Scoped Timing Constraints Guidelines - 2023.2 English

Vivado Design Suite User Guide: Using Constraints (UG903)

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2023.2 English

To avoid negatively impacting the top-level design, it is important to make sure that timing constraints written for the IP or sub-module do not propagate beyond its boundary, except for clock definition in some cases.

For example, consider the case in which a false path constraint is defined in the IP XDC between two clocks that come into the IP. The IP includes proper circuitry for asynchronous clock boundaries, but perhaps not for the rest of the design. This is a problem if the two clocks are related and must be timed together in the rest of the design to have proper hardware functionality.

Also, as discussed in XDC Precedence, a timing exception defined in the IP XDC file can have higher precedence than top-level constraints and can override them, which is undesired. To avoid this situation, AMD recommends that you apply the constraints to netlist objects local to the IP. In case of a false path between two global clocks, the false path must be applied from a group of startpoint cells inside the IP to another group of endpoint cells inside the IP as well. This technique is referred to as point-to-point exceptions instead of global exceptions.