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AR# 31832

13.x Constraints - Is there a way to globally set IOSTANDARD constraints and not have to set it for each individual I/O?


The ISE tools set default I/O standards as LVCMOS25. Is there a way to change it to other I/O standards?


Add the below constraint in UCF file:


This constraint will overwrite the I/O standard constraints in XCF or HDL codes and set all the I/Os to USER_DEFAULT_IOSTANDARD.

If there are other IOSTANDARDs in the design, you need to add corresponding IOSTANDARD constraints to overwrite this global one. For example, if assigning the USET_DEFAULT_IOSTANDARD is a single-ended standard and there are some differential I/Os in the design, there would be an error during MAP process; you must manually specify the correct I/O standard in the UCF for those differential I/Os.

The DEFAULT IOSTANDARD constraints will not be supported.


Note: In the latest ISE 13.x software, there have been changes to the propagation rules of constraints on nets. This new propagation rules may prevent the above constraint from working as expected. The above constraint may provide inconsistent results on your given design.The use ofa wildcard to specify an IOSTANDARD is not ideal, since the wildcard will match to internal nets. Internal nets do not support I/O Standard constraints. It is recommended to specify the IOSTANDARD for every I/O in your design.

TheDEFAULT constraintdoessupport for DRIVE and SLEW; however, this does not apply to IOSTANDARD.

WARNING:NgdBuild:1421 - 'DEFAULT IOSTANDARD' constraint in file
'example_top.ucf' at line '41' will not globally set the iostandard for the

A suggested way to specify each I/O is to use the "Back Annotate Pin Locations" under the PAR process. This lists out all of the pins locked down from a successful PAR run, to which you can add IOSTANDARDs to each.


Linked Answer Records

Associated Answer Records

Answer Number Answer Title Version Found Version Resolved
37214 Virtex-6 FPGA Design Assistant - Troubleshoot common block RAM/FIFO problems N/A N/A
AR# 31832
Date Created 01/15/2009
Last Updated 12/15/2012
Status Active
Type General Article
  • ISE Design Suite - 13.1
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