Enumeration proves basic wiring and reset timing, not margin under worst-case noise and temperature. Under full load, switching current increases rail ripple and ground bounce, and junction temperature rises—both reduce eye margin and can trigger retries, AER (Advanced Error Reporting), and retrains.

• Correlate AER/error counters with temperature ramp and power bursts.
• Check rail droop at the endpoint/retimer and refclk quality during load steps.
• Look for retrain frequency spikes and whether errors disappear at a lower Gen speed.

Start with the three inputs that silently differ across hosts: reference clock, reset behavior, and slot power quality. A “good” host can hide marginal module timing; a different host exposes it.

• Refclk: frequency accuracy, jitter, spread-spectrum settings, routing quality.
• Reset: PERST# timing relative to power-good and clock presence; any glitches on warm/cold boots.
• Power: inrush limits, input ripple, and transient response under burst load.

Average power can look safe while brief peaks violate UV (undervoltage) or OC (overcurrent) thresholds. AI workloads often create fast di/dt bursts; cable/connector impedance and VRM control loop limits turn that into droop. Some protections also react to fast spikes even if the average stays low.

• Measure peak current and droop during worst-case workload transitions.
• Verify inductor saturation margin and OCP mode (latch vs retry).
• Align protection timestamps with workload phases to avoid false conclusions.

Separate “fast” events from “slow” events. Power transients cause immediate frequency dips tied to droop/alerts; thermal limits usually show a lag as temperature accumulates. The fastest way is to correlate frequency/throttle flags with rail droop and temperature at the same timestamp.

• If dips coincide with droop/UV events → power transient is primary.
• If dips follow temperature rise and persist until cooling → thermal limit dominates.
• Ensure telemetry captures peaks (not only averages), otherwise the signature is hidden.

The bottleneck is often not the USB PHY “spec,” but the bridge implementation, memory movement, or thermal throttling. Bridges may share internal bandwidth across lanes, add protocol overhead, or downshift under heat. Some designs use USB as a management/fallback path, not a sustained data path.

• Confirm negotiated USB mode (Gen, lanes) and sustained, not burst, throughput.
• Check bridge temperature and whether throttling or retries appear over time.
• Verify host DMA path and local memory contention during heavy workloads.

Retimers amplify both signal integrity and design mistakes. Common causes are: noisy retimer supply (poor local decoupling), reference plane discontinuities around the device, incorrect orientation/lane mapping, or bad return paths through connectors. A retimer also adds its own refclk/reset/config requirements.

• Audit retimer power rails for ripple during bursts; add/relocate high-frequency decoupling.
• Check reference plane continuity and via stubs around high-speed pairs.
• Validate refclk/reset/config strap timing and whether defaults match the target link.

“Looks fine” often means the sensor is not tracking the hotspot. A board sensor can lag a die hotspot, and a die sensor can miss VRM/retimer hotspots that degrade link margin. First determine which component’s temperature most correlates with the errors, then check whether sensors capture that location and dynamics.

• Compare die/board/VRM-zone readings during a controlled thermal ramp.
• Check for time lag: if errors precede the reported temperature rise, the sensor is not representative.
• Validate heatsink contact consistency; poor TIM compression creates localized hotspots.

Assembly method changes thermal contact resistance more than many expect. Torque pattern, standoff tolerance, clip preload, and TIM thickness/voids alter real contact area. Small differences create large hotspot changes, which can force throttling or increase link errors even if average temperature seems similar.

• Standardize torque and sequence; control TIM thickness and compression.
• Check flatness/warpage and contact imprint (coverage) to spot partial contact.
• In production, screen by thermal soak + performance stability (not only idle temperature).

The common issue is added capacitance/inductance and an unintended return path near the connector. Even “low-C” devices add parasitics; placed with long stubs or poor reference grounding, they create discontinuities that reflect energy and shrink eye margin. The fix is usually placement and return-path discipline, not “a different TVS brand.”

• Minimize stub length; place protection devices with a tight, low-inductance return to the reference plane.
• Avoid breaking the reference plane under high-speed pairs at the connector edge.
• Re-validate at target speed across temperature and burst-current conditions.

Capture the minimum evidence set that reveals ordering and margin. The fastest triage is to record input rail behavior, a critical downstream rail droop, and reset timing relative to power-good and clock. This identifies whether failures are caused by inrush/UV events, sequencing, or reset/strap sampling windows.

• Waveform #1: input (VIN) ripple/dip during inrush and workload start.
• Waveform #2: critical rail droop (core/IO/PHY) with enough bandwidth to catch spikes.
• Waveform #3: PG + module reset + PERST# timing (include refclk presence if possible).

Peaks disappear when sampling is too slow, averaging windows are too long, or alerts are not latched. For bursty loads, design telemetry around peak capture and event-triggered snapshots: sample faster than the transient of interest, use short-window max/peak registers, and generate interrupts/alerts that freeze evidence at the moment of failure.

• Define the fastest event to catch (droop, OC spike) and set sampling accordingly.
• Prefer peak-hold / max registers and latched alert flags over rolling averages only.
• Time-align power/thermal readings with error counters or throttle reasons.

Cold-boot-only failures often point to timing windows and analog margins that improve after warm-up. Strap sampling can be sensitive to slow-rising rails and weak pulls at low temperature; sequencing issues can shift PG timing and violate reset/clock assumptions. The most efficient approach is to compare cold vs warm: rail rise times, PG edges, and strap/default states.

• If default mode differs between cold/warm → strap sampling window or pull strength is suspect.
• If rails rise slower and PG shifts on cold → sequencing/inrush/UV margin is suspect.
• Turn it into a screen: cold-soak + first-boot pass/fail plus logged evidence.