Corner Cases and Stress Tests

Corner Cases and Stress Tests

This supplemental lesson extends rtl for timing with corner cases, reset, and stress patterns. Study it after the core sub-lessons in this topic.

Section context: timing. Use the diagrams, code, and exercises below together with the main lessons — they are not optional fluff; they mirror what senior engineers attach to block delivery packages.

Learning objectives

• Reproduce the worked example without looking at the solution

• Explain one failure mode and how your TB catches it

• State one timing or protocol assumption explicitly

• Document one integration dependency on other blocks

Quick reference diagram

TOPIC: rtl-for-timing
LESSON: Corner Cases and Stress Tests
FLOW: spec → RTL → TB → lint → review → merge

Deep dive

This lesson deepens Corner Cases and Stress Tests within timing. Senior reviewers expect you to connect mechanism to STA graphs, slack, constraints, and RTL structure for closure — not just define terms.

Start from the spec invariant: what must always be true each cycle? Write it as a Boolean relation, timing budget, or protocol rule before coding. That invariant becomes your reference model, assertion, or waveform check.

At tape-out quality, every block needs a sign-off story: SDC reviewed, exceptions justified, setup/hold clean. Treat this lesson as building one paragraph of that story for your project documentation.

Architecture and signal flow

CORNER CASES AND STRESS TESTS

  inputs ──► [timing] ──► mechanism ──► outputs
                │                │
           rtl-for-timing     verify in sim + lint

Worked example (Verilog/SystemVerilog)

Synthesizable pattern for this topic — simulate locally and compare against your reference.

// pipeline registers on long paths

Step-by-step design procedure

1. Write the spec invariant (truth table, timing, or protocol rule).

2. Sketch block diagram — inputs, outputs, clock/reset domains.

3. Code the minimal correct version (no optimizations yet).

4. Run self-checking TB with corners: min, max, reset, idle.

5. Lint and review: width, latch, clock, CDC if applicable.

6. Iterate for timing/area only after functionally proven.

Timing and resource trade-offs

METRIC          TYPICAL LEVER
Logic levels      algebra / pipelining
Register count    retiming / sharing
Wire fanout       duplication / pipeline
Power             clock gating / operand isolation
Debug visibility  status flags / SVA / waveform probes

Debug checklist

• Compare DUT vs reference on every stimulus vector

• Capture first cycle of mismatch — not last

• Log seed and plusargs for random regressions

• Check reset release and clock alignment in TB

• If waveform is ambiguous, add temporary assertions

Interview angle

Setup failure fix order: RTL vs constraint vs physical?

MODEL ANSWER SKELETON
1. MECHANISM — one-sentence technical truth
2. MOTIVATION — why this structure vs alternatives
3. WHEN TO USE / SKIP — scope and assumptions
4. PITFALL — common junior mistake
5. EXAMPLE — Verilog or waveform scenario

Practice exercise

Extend the worked example for "Corner Cases and Stress Tests": add one corner case, write a self-checking test, and document one intentional pitfall you avoided. Timebox: 30–45 minutes.

Key takeaways

• Connect Corner Cases and Stress Tests to STA graphs, slack, constraints, and RTL structure for closure.

• Spec → diagram → code → TB → lint is the default loop.

• Document assumptions at block boundaries (width, signedness, latency).

• Interview answers need mechanism + trade-off + example.

Common pitfalls

• Skipping reference model — passes wrong together with DUT.

• Optimizing before correct — ECO cost goes up 10×.

• Ignoring tool warnings — lint/CDC/STA warnings are technical debt.

• Undocumented clock/reset domain — integration surprises.

Extended design scenario

Scenario for Corner Cases and Stress Tests : Hold failure after buffer — review clock skew on fast path.

Scenario resolution outline

1. Reproduce with minimal TB — one stimulus, one check.

2. Isolate failing cone (logic, FSM state, or bus beat).

3. Fix root cause — not symptom — in RTL or TB alignment.

4. Add regression test that fails without the fix.

5. Document invariant in comment or SVA for permanence.

Additional simulation pattern

// Annotate failing path in STA report
// launch: u_reg/CK -> capture: u_out/D

Synthesis and sign-off notes

• Elaborate clean — no OOM from unconstrained generate

• Constraints cover all clocks and I/O delays

• Cross-check RTL parameters vs integration top

• Attach sim log + seed to code review

Lab exercise (45–60 min)

Implement or extend the worked example for "Corner Cases and Stress Tests". Add two new test vectors that target different branches. Write a one-paragraph sign-off note covering function, corners, and what you would still verify in SoC context.

Further reading in this course

Next topics in Part: follow nav_order in sidebar.
Cross-part: timing ↔ sequential, CDC ↔ senior interview,
combinational ↔ foundations number systems.

Extended theory

Timing is the bridge between RTL intent and silicon reality. STA does not find functional bugs—it proves that your register-to-register contracts close at the target frequency with margin.

For "Corner Cases and Stress Tests", the invariant you defend in review is: behavior matches spec under all legal input sequences, reset flows, and backpressure patterns—not just the happy path shown in introductory diagrams.

Write the invariant as a comment above the module or as an SVA property when possible. Future you (and formal tools) will treat it as the contract.

Waveform reading guide

WAVEFORM NARRATIVE — Corner Cases and Stress Tests

cycle 0: reset asserted, outputs safe/idle
cycle 1-2: reset held, clocks running
cycle 3: reset released, first legal inputs
cycle 4+: check output latency (N cycles)
mark FIRST mismatch cycle — not last

Second worked example

Alternate pattern emphasizing debug, coverage, or integration:

// SDC fragment (conceptual)
// create_clock -name clk -period 10 [get_ports clk]
// set_input_delay 2 -clock clk [all_inputs]

Comparison: naive vs production

NAIVE APPROACH              PRODUCTION APPROACH
quick hack, one sim vector    self-checking TB + corners
ignore lint warnings          zero new waivers
implicit widths               explicit casts/parameters
undocumented latency          latency in module header comment

Additional interview questions

• Explain Corner Cases and Stress Tests to a verification engineer — what would they assert?

• What breaks first at high frequency or low voltage?

• What is your rollback plan if synthesis QoR is unacceptable?

• How would you debug this block with only a 32-bit GPIO trace?

Follow-up interview model answer

Q: What is the #1 mistake with Corner Cases and Stress Tests?
A:
  MECHANISM: [core rule in one line]
  MOTIVATION: why teams care in tape-out
  PITFALL: what juniors do wrong
  EXAMPLE: one Verilog line or one waveform event

Hands-on lab part 2

1. Fork the worked example; add one assertion or SVA cover.

2. Inject a bug deliberately; confirm TB or assertion catches it.

3. Write 5-bullet PR description for your change.

4. Peer review: can a teammate enable the block without asking you?

Sign-off evidence checklist

• Directed sim log attached (PASS, seed noted)

• Lint report clean for touched files

• If sequential: reset + clocking section in README

• If bus-facing: protocol cheat sheet in module doc

• If timing-critical: note expected critical path endpoint