Logically Exclusive Clock Groups with Interaction - 2021.2 English

Vivado Design Suite User Guide: Using Constraints (UG903)

Document ID
UG903
Release Date
2021-11-17
Version
2021.2 English

The Timing Constraints wizard identifies logically exclusive clocks that have timing paths between each other elsewhere than just on the logic connected to the shared clock tree. The following figure shows an example where clkA and clkB have a shared clock tree portion, and also have a timing path from the shared clock tree to clkA only.

Figure 1. Example of a Design with Logically Exclusive Clocks with Interaction

Because only the clock domain crossing paths of the shared clock tree must be ignored, the wizard recommends to create generated clocks that are copies of clkA and clkB but that only exist on the shared clock tree. The clock groups constraint is applied to the generated clocks only, so that the paths outside the logic of the shared clock tree can still be normally timed. The following figure illustrates the wizard recommended constraints for the example above.

Figure 2. Recommended Constraints for Logically Exclusive Clocks with Interaction