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Figure 4 – You can generate timing reports to debug specific nodes in the design.
sign that is not performing using a hierarchical program-management flow. Iterate, fix and merge back the results. • Have the synthesis software communicate to the Vivado place-and-route tool that it is to place the critical path on the same die of a multi-SLR device such as the Virtex®-7 2000T FPGA to avoid cross-SLR delays. STEP 3: GAINING FINAL TIMING CLOSURE General timing can be reported post-synthesis and after placement and routing (Figure 4). For example, Synplify software allows you to report upon specific parts of the design of interest using a TCL command (report_timing). To improve timing QoR further, we recommend that you correlate post-synthesis and post-P&R timing results, specifically the slack margins for given start points and endpoints on timing-critical paths. In Synplify Premier
synthesis software, for example, you can display the post-synthesis and P&R timing reports side-by-side to read the timing results. The correlation tool does a side-byside comparison of the status of endpoints, start points and required periods. Paths are reported against the end clock. Paths for which pre- and postP&R timing do not correlate well to within your specified “slack margin” criteria will be flagged as “correlation mismatches” so that you can take action on them. A typical action would be to specify so-called “-route” constraints to the synthesis tool that tighten timing-path constraints only during the synthesis phase. Here’s an example: DC Constraints input file to synthesis: F set_clock_route_delay {c:clka} 1.4 These constraints make synthesis “work harder” to meet timing performance on those paths, resulting in better correlation and QoR.
The nice thing about the timing-correlation capability is that you can drill down to view the exact paths that are causing problems, for example, changing the number of paths that you want to be displayed per endpoint. You can search for specific clocks or instances of interest and have their timing paths displayed. Clocks are also compared and displayed to further assist in timing correlation (see Figure 5). As you can see, there are certain steps you need to take in order to achieve better timing performance in a reasonable amount of time in your Vivado Design Suite flow. The methodology we have outlined will intercept clock and constraints setup issues early, while also offering a variety of techniques to tune and correlate timing in your design and its RTL to get fast timing closure. For more information and examples, please visit http://www.synopsys.com/ fpga.
Figure 5 – A timing correlation reports allow you to compare timing mismatches post-synthesis and post-P&R, displaying and filtering path timing results and clocks side-by-side. Third Quarter 2014
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