Ensure that all the timing constraints for the core were properly incorporated from the example design and that all constraints were met during implementation.
• Does it work in post-place and route timing simulation? If problems are seen in hardware but not in timing simulation, this could indicate a PCB issue. Ensure that all clock sources are active and clean.
• If you are using MMCMs in the design, ensure that all MMCMs have obtained lock by monitoring the LOCKED port.