The design has a DCM with no feedback. It is my understanding that the software defaults to Stepping Level 1, but the behavior of the DCM in the lab indicates that Stepping Level 0 is being used. What is wrong?
It has been determined that MAP is defaulting to the incorrect Stepping Level 0 for Virtex-4 LX and SX devices. The Stepping Level for FX devices is correct.
For more information on the implications of using incorrect Stepping Levels, see (Xilinx Answer 21605).
This problem is scheduled to be fixed in ISE version 9.1i SP3.
In the meantime, the problem can be avoided by setting the Stepping Level explicitly in the UCF file.
Virtex-4 LX and SX:
CONFIG STEPPING = 1 ;
CONFIG STEPPING = 0 ;
Once the default Stepping Levels have been corrected, designers with Step 0 LX and SX devices will need to specify the Stepping explicity. The default for FX devices will continue to be 0.