How do you choose the activity scenario for dynamic IR signoff?

Short Answer

Use vectorless analysis to find weak areas across the whole design, then sign off with VCD windows chosen for the highest power and the fastest change in current, not whatever the testbench happened to dump. The danger is optimistic vectors: a reset sequence or a light test gives a clean result that real traffic never matches. Document which windows were run and why each is the worst for its mode.

Technical Explanation

• Vectorless analysis builds a switching scenario from toggle rates and STA timing windows, then scales it to meet the total average power given in the GSR. It covers every instance but spreads activity, so a local burst can come out milder than in silicon.
• Accelerated dynamic analysis is quicker, but the manual gives an average 10-15% loss in dynamic voltage drop accuracy and says it is not for signoff. Use it for early prototyping only.
• VCD analysis takes switching entirely from the dump. Any net missing from the VCD is treated as inactive, so partial coverage quietly lowers the result. Timing in the VCD is accurate only when delays were back-annotated during simulation.
• RedHawk can pick the window: with SELECT_RANGE (RH) in the VCD_FILE (RH) keyword it computes power in a sliding window of the DYNAMIC_SIMULATION_TIME (RH) width and lists the worst cycles in adsRpt/worst_power_cycle.rpt (RH).
• Highest average power is not always deepest droop. A window where many cells switch in the same edge, such as a clock gate enabling a large bank, gives a bigger step in current even at lower average power. RTL VCD pruning supports a WORST_DPDT_CYCLE (RH) selection mode aimed at that case.
• In ICC2, analyze_rail -voltage_drop dynamic_vcd (ICC2) takes the dump through -switching_activity (ICC2), with optional start and end times to use one window; without them all time values are used. dynamic_vectorless needs no activity file.
• Mixed mode lets block VCDs sit under a vectorless top level, useful when only some blocks have good vectors.

What To Check

• VCD coverage: fraction of nets and instances that toggle in the dump.
• Whether VCD timing came from an SDF-annotated simulation.
• The worst-power and worst current-step windows, and which one was run.
• Vectorless versus VCD droop on the same regions, to spot optimistic vectors.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): The worst VCD window droops 69 mV and vectorless 61 mV against a 72 mV budget, with 96% of instances covered by the dump.
• Suspicious (illustrative): VCD droop is half the vectorless result, which usually means the window is idle or coverage is poor.
• Hard stop: Signoff on a single VCD window with no record of how it was selected, or a dump with most nets missing.

What The Interviewer Is Testing

• Awareness that missing nets in a VCD are treated as inactive.
• Knowledge of critical cycle selection in RedHawk.
• Separating worst average power from worst current step.

Follow-up Question & Model Response

"The design team has no gate-level VCD in time for signoff. What do you do?"

Candidate Model Response: Run vectorless with the best toggle rates available and a GSR total power taken from the PrimePower estimate for the heaviest mode, so the scenario is scaled to a realistic worst case. Add a GSC file to force the known worst blocks active together. If an RTL VCD exists, RedHawk can use it with cycle selection and pruning. State the gap in the signoff record, and add margin to the budget until a gate-level window is run.