What are PVT corners, modes, and MMMC scenarios?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
A mode describes how the design is operating (functional, scan-shift, test); a corner describes the analysis conditions (process/voltage/temperature plus the RC model); a scenario is simply one mode paired with one corner. Process, voltage, and temperature all shift cell delay, but interconnect matters too — a corner setup needs matching cell models, matching parasitic assignments, and whatever OCV/derating methodology the signoff plan requires.
Technical Explanation
- A mode describes how the design is operating (functional, scan-shift, test); a corner describes the analysis conditions (process/voltage/temperature plus the RC model); a scenario is simply one mode paired with one corner.
- Process, voltage, and temperature all shift cell delay, but interconnect matters too — a corner setup needs matching cell models, matching parasitic assignments, and whatever OCV/derating methodology the signoff plan requires.
- Don't assume every mathematically possible mode×corner combination needs to be active — the approved analysis matrix defines which scenarios are actually required and which checks (setup, hold, leakage) are turned on in each one.
- A scenario existing in the database proves nothing by itself — its required analysis type could be silently disabled. Audit both the scenario definitions and their active status, and remember that temperature inversion and clock/data interaction can make the "obviously worst" corner not actually be the limiting one — use the qualified, approved combinations rather than intuition.
Command Checks & Actions
create_scenarioDefines one specific mode + corner + library + parasitics combination -- the literal unit an MMMC signoff run is built from. Everything else in an MMMC flow exists to create, merge, or report across a set of these.
current_scenario -allFocuses subsequent commands across every scenario at once, which is how you confirm a given check was actually run everywhere it needs to be, not just in the scenario you happened to be looking at.
report_global_timingMerges WNS/TNS across every scenario currently in focus into one view -- the fastest way to see whether a single PVT corner or mode is the actual outlier driving a signoff failure.
Common Mistake
The Trap: Candidates often mistakenly treating a mode as just a temperature, or creating many scenarios without checking which ones actually participate in optimization.
Follow-up Question & Model Response
"Why can timing look clean in one scenario and fail in another?"
Candidate Model Response: Different edge relationships, models, loads, exceptions, and early/late behavior change the analyzed problem.
Practical Example
Tapeout Scenario: FUNC_SLOW might pair functional constraints with one slow operating corner, while SCAN_FAST pairs shift constraints with another corner. Their clocks and active checks can differ; neither can replace the other merely because both produce a timing report.
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