What are the advantages and disadvantages of using path-based analysis vs. block-based analysis in STA?

1 Path-based analysis

Path-based analysis is a method that considers every individual path in the circuit and computes its delay based on the logic gates, wires, and parasitics along the path. It also accounts for the effects of crosstalk, process variations, and temperature on the delay. Path-based analysis can provide accurate and detailed information about the critical paths and the slack of each path. However, it can also be very time-consuming and memory-intensive, as the number of paths grows exponentially with the circuit size and complexity. Moreover, it can generate false paths, which are paths that never occur in reality due to logic or functional constraints, and lead to unnecessary pessimism or optimism.

3 Choosing the best method

The choice of using path-based analysis or block-based analysis depends on several factors, such as the design size, complexity, stage, and goal. Generally, path-based analysis is more suitable for small or medium-sized designs, where the number of paths is manageable and the accuracy is important. It can also be used for final verification or optimization, where the timing margins are tight and the details are needed. On the other hand, block-based analysis is more suitable for large or complex designs, where the number of paths is overwhelming and the speed is essential. It can also be used for early estimation or exploration, where the timing budget is loose and the trends are sufficient.

4 Combining both methods

In some cases, it may be beneficial to combine both methods and use them in different stages or levels of the design. For example, one can use block-based analysis for the top-level or system-level design, where the blocks are abstracted and the overall timing is estimated. Then, one can use path-based analysis for the lower-level or block-level design, where the paths are detailed and the individual timing is verified. This way, one can leverage the advantages of both methods and achieve a balance between accuracy and efficiency.

5 Best practices and tips

No matter which method you choose, there are some best practices and tips that can help you with STA. For example, realistic and consistent delay models and libraries should be used for logic gates, wires, and parasitics. Additionally, proper timing constraints and annotations should be utilized to specify the design requirements and assumptions. Furthermore, crosstalk analysis and noise margin should be employed to account for interference between signals. Statistical or probabilistic methods can also be used to account for variations in process, voltage, and temperature. Additionally, incremental or hierarchical methods can be employed to reduce analysis time and memory. Finally, reports and graphs should be utilized to visualize timing results and identify critical paths and slack.

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?