When I implement an external feedback loop using a Virtex-6 MMCM, I see the following warning reported by PAR, even if the FEEDBACK constraint is properly applied:
WARNING:Timing:3262 - Feedback on DCM or PLL Qam1/CLKcontroller/mmcm_adv_inst
forms an incomplete loop. The Tdcmino calculation will be invalid. If the DCM
or PLL uses external feedback, please apply the FEEDBACK constraint to
indicate the external board delay. Consult the Constraints Guide for further
information on how to apply the FEEDBACK constraint.
This is a known issue. A tactical patch has been released for this issue. Please consult the "AR33583_Timing_11.3_AllPlatforms_ReadMe.txt" file in the ZIP file for details on how to deploy the patch.
This issue is resolved in the 11.4 release of the software.