3.2.2. Synchronizer Data Toggle Rate in MTBF Calculation
If multiple clocks apply, the highest frequency is used. If no source clocks can be determined, the data rate is taken as 12.5% of the synchronization clock frequency.
If you know an approximate rate at which the data changes, specify it with the Synchronizer Toggle Rate assignment in the Assignment Editor. You can also apply this assignment to an entity or the entire design. Set the data toggle rate, in number of transitions per second, on the first register of a synchronization chain. The timing analyzer takes the specified rate into account when computing the MTBF of that register chain. If a data signal never toggles and does not affect the reliability of the design, you can set the Synchronizer Toggle Rate to 0 for the synchronization chain so the MTBF is not reported. To apply the assignment with Tcl, use the following command:
set_instance_assignment -name SYNCHRONIZER_TOGGLE_RATE <toggle rate in transitions/second> -to <register name>
In addition to Synchronizer Toggle Rate , there are two other assignments associated with toggle rates, which are not used for metastability MTBF calculations. The I/O Maximum Toggle Rate is only used for pins, and specifies the worst-case toggle rates used for signal integrity purposes. The Power Toggle Rate assignment is used to specify the expected time-averaged toggle rate, and is used by the Power Analyzer to estimate time-averaged power consumption.
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