When I perform static timing analysis in 4.1i, the percent coverage seems to have decreased from previous software versions. Why?
In 4.1i, paths crossing between flip-flops clocked by unrelated clocks are no longer covered in the PERIOD constraint. Thus, the coverage has gone down, as these paths are no longer covered. A FROM TO or a relational PERIOD constraint is now necessary to analyze these paths.
Please see (Xilinx Answer 11850) for more information.