← Formal Verification Interviews
Interview Question · Project Defence
Defending a Formal Proof You Closed
The Question
The interviewer asks
Walk me through a formal proof you closed on a real block. Cover the properties, the assumptions, the hard part, and how you knew you were done.
You have two and a half minutes. Speak or type, then press Get Feedback under your answer. No credits needed.
Speech-to-text runs in your browser. Chrome and Edge may send audio to their own speech service.
You can edit your answer before you send it. Feedback uses AI and checks your answer only against our written answer. No credits are used and your answer is not saved.
Reveal the Model Answer
A strong answer names the block and its spec rows, then the property kinds: end-to-end checks, glue invariants and covers. It lists each assumption with its source and how the system checks it. It names the convergence problem and the technique that solved it. It ends with the evidence for done: full or bounded with the required depth, reached covers, and a fault-injection or mutation result.
Read the diagram as text
- AW, W, AR in: assume: VALID holds, payload stable
- axil_regs: CTRL, DATA (RW); STATUS, ID (RO); 215 flop bits
- B, R out: assert: VALID holds, payload stable, response and ID right
- write scoreboard: accepted AW and W beats in order, two deep
- watched word: `$anyconst` index; four byte-lane shadows
- read in flight: address, ID, shadow value at acceptance
- AW, W, AR in → axil_regs
- axil_regs → B, R out
- write scoreboard → watched word: B taken
- watched word → read in flight: AR taken
- read in flight → B, R out: compare
The whole model, block and bench: 498 flop bits. The bench reads only the block's ports.
One bug found by a proof, ready to defend: the property, the depth and the step where it failed.
Self-Check
Tick each key point you covered in your answer.
What Interviewers Listen For (Mid-Level)
A good mid-level answer tells a structured story with real numbers. It gives the depth, the covers, an assumption that proved wrong and a bug found.
What Interviewers Listen For (Senior)
A good senior answer says what was out of scope and why, and what trade-offs the schedule forced. It says what it would change, and how the bench was reused after tape-out.
Cross-Questions
Where the interviewer goes next. Open a node to practise that follow-up.
What IfProve ItBreak ItCompareScale It
- Defending a Formal Proof You Closed
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What IfFive Minutes with a CounterexampleL1 JuniorL2 Mid-Level
The interviewer hands you the counterexample shown with the answer, from an APB register block, and five minutes. What do they want to hear?
2:30You have two and a half minutes. Speak or type, then press Get Feedback under your answer. No credits needed.
Speech-to-text runs in your browser. Chrome and Edge may send audio to their own speech service.
You can edit your answer before you send it. Feedback uses AI and checks your answer only against our written answer. No credits are used and your answer is not saved.
Reveal the Model Answer
The failing step, the cause, and the fix, in that order, plus the causes you ruled out and how. Say whether it is an RTL bug, a missing assumption, a wrong property or an unmodelled reset. Point at the signal and step that decided it. Thinking aloud in that structure matters more than reaching the answer first.
Drill trace E from lesson F9.4 (measured): a completed write of 0 to the interrupt-status register, and the status bits afterwards. The cause is in the RTL: a W1C field built as read-write.
Trace E Measured: FAIL at reported step 4,
bmcdepth 20, reset assumed in step 0 unless the cause is the reset.Self-Check
Tick each key point you covered in your answer.
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Break ItAn Assumption That Turned Out WrongL2 Mid-LevelL3 Senior
Tell me about an assumption in one of your formal benches that turned out to be wrong. How was it found?
2:30You have two and a half minutes. Speak or type, then press Get Feedback under your answer. No credits needed.
Speech-to-text runs in your browser. Chrome and Edge may send audio to their own speech service.
You can edit your answer before you send it. Feedback uses AI and checks your answer only against our written answer. No credits are used and your answer is not saved.
Reveal the Model Answer
A good story names the assumption, why it seemed right, and the evidence that broke it. That evidence might be an unreached cover, a bound assertion firing in system simulation, or a lab failure. It ends with what changed in the process, such as covers per assumption or binding assumptions into simulation from day one.
Self-Check
Tick each key point you covered in your answer.
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Scale ItCatching Wrong Assumptions EarlyL2 Mid-LevelL3 Senior
What would you change in the flow so that the next wrong assumption is found in days, not in the lab?
2:30You have two and a half minutes. Speak or type, then press Get Feedback under your answer. No credits needed.
Speech-to-text runs in your browser. Chrome and Edge may send audio to their own speech service.
You can edit your answer before you send it. Feedback uses AI and checks your answer only against our written answer. No credits are used and your answer is not saved.
Reveal the Model Answer
Give every assumption a written reason from the spec and a cover that must stay reached. Bind every block assumption into system simulation as an assertion from day one. Review the list at each milestone with someone from the neighbouring block. Then a wrong assumption shows up early, as a firing check or an unreached cover.
Self-Check
Tick each key point you covered in your answer.
Wrong Altitude
"We review the assumptions at sign-off."
By sign-off the block has been proven for months under them. A check that fires in simulation finds the error while it is still cheap.
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Prove ItSolve F9.4.1: Five Traces at the InterviewSimulator-graded
Show it, don't just say it: this Silicon Lab problem is graded by the simulator.
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CompareBug Hunting vs Sign-Off GoalsL2 Mid-LevelL3 Senior
Asked if their project was bug hunting rather than sign-off
Why do bug hunting and sign-off need different exit criteria?
2:30You have two and a half minutes. Speak or type, then press Get Feedback under your answer. No credits needed.
Speech-to-text runs in your browser. Chrome and Edge may send audio to their own speech service.
You can edit your answer before you send it. Feedback uses AI and checks your answer only against our written answer. No credits are used and your answer is not saved.
Reveal the Model Answer
Bug hunting aims to find failures fast. So it accepts over-constraints, shallow bounds and partial property sets, and it ends when the bug rate drops. Sign-off claims the block is correct against its spec. So it needs complete properties, justified assumptions with covers, and proofs or argued bounds. Mixing the two leads to a bug-hunting bench presented as a sign-off.
Self-Check
Tick each key point you covered in your answer.
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Scale ItThe Formal Bench After Tape-OutL3 Senior
What happens to your formal bench after tape-out?
2:30You have two and a half minutes. Speak or type, then press Get Feedback under your answer. No credits needed.
Speech-to-text runs in your browser. Chrome and Edge may send audio to their own speech service.
You can edit your answer before you send it. Feedback uses AI and checks your answer only against our written answer. No credits are used and your answer is not saved.
Reveal the Model Answer
Keep it alive. When the lab reports a symptom, write it as a cover and the lab observations as assumptions. The shortest trace is a hypothesis for the root cause. Never assume away what the lab did not observe, since it may be the cause. Then prove the fix under the same environment before the respin.
Self-Check
Tick each key point you covered in your answer.
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Prove ItSolve F5.9.1: Read the Lab Constraint ExperimentSimulator-graded
Show it, don't just say it: this Silicon Lab problem is graded by the simulator.
What People Get Wrong
Common wrong answers and why they fail.
Delivery
Describes the tool flow and commands but never the properties or the bugs found.
The interviewer is testing judgement about what to prove. A story without properties, assumptions and bugs shows only that a script ran.
Buzzword Without Mechanism
Says "we got full proofs" without saying what was assumed.
A proof is only as strong as its assumptions. Without them the claim cannot be weighed, and a good interviewer will ask next.
Wrong Altitude
A senior candidate who cannot say which bugs the proof could not have found.
A lead knows the limits of a sign-off, such as logic outside every cone or behaviour past the bound. That knowledge separates a lead from an operator.