Keep it Registered - 9.0 English

Tri-Mode Ethernet MAC LogiCORE IP Product Guide (PG051)

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9.0 English

To simplify timing and increase system performance in an FPGA, keep all inputs and outputs registered between your application and the core. This means that all inputs and outputs from your application should come from, or connect to a flip-flop. While registering signals might not be possible for all paths, it simplifies timing analysis and makes it easier for the AMD tools to place-and-route the design.