Non-FD Primitive - 2023.2 English

Vivado Design Suite User Guide: Design Analysis and Closure Techniques (UG906)

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2023.2 English

In the Non-FD Primitive example presented in the following figure, a CDC is occurring between a FDRE and a RAMB while no synchronization logic exists inside the RAMB primitive. Even if a single stage flip-flop connected to clk_b is inserted in front of the RAMB, it is still considered an inadequate synchronizer due to the routing distance between the FDRE and RAMB cells.

Figure 1. Non-FD Primitive Example

Note: This rule does not include the HARD_SYNC macro, which is detected and covered by CDC-18.