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Use register replication to reduce high fanout. So be sure the signal is registered in your RTL or else the synthesis/PAR tool won't be able to reduce the fanout on that net. Just enable register replication and set your fanout limit in the synthesis/PAR tool.
Logic replication occurs early in the placement process for structures that fan out to other logic elements that cannot (for any number of reasons) reside in the same proximity. The problem this addresses is illustrated in below figure_1.
In this scenario, the output of D2 fans out to two structures that are relatively distant from one another. Regardless of where the driver is placed, the resultant route will be lengthy to one of the driven elements. To eliminate potentially long routing delays, logic duplication will replicate the driver as shown in below figure_2.
Logic duplication should be used only on critical path nets with multiple loads that cannot be physically localized.
The effect of this duplication is that an individual driver can be placed closer to each load, thereby minimizing the route length and delay of the route. Clearly,this optimization will reduce routing delays but will increase area. This optimization may cause inferior results if the utilization of the device is already high. If the placement tool is not intelligent enough to only perform this on critical-path routes, this option may need to be disabled and duplication added to the RTL with corresponding “don’t touch” attributes to ensure that the synthesis does not optimize the duplicate structures.
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