We have detected your current browser version is not the latest one. Xilinx.com uses the latest web technologies to bring you the best online experience possible. Please upgrade to a Xilinx.com supported browser:Chrome, Firefox, Internet Explorer 11, Safari. Thank you!

AR# 41746

XST - WARNING:Xst:647 - Input <> is never used.


I see the following warning in the XST report:

WARNING:Xst:647 - Input <***> is never used. This port will be preserved and left unconnected if it belongs to a top-level block or it belongs to a sub-block and the hierarchy of this sub-block is preserved.

However, when I open the design in Technology Schematic or in FPGA Editor I see that the port is connected in the design.

Is this a known issue?


There is a known issue with XST incorrectly highlighting this warning.

This issue is fixed in ISE 14.1.


In the future, as a good practice to determine if these XST warnings are valid or not, you can check the MAP report.

If the input is not being used, the MAP report should report that the input has no load and a corresponding message should appear in Section 3.

For example:

 INFO:LIT:243 - Logical network xxx has no load.


AR# 41746
Date Created 04/13/2011
Last Updated 10/09/2014
Status Active
Type General Article
  • ISE Design Suite - 13
  • ISE Design Suite - 13.1