What are the benefits and challenges of path grouping for timing closure?

1 Benefits of path grouping

Path grouping can offer several advantages for your STA process, such as reducing the number of false paths and pessimistic timing violations that may arise from applying global constraints to heterogeneous paths. It can also improve accuracy and efficiency of timing analysis by focusing on the relevant paths for each group, as well as simplify the timing reports and debugging by filtering and sorting the paths by group. For example, you can group the paths that cross multiple clock domains and apply appropriate clock domain crossing (CDC) constraints and checks to them, or group the paths that are critical for performance and apply more aggressive optimization and slack budgeting to them. Additionally, you can group the paths that are related to a specific functionality or module and generate separate reports and summaries for them.

2 Challenges of path grouping

Path grouping can also present some challenges for your STA process, such as increasing the complexity of defining and managing the path groups and their constraints. This could involve creating and updating multiple constraint files and scripts for each group, and ensuring they are consistent and compatible with each other and with design changes. Additionally, there is a risk of missing or incorrect paths or constraints in the path groups, which can lead to inaccurate or incomplete timing analysis and closure. It is important to consider the trade-offs between granularity and scalability of the path groups; creating more fine-grained groups can capture nuances and variations of the paths, but may also increase the number of groups and computational/memory resources required for analysis.

3 Best practices for path grouping

In order to take advantage of path grouping while avoiding its challenges, you should plan and document your path grouping strategy before beginning the STA process. This includes identifying criteria and objectives for creating and naming path groups, as well as how they relate to the design specifications and requirements. Automated tools and methods should be used to create and update the path groups and their constraints, instead of manual or ad hoc processes. Additionally, validation and verification of the path groups and their constraints should be done regularly and thoroughly, especially after any design or constraint changes. Tools that can check and report consistency and completeness of the path groups, as well as flag errors or warnings, are helpful in this regard.

4 Examples of path grouping

Path grouping is a powerful technique that can be applied to the STA process. It involves grouping paths by their clock domains, logic depth, criticality, functionality, or module. For example, clock-to-clock paths can be grouped by their clock domains and have clock skew, jitter, and uncertainty constraints and checks applied to them. Data-to-data paths can be grouped by their logic depth or criticality and have setup and hold time constraints and checks applied. Input-to-output paths can be grouped by their functionality or module and have input delay, output delay, and drive strength constraints and checks applied. By understanding the benefits and challenges of path grouping, as well as following best practices and examples, you can use it effectively in your STA workflow to achieve timing closure.

5 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?