oPnR Target: The target set in the PnR tool (e.g., via SDC set_max_transition command) is usually a fraction of the library limit, often around 70-80% of Lib_Max_Trans.
- Provides design margin. Gives the PnR tool more headroom to work with. Helps improve correlation between PnR timing estimates and signoff STA results, which use more accurate extraction and timing models. Tighter transition control generally improves noise immunity.
Decision Process: The exact percentage or value is often project-specific or methodology-defined, based on:
Technology node characteristics.
Foundry recommendations.
Past project experience and correlation data.
Design goals (performance vs. power vs. area).