Constraining Asynchronous Signals - 2022.1 English

Vivado Design Suite User Guide: Using Constraints (UG903)

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2022.1 English

The set_max_delay command can also be used to constrain asynchronous signals that do not have a clock relationship, but which require maximum delay.

For example, timing paths between two asynchronous clock domains can be disabled with the set_clock_groups command (recommended) or the set_false_path command (not recommended). This assumes that you have properly designed the inter-clock domains with, for instance, a double registers synchronizer or a FIFO. However, you must still ensure that the path delay between the two clock domains is not unnecessarily high.

In some multi-bit CDC scenarios the skew between the bits must be within certain requirements. Even though the skew can be constrained through the Bus Skew constraint (set_bus_skew), it must be ensured that the path delay between the two clock domains is not unnecessarily high. This can be done by replacing the set_false_path or set_clock_groups constraints inside the source XDC file on the relevant path(s) with set_max_delay –datapath_only. Refer to CDC Constraints for further information on constraining CDC paths.

Note: There is runtime impact between a False Path constraint and a Max Delay constraint because the paths are timed with Max Delay.

If a maximum delay must be specified for some or for all the paths between two clock domains, then you must use the command set_max_delay -datapath_only to constrain those paths. In this case, set_clock_groups cannot be used to define the two clock domains as asynchronous, as it supersedes the set_max_delay constraint in terms of constraint priority. Other cross clock domains paths must then be constrained with a combination of set_false_path or set_max_delay constraints.

See the following example:

set_max_delay <delay> -datapath_only -from <startpoints_source_clock_domain> \
-to <endpoints_destination_clock_domain>