What are the common pitfalls and challenges of false and multi-cycle paths verification?

1 False paths definition

A false path is a timing path that is never activated or has no effect on the circuit behavior under any input condition, such as a test mode path or a redundant logic path. However, incorrectly defining false paths can lead to timing violations or functional failures. To avoid this, be careful not to define false paths too broadly with wildcards or without considering the context of the path, as this can mask real timing problems or introduce unnecessary pessimism in the analysis. Similarly, ensure that the design intent and functional specification are taken into account when defining false paths. Additionally, validate the false paths with simulation or formal methods to avoid undetected errors or inconsistencies.

2 Multi-cycle paths definition

A multi-cycle path is a timing path that takes more than one clock cycle to propagate the data from the source to the destination. For example, a pipeline stage or a slow interface can be declared as multi-cycle paths in STA. However, defining multi-cycle paths incorrectly can lead to timing violations or functional failures. There are several challenges when it comes to defining multi-cycle paths, such as ensuring the correct cycle count, clock domain, and edge. Additionally, you must define the correct constraints, such as setup, hold, recovery, removal, or skew. Finally, you must take into account any exceptions, such as false paths, max delays, or min delays. This can be especially difficult when dealing with special cases like reset, enable, or bypass signals.

3 False and multi-cycle paths verification

Verifying the correctness and completeness of false and multi-cycle paths can be done through various methods, such as simulation, formal methods, and reporting. Simulation can be used to check the waveform or coverage results, while formal methods like property checking or equivalence checking can help to verify the logic and timing. Generating reports like timing reports, exception reports, or crosscheck reports can also help to verify the consistency and accuracy of the false and multi-cycle paths.

4 Common verification issues

Despite the verification methods, there can be some issues that can affect the quality and reliability of false and multi-cycle paths verification. These issues often include incomplete or incorrect test vectors, inadequate or inaccurate tools, and human errors or oversights. For instance, the test vectors used for simulation or formal verification may not cover all possible input scenarios or match the design intent. Additionally, the tools used for simulation, formal verification, or reporting may not support all features or formats of false and multi-cycle paths definition. Lastly, the verification engineer may make mistakes when defining, verifying, or reviewing the false and multi-cycle paths.

5 Best practices and tips

In order to avoid or minimize problems, there are some best practices and tips for false and multi-cycle paths verification. It is important to follow a systematic and consistent methodology for defining, verifying, and reviewing the false and multi-cycle paths, such as using templates, checklists, guidelines, or scripts. Furthermore, it is beneficial to communicate and collaborate with the design team, the functional team, and the tool vendor to ensure that the false and multi-cycle paths are aligned with the design intent, the functional specification, and the tool capabilities. Additionally, you should review and update the false and multi-cycle paths regularly and frequently throughout the design cycle. Utilizing version control, change management, or regression testing can help track and validate any changes.

6 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?