SerDes & High-Speed I/O · All levels
SI/PI Co-Design: Tricky Q&A
Senior interview and review questions for SI/PI Co-Design.
Section Q&A bank
Use these drills after completing all topics in SI/PI Co-Design. Answer with workload context, mechanism proof, artifact, owner, and release decision.
How can you tell PI noise from channel ISI in an eye closure symptom?
diagram
[INT][SERDES][SI-PI-CO-DESIGN]
Q: How can you tell PI noise from channel ISI in an eye closure symptom?
A:
PI noise often correlates with PRBS density, simultaneous switching, and rail ripple frequency; toggling idle vs traffic changes eye without coefficient updates. ISI is pattern-structure dependent but stable if supplies are quiet. Measure rail ripple synchronized to data rate harmonics and compare eye on quiet vs stressed power load.
FOLLOW-UP TRAP: Retraining RX when the root cause is PDN resonance under traffic.Why is refclk routing treated as part of the SerDes link budget?
diagram
[INT][SERDES][SI-PI-CO-DESIGN]
Q: Why is refclk routing treated as part of the SerDes link budget?
A:
PLL multiplies refclk jitter to the line; a mediocre reference cannot be fully cleaned without wander tradeoffs. Skew between redundant clocks causes phase steps. Refclk quality is therefore a first-class spec alongside channel s-params.
FOLLOW-UP TRAP: Using generic clock buffer without phase noise data at product offsets.What layout choice most often increases common-mode EMI on high-speed lanes?
diagram
[INT][SERDES][SI-PI-CO-DESIGN]
Q: What layout choice most often increases common-mode EMI on high-speed lanes?
A:
Broken or narrow return paths, asymmetric via fields, and connector shield discontinuities force return current on unintended paths, raising common-mode radiation. Differential routing mismatch converts differential mode to common mode. Fix geometry before relying on SSC alone.
FOLLOW-UP TRAP: Applying SSC to pass EMC while ignoring return-path discontinuity.How should thermal throttling interact with link health monitoring?
diagram
[INT][SERDES][SI-PI-CO-DESIGN]
Q: How should thermal throttling interact with link health monitoring?
A:
Throttle policies should precede hard failure: reduce swing, widen FEC, or schedule retrain during controlled windows while logging margin trend. Abrupt thermal events without telemetry cause fleet flaps. Correlate temperature sensors with coefficient wander and BER.
FOLLOW-UP TRAP: Hard power cut on thermal trip without graceful link downgrade.Q&A drill guide
diagram
WORKLOAD -> SerDes SYMPTOM -> TIMING/QUEUE METRIC -> ROOT CAUSE -> FIX -> REGRESSIONSketch while answering
diagram
SI PI CO DESIGN
power-integrity-noise -> reference-clock-quality -> closureKey takeaways
Always tie controller and PHY counter shifts to application latency and throughput outcomes.
Lock firmware timing profile, thermal condition, and DIMM state before comparing SerDes captures.
Common pitfalls
Chasing peak bandwidth while ignoring p99 latency and fairness tails.
Changing timing guardbands without separating SI noise from scheduling issues.
Declaring closure without reliability gates, fault injection, and regression replay.