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AR# 25295

9.1i XST - "ERROR:Parsers:11 - Encountered unrecognized constraint while parsing"


Case 1: XST fails while annotating a XCF constraint file and errors out on an unrecognized constraint or a missing signal:

"Annotating constraints using XCF file ./accum_test_cw.xcf'

ERROR:Xst:1587 - Line 7: No signal name matches ce_2_sg_x12* pattern.

ERROR:Parsers:11 - Encountered unrecognized constraint while parsing.

ERROR:Xst:1341 - XCF parsing failed"

Case 2: NGDBuild fails with a similar error when annotating a UCF constraints file.


These errors occur because the XCF or UCF constraint is set on an "optimizable" net.

For the XCF, a timing constraint on a net does not guarantee that XST will preserve this net and, consequently, XST generates the error in Case 1 above. For a UCF constraint, whatever the constraint is, XST is not reading this file and, consequently, any object (FF, LUT, Signal ...) is potentially "optimizable".

For example, if you have two signals that are connected through hierarchy and are equivalent, when flattening the design, XST cannot determine which signal should be preserved. XST depends on how the optimization is done. If there is a UCF or XCF timing constraint on the optimized signal, XST or NGDBuild fails.

To work around this issue, if a constraint is set on an "optimizable" net, use the KEEP property on that net. For information on using the KEEP property, refer to the Constraints Guide available at:


This is a known issue and will be fixed in a future release.

AR# 25295
Date 12/15/2012
Status Active
Type General Article