I have placed two PERIOD constraints in my design. One PERIOD constraint constrains a unique synchronous-to-synchronous elements path, and this constraint is not analyzed by the tool. The analyzed PERIOD constraint has no relationship to the one not analyzed.
Why is the first PERIOD constraint not analyzed by the tool?
When each period constraint is analyzed in isolation, they appear to cover the correct portion of the design. However, when run simultaneously, one period constraint analyzes the entire path and the other has zero paths analyzed. That is, each time group shares elements, and the last period constraint in the PCF file takes priority.
To work around this issue:
1. Create a FROM:TO constraint to replace the tighter of the two PERIOD constraints.
2. Use the PRIORITY keywords on the two PERIOD constraints and set the priority prior to implementation.
For more details on timing constraints, please see the Timing Constraints User Guide: http://www.xilinx.com/support/documentation/sw_manuals/xilinx11/ug612.pdf