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.