Supplemental interview prep for coding whiteboard. Answers use the model chain: mechanism → motivation → when-to-use → pitfall → example.
Answer out loud in under 90 seconds per question
Draw one diagram or waveform per answer
Name a real project analog even if simplified
List what evidence you'd show (sim, STA, silicon)
This lesson deepens Follow-up and Whiteboard Drills within senior. Senior reviewers expect you to connect mechanism to power, DFT, debug, ECO, and staff-level trade-offs — 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: evidence bundle: sim, STA, CDC, silicon correlation. Treat this lesson as building one paragraph of that story for your project documentation.
FOLLOW-UP AND WHITEBOARD DRILLS
inputs ──► [senior] ──► mechanism ──► outputs
│ │
interview verify in sim + lintSynthesizable pattern for this topic — simulate locally and compare against your reference.
// model answer: mechanism -> trade-off -> exampleWrite the spec invariant (truth table, timing, or protocol rule).
Sketch block diagram — inputs, outputs, clock/reset domains.
Code the minimal correct version (no optimizations yet).
Run self-checking TB with corners: min, max, reset, idle.
Lint and review: width, latch, clock, CDC if applicable.
Iterate for timing/area only after functionally proven.
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 probesCompare 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
Walk through last timing ECO you performed.
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 scenarioExtend the worked example for "Follow-up and Whiteboard Drills": add one corner case, write a self-checking test, and document one intentional pitfall you avoided. Timebox: 30–45 minutes.
Connect Follow-up and Whiteboard Drills to power, DFT, debug, ECO, and staff-level trade-offs.
Spec → diagram → code → TB → lint is the default loop.
Document assumptions at block boundaries (width, signedness, latency).
Interview answers need mechanism + trade-off + example.
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.
Scenario for Follow-up and Whiteboard Drills: ECO adds buffer — re-run STA min/max, verify no hold regression.
Reproduce with minimal TB — one stimulus, one check.
Isolate failing cone (logic, FSM state, or bus beat).
Fix root cause — not symptom — in RTL or TB alignment.
Add regression test that fails without the fix.
Document invariant in comment or SVA for permanence.
// Wave marker: first failing cycle
$display("FAIL t=%0t sig=%h", $time, sig);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
Implement or extend the worked example for "Follow-up and Whiteboard Drills". 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.
Next topics in Part: follow nav_order in sidebar.
Cross-part: timing ↔ sequential, CDC ↔ senior interview,
combinational ↔ foundations number systems.Senior RTL is closure across disciplines: your RTL change must survive sim regressions, STA corners, CDC review, and sometimes scan/power validation.
For "Follow-up and Whiteboard Drills", 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 NARRATIVE — Follow-up and Whiteboard Drills
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 lastAlternate pattern emphasizing debug, coverage, or integration:
// ECO note in RTL comment
// ECO-2024-01: added buffer u_fix on path u_alu/outNAIVE 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 commentExplain Follow-up and Whiteboard Drills 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?
Q: What is the #1 mistake with Follow-up and Whiteboard Drills?
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 eventFork the worked example; add one assertion or SVA cover.
Inject a bug deliberately; confirm TB or assertion catches it.
Write 5-bullet PR description for your change.
Peer review: can a teammate enable the block without asking you?
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