CDC-50003: CE-Type CDC Missing Skew Constraints

When a multi-bit data transfer to a DFFE (D Flip-Flop with Enable) register crosses asynchronous clock domains, and the enable signal to the DFFE is synchronous with the latching domain, you must constrain the skew of the bits on the data bus. Otherwise, there is no guarantee that all bits of the bus latch on the same clock cycle.

Recommendation

Apply a set_max_skew constraint to each bit of a DFFE bus to ensure that all bits latch on the same clock cycle.

In the following example, the enable signal is synchronized, but requires a max skew constraint on the n-bit data transfer:

Figure 1. Example Transfer to a Multi-Bit Register Controlled by a Clock Enable Pin

Severity

High

Stage

Plan, Place, Route, Finalize

Device Family

  • Intel® Stratix® 10
  • Intel® Agilex™
  • Intel® Cyclone® 10 GX
  • Intel® Arria® 10