説明

XST does not preserve port names with a leading "_" (underscore) in Verilog, and issues the following warning: 

"WARNING:Xst:921 - "file", line xx: Renaming <_name> to <X_name>"

ソリューション

To remain compliant with the various netlists formats associated with the implementation tools, XST adds an "X" at the beginning of a signal/port name that begins with an underscore ( _ ).