Description

If I try to apply a PERIOD constraint for an OFFSET OUT/BEFORE constraint on a shift register LUT that is being used as part of a chain, a fatal error reports that the PERIOD constraint cannot be found.

(If the design uses only the shift register as part of a chain, and the last stage is packed into the slice with the previous stage, TRACE will not be able to validate the D input of that BEL as a path endpoint.)

Solution

Currently, you can work around this issue by not applying an OFFSET OUT/BEFORE constraint on a shift register LUT that is being used as part of a chain for Virtex-II.

For more information, please also see (Xilinx Answer 5489).

This issue is fixed in the 11.1 release of the software.