What's the difference between static and dynamic IR drop analysis?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Static IR drop uses each cell's average current over a cycle, so it shows the DC drop caused by grid resistance and is good at finding weak straps, missing vias and poor supply placement. Dynamic IR drop follows the current as it changes in time, when many cells switch together near a clock edge, so it catches short, deep dips shaped by decap, grid capacitance and the package. Run static first as a grid-quality check, then dynamic once placement, the clock tree and timing windows are real.
Technical Explanation
- Static: power calculation gives an average current per instance, those currents are placed on the resistive PG network, and node voltages are solved. Capacitance and inductance do not enter a DC solution, so decap cannot change a static result.
- Dynamic: the tool builds time-varying current waveforms for switching instances, from the libraries or more accurately from APL characterisation, and simulates the grid with its capacitance and decap over the analysis window. The result is a voltage waveform at each node, and the number that matters is the worst dip.
- In ICC2,
analyze_rail -voltage_drop(ICC2) takesstatic,dynamic,dynamic_vectorlessordynamic_vcd. Withdynamic_vcdyou pass the activity with-switching_activity; VCD, FSDB, SAIF and ICC_ACTIVITY are accepted. Outside rail scenarios,-netsis required unless you reuse a script with-redhawk_script_file. - Dynamic needs more inputs.
rail.apl_files(ICC2) supplies current waveforms and intrinsic parasitics, andrail.switch_model_files(ICC2) supplies power-switch models. In standalone RedHawk the STA file is required for dynamic analysis and optional for static. - Standalone RedHawk runs the same ideas as
perform analysis -static(RH),perform analysis -vectorless(RH) andperform analysis -vcd(RH), each afterperform pwrcalc(RH) andperform extraction(RH). Standalone RedHawk needs an Ansys licence. - The dynamic dip sits on top of the static drop, so a static violation almost always means a real grid problem, while a clean static result says nothing about a clock-edge dip.
- RedHawk Fusion does not support RedHawk signoff features such as hierarchical analysis or dynamic analysis with lumped or SPICE packages. With RedHawk signoff licences,
rail.allow_redhawk_license_checkout(ICC2) enables a listed subset in ICC2, including dynamic analysis with package models; the rest needs standalone RedHawk.
What To Check
- Worst static drop per net and where it sits, against the static budget.
- Worst dynamic dip, its location and its time relative to the clock edge.
- That dynamic inputs are present: APL or library current data, timing windows, and an activity file for VCD mode.
- Whether the dynamic run needs a package model, which means RedHawk signoff licences through
rail.allow_redhawk_license_checkout(ICC2) or standalone RedHawk.
Command Checks & Actions
analyze_rail -voltage_drop static -nets {VDD VSS}Static drop from average currents; the grid-quality check.
analyze_rail -voltage_drop dynamic_vectorless -nets {VDD VSS}Dynamic drop without a VCD, using toggle-based switching scenarios.
analyze_rail -voltage_drop dynamic_vcd -switching_activity {VCD top.vcd} -nets {VDD VSS}Dynamic drop driven by gate-level simulation activity.
open_rail_resultLoads the rail results so the static or dynamic maps can be displayed.
perform pwrcalcStandalone power calculation that feeds both analyses.
perform analysis -staticStandalone static IR and EM analysis.
perform analysis -vectorlessStandalone vectorless dynamic voltage drop analysis.
Healthy, Suspicious & Hard-stop Results
- Healthy (illustrative): Static worst 22 mV and dynamic worst 68 mV on a 0.75 V supply, against budgets of 40 mV static and 75 mV dynamic.
- Suspicious (illustrative): Static 22 mV but a 72 mV dynamic dip in one region, 3 mV under budget, or a dynamic run made without timing windows or with default toggle rates only.
- Hard stop: Static drop already over budget, which is a grid problem, or dynamic dips over budget on timing-critical cells, or results from a run that reported missing current data.
Common Mistake
The Trap: Treating a clean static result as IR signoff for a high-activity block. Static averages the current over the cycle, so a 20 mV static drop can hide a 100 mV dip that happens every time a large bank of flops and clock buffers switches together. The block passes review, and silicon shows speed failures only under heavy workloads, which is expensive to reproduce and fix.
What The Interviewer Is Testing
- Can the candidate explain average versus instantaneous current, and which physical elements each analysis includes?
- Picking the right
analyze_rail(ICC2) mode and the inputs each one needs. - Knowing where RedHawk Fusion stops and standalone RedHawk is required.
Follow-up Question & Model Response
"When would you choose vectorless dynamic over VCD-based dynamic?"
Candidate Model Response: Vectorless is the choice when no representative VCD exists yet, or when you want a switching scenario that is not tied to one test. It builds the scenario from toggle rates and timing windows, so its quality depends on those settings. VCD-based analysis is better when simulation of a known worst workload exists, but it only covers the window you select. Many teams run both and compare the hotspots.
Practical Example
Tapeout Scenario: (illustrative) A 0.75 V block shows a static worst drop of 22 mV (2.9%) at the corner farthest from the bumps. Vectorless dynamic shows an 84 mV dip about 120 ps after the rising clock edge, in a cluster of 400 flops and six clock buffers. Against a 75 mV (10%) dynamic budget it fails by 9 mV. Spreading the clock buffers and adding decap around the cluster brings the dip to 68 mV, while the static number barely moves.
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