Intel® Hyperflex™ Architecture High-Performance Design Handbook

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ID 683353
Date 10/04/2021
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2.2.2.2. High Fan-Out Clock Enable

Avoid high fan-out signals whenever possible. The high fan-out clock enable feeds a large amount of logic. The amount of logic is so large that the registers that you retime are pushing or pulling registers up and down the clock enable path for their specific needs. This pushing and pulling can result in conflicts along the clock enable line. This condition is similar to the aggressive retiming in the Synchronous Resets Summary section. Some of the methods discussed in that section, like duplicating the enable logic, are also beneficial in resolving conflicts along the clock enable line.

You typically use these high fan-out signals to disable a large amount of logic from running. These signals might occur when a FIFO’s full flag goes high. You can often design around these signals. For example, you can design the FIFO to specify almost full a few clock cycles earlier, and allow the clock enable a few clock cycles to propagate back to the logic that disables. You can retime these extra registers into the logic if necessary.

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