What are some common pitfalls or challenges in path group creation and analysis?

1 Path group definition

One of the first challenges in path group creation is how to define the path group itself. You need to specify the sources, destinations, and optionally, the through or from pins of the paths you want to group. Depending on the tool and the syntax, this can be done using regular expressions, wildcards, attributes, or other methods. However, you need to be careful not to include unwanted paths or exclude relevant ones. For example, if you use a wildcard like *clk* to match all clock signals, you might also match signals that are not clocks, such as clock_enable or clock_divider. Similarly, if you use an attribute like is_clock to filter clock signals, you might miss some signals that are not marked as clocks, such as generated clocks or gated clocks. Therefore, you should always verify your path group definition and check if it matches your intended scope and purpose.

2 Path group constraints

Another challenge in path group creation is how to apply constraints to the path group. Constraints are rules or specifications that define the timing requirements or assumptions for a set of paths. For example, you can set a clock period, a clock uncertainty, a setup or hold margin, or a false path for a path group. However, you need to be aware of how these constraints interact with the existing constraints in your design and how they affect the path group analysis. For example, if you set a clock period for a path group that is different from the global clock period, you might create a mismatch or a conflict that can lead to incorrect timing results. Similarly, if you set a false path for a path group that contains some critical paths, you might miss some timing violations or optimization opportunities. Therefore, you should always validate your path group constraints and check if they are consistent and reasonable for your design goals.

3 Path group analysis

The final challenge in path group creation is how to perform analysis on the path group. Analysis is the process of checking the timing performance or quality of a set of paths. For example, you can report the slack, the delay, the skew, or the power of a path group. However, you need to be careful not to over-analyze or under-analyze the path group. For example, if you report the slack of a path group that contains many false paths or multicycle paths, you might get misleading or irrelevant information that can waste your time and resources. Similarly, if you report the delay of a path group that contains many paths with different clock domains or modes, you might get inaccurate or incomparable information that can confuse your interpretation and decision. Therefore, you should always select the appropriate analysis commands and options for your path group and check if they are meaningful and useful for your design verification and optimization.

4 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?