Fixing Large Hold Violations Prior to Routing - 2023.2 English

UltraFast Design Methodology Guide for FPGAs and SoCs (UG949)

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2023.2 English

For paths that have large hold violations (> 0.4 ns), it is advantageous to reduce the hold violations prior to routing the design, making it easier for the router to fix the remaining smaller hold violations using route detours. Reducing hold violations prior to routing can be beneficial if hold fixing has been identified as a source of routing congestion. The phys_opt_design hold fixing options each use different resources and have specific targets. It is important to use the proper option depending upon the device utilization and desired impact. Prior to running phys_opt_design for hold fixing, it is important to validate that the design has properly constrained clocktrees for minimal skew.

The insertion of negative-edge triggered registers between sequential elements can split a timing path into two half period paths and significantly reduce hold violations. You can insert the negative-edge triggered registers using the -insert_negative_edge_ffs option during the phys_opt_design implementation step. Only paths with flip-flop drivers and at most one LUT in between the sequential elements are considered for this optimization. The setup slack on the paths must be sufficiently positive after the optimization or else the optimization is discarded.

The following figure shows a negative-edge triggered register inserted after a flip-flop driving a CMAC block. Before the optimization, the hold slack between the flip-flop and the driver was -0.492 ns. After the insertion of the negative-edge triggered register (highlighted in blue), the setup and hold slack are both positive.

Figure 1. Fixing Hold Violation with Negative Edge Register Insertion

You can also insert LUT1 delays onto datapaths to reduce hold violations. To insert LUT1 delays, use one of the following options during the phys_opt_design implementation step:

Performs LUT1 insertion and only considers paths that are the largest WHS violators with sufficient positive setup slack.
Performs LUT1 insertion in a more aggressive manner than the standard -hold_fix option. The -aggressive_hold_fix optimization considers many hold violating paths for LUT1 insertion and can be used to significantly reduce design THS at the expense of LUT utilization.
Note: The phys_opt_design -directive ExploreWithAggressiveHoldFix directive runs the Explore directive along with the -aggressive_hold_fix as a single optimization.

The following figure shows a LUT1 delay inserted after a flip-flop driving an ILKN block. Before the optimization, the path from the flip-flop to the ILKN is the WHS path in the design with -0.277 ns hold slack. After the insertion of the LUT1 delay (highlighted in blue), the hold slack is positive and the setup slack remains positive.

Figure 2. Fixing Hold Violation with LUT1 Delay Insertion