When I design with an OBUFDS and use LVPECL, the tools do not utilize the DIFFO_OUT in the DIFFM and the DIFFO_IN in the DIFFS. Why?
The DIFFO_IN connection, which is used for LVDS outputs, is not supported in the hardware for LVPECL_33 outputs. For LVPECL_33 outputs, normal output paths are used to drive both DIFFM and DIFFS paths. A MAXSKEW constraint should be used to minimize skew between the DIFFM and DIFFS paths.