met if the core is constrained to this window, as it needs to be placed close to the GTs. The timing failures should not be seen in larger designs where the low utilization window would not get set. To work around this issue and disable the low device utilization window, the following environment variable can be set:
% setenv PAR_USE_LOWUTILHEUR 0
Alternatively an area group can be added to the UCF:
AREA_GROUP "xaui_core" RANGE = "CLOCKREGION_XiYj:CLOCKREGION_XiYj";
INST "xaui_block/*" AREA_GROUP = "xaui_core";
Where "i" and "j" are integers that constrain the clock region to be next to the chosen transceivers.