In Synplicity, when the global constraint "xc_fast_auto" is set, Synplicity writes the LVTTL IOSTANDARD attribute for all ports into the netlist. This constraint will overwrite any specific IOSTANDARD attribute that is assigned in the HDL.
The constraint "xc_fast_auto" controls the instantiation of fast output buffers in Virtex designs. Subsequently, the xc_fast_auto constraint can be overridden on an individual basis by the xc_padtype constraint.
For a brief description of these constraints, please see the Constraint Guide in the "Synplicity Constraints" section of the online software manuals:
For a complete description of how to apply these constraints, please refer to the online help within the Synplify/Synplify Pro tool.