1.3.3. Reducing Critical Path Delay
By default, the Intel® Quartus® Prime Fitter works to meet the timing requirements, and stops when the requirements are met. Therefore, realistic constraints are crucial for timing closure.
Under-constrained designs can lead to sub-optimal results. For over-constrained designs, the Fitter might over-optimize non-critical paths at the expense of true critical paths. In addition, area and compilation time may also increase.
For designs with high resource usage, the Intel® Quartus® Prime Fitter might have trouble finding a legal placement. In such circumstances, the Fitter automatically modifies settings to try to trade off performance for area.
The Intel® Quartus® Prime Fitter offers advanced options that can help improve the design performance when you properly set constraints. Use the Timing Optimization Advisor to determine which options are best suited for the design.
If you use incremental compilation, you can help resolve inter-partition timing requirements by locking down results, one partition at a time, or by guiding the placement of the partitions with Logic Lock (Standard) regions. You might improve the timing on such paths by placing the partitions optimally to reduce the length of critical paths. Once the inter-partition timing requirements are met, use incremental compilation to preserve the results and work on partitions that have not met timing requirements.
In high-density FPGAs, routing accounts for a major part of critical path timing. Because of this, duplicating or retiming logic can allow the Fitter to reduce delay on critical paths. The Intel® Quartus® Prime software offers push-button netlist optimizations and physical synthesis options that can improve design performance at the expense of considerable increases of compilation time and area. Turn on only those options that help you keep reasonable compilation times and resource usage. Alternately, you can modify the HDL to manually duplicate or adjust the timing logic.