How do you run rail analysis across multiple scenarios?

Short Answer

RedHawk Fusion analyses only the current design scenario by default, so multiple scenarios need rail scenarios. Enable them with app options, mark the design scenarios for IR drop, create each rail scenario with create_rail_scenario (ICC2) before configuring it with set_rail_scenario (ICC2), then run them together with analyze_rail -rail_scenarios (ICC2). Pick scenarios by current, not by timing corner names.

Technical Explanation

• Enable the feature with rail.enable_new_rail_scenario (ICC2) set to true (default false). rail.enable_parallel_run_rail_scenario (ICC2), default true, runs rail scenarios in parallel.
• Mark each design scenario for rail analysis with set_scenario_status func_cbest -ir_drop true (ICC2).
• Order matters: rail scenarios must be created with create_rail_scenario (ICC2) before set_rail_scenario (ICC2) sets their attributes. Several rail scenarios can map to one design scenario, and they are saved in the design library.
• set_rail_scenario (ICC2) takes the analysis type through -voltage_drop, the nets, and optional extra GSR files. -twf_generation and -ipf_generation (ICC2) choose whether ICC2 or PrimeTime and PrimePower generate the timing window and instance power files.
• Host setup comes from set_host_options (ICC2): -submit_command {local} for one machine, or a grid protocol and submit command. -submit_to_other_machines (ICC2) sends the scenarios out, but RedHawk-SC Fusion does not support that option.
• If you need only one scenario other than the current one, rail.scenario_name (ICC2) is simpler. IR-driven placement and IR-driven CCD honour it.
• Choose scenarios by what stresses the grid: the highest-power mode and corner for IR and EM, and the slow corner if you want IR feeding back into timing. Timing corner names alone do not tell you which one draws the most current.

What To Check

• That every signoff power mode has a rail scenario.
• Scenario status for IR drop on each mapped design scenario.
• Results for every rail scenario, not only the one loaded in the GUI.
• Option support in the Fusion flavour you are running.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): Four rail scenarios cover two modes at two corners, all complete, worst dynamic drop 69 mV against 72 mV.
• Suspicious (illustrative): Only two of four scenarios have results, or both scenarios point at the same design scenario by mistake.
• Hard stop: Signoff reported from the current scenario alone on an MCMM design.

What The Interviewer Is Testing

• Knowledge of the default: current scenario only.
• Does the answer keep the create-before-set order?
• Choosing scenarios by current draw rather than by name.

Follow-up Question & Model Response

"How do you decide which design scenario draws the most current?"

Candidate Model Response: Run power analysis per scenario first, for example with PrimePower or the ICC2 power reports, and compare total and per-region power. Leakage-heavy blocks often peak at the hot, fast corner, while switching-heavy blocks peak in the mode with the highest clock frequency and activity. The worst IR scenario and the worst timing scenario are often different, so keep both if IR feeds into timing.