Intel® Quartus® Prime Pro Edition User Guide: Design Optimization

ID 683641
Date 10/04/2021

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Document Table of Contents Duplicate Registers for Fan-Out Control

Often, timing failures can occur due to the influence of signals that are not directly involved in the failing transfers. This condition tends to manifest when off-critical nets, most commonly with a high fan-out, span a large distance and consequentially, warp the optimization of other paths around them.

Duplicating the sources of these types of globally-influential signals can help to disperse them across many hops, or even across many clock cycles, and focus more on local transfers.

For example, by duplicating a high fan-out signal in the form of a tree of registers, you can disperse the signal over several clock cycles. As the signal progresses down the tree, it progressively feeds more into local copies of the original registers, such that any individual register's destinations are well-localized and its influence on register optimization is minimal. The key to this optimization is to determine how to assign the original signal’s fan-outs among the duplicates. If any individual register requires driving a large distance, the benefit of the tree can be removed.

You can manually create a register tree and group the endpoints in the RTL by leveraging your system-level knowledge about how best to disperse the signal throughout your design, but it can be time consuming and have a widespread impact. For more information about manually creating a register tree, refer to Manual Register Duplication.

You can create register trees automatically in one of the following ways.

Each method has its own methodology to determine the number of duplicates to create and how to assign the fan-outs between the duplicates.