Fan-out is defined as the number of nodes driven by an instance or top‑level port. High fan-out nets can cause significant delays that result in an unroutable net. On a critical path, high fan-out nets can cause longer delays in a single net segment that result in the timing constraints not being met. To prevent this behavior, each device family has a global fan‑out value set in the Precision Synthesis software library. In addition, the Intel® Quartus® Prime software automatically routes high fan-out signals on global routing lines in the Intel device whenever possible.
To eliminate routability and timing issues associated with high fan-out nets, the Precision Synthesis software also allows you to override the library default value on a global or individual net basis. You can override the library value by setting a max_fanout attribute on the net.