184.108.40.206.2. Synchronizer Chains
If the number of synchronizer chains found is different from what you expect, or if the length of the shortest synchronizer chain is less than you expect, you might have to add or change Synchronizer Identification settings for the design. The report also provides the Worst Case Available Settling Time, defined as the available settling time for the synchronizer with the worst MTBF.
You can use the reported Fraction of Chains for which MTBFs Could Not be Calculated to determine whether a high proportion of chains are missing in the metastability analysis. A fraction of 1, for example, means that MTBF could not be calculated for any chains in the design. MTBF is not calculated if you have not identified the chain with the appropriate Synchronizer identification option, or if paths are not timing-analyzed and therefore have no valid slack for metastability analysis. You might have to correct your timing constraints to enable complete analysis of the applicable register chains.