General Description: Contrary to the information stated in the XST User Guide, XST will produce an incorrect netlist constraint file (.ncf) if the IOSTANDARD attribute is declared on a port in VHDL or Verilog. This results in the following errors in NGDBuild:
"ERROR:NgdBuild:393 - Could not find INST(S) '<instance_name>' in design '<design_name>'.
ERROR:NgdBuild:134 - Unable to process the "<design_name>.ncf" constraints file; please see the above messages for details. Check to make sure the file exists and that you have read permission for it. If this is the case, check the syntax of the file using the "Attributes, Constraints, and Carry Logic" section of the Libraries Guide as a reference.
ERROR:NgdBuild:276 - edif2ngd exited with errors (return code 1).
ERROR:NgdBuild:28 - Top-level input design file "<design_name>.edn" cannot be found or created. Please make sure the source file exists and is of a recognized netlist format (e.g., NGO, EDIF or XNF)."
The work-around for both VHDL and Verilog is to place the IOSTANDARD attribute on a temporary signal in the architecture/module. The temporary signal is then connected to the appropriate port.
Another option is to instantiate the appropriate buffer.
NOTE: This problem was fixed in the 4.1i software release.