Extended case study

Signoff review: local droop at analog islands, guardband consumption, reset excursions regressed after a change touching IR at Analog Islands.

Background

Team had prior closure; latest integration run shows instability concentrated in one operating condition.

Symptoms observed

• local droop at analog islands, guardband consumption, reset excursions degradation

• Cross-team disagreement on root cause

• Directed tests pass but system scenario fails

Investigation timeline

1. Hour 0: freeze data tags, firmware build, and environmental conditions

2. Hour 1: diff boundary assumptions and handoff revisions

3. Hour 2: isolate first failing mode and trigger sequence

4. Hour 3: correlate waveform/log/report evidence

5. Hour 4: classify root cause and ownership

6. Hour 5: apply bounded fix

7. Hour 6: execute full regression and issue signoff memo

Root cause

Root cause tied to IR at Analog Islands: IR drop localized near analog islands distorts bias points and clock quality; strap topology and domain partitioning define resilience under burst load.

Fix and validation

• Bounded RTL/config/layout correction

• Re-run IR map around islands, dynamic droop trace, power-grid review memo

• Full cross-domain regression matrix

Lessons learned

• Tag everything

• Mechanism before commands

• No signoff without owner agreement

CASE STUDY — IR at Analog Islands
baseline metric / regressed metric / post-fix metric

Sequence under stress

AMS BOUNDARY SEQUENCE — IR at Analog Islands

analog macro -> wrapper / boundary cell -> synchronizer or sampler -> digital consumer
      |                 |                         |                     |
  analog assumptions    legal voltage/state       clock/reset contract   protocol/data validity

metric under watch: local droop at analog islands, guardband consumption, reset excursions

AMS deep dive

AMS signoff is a combined noise, IR, timing, and package decision.

Concept diagram

BOUNDARY SIGNOFF

noise + IR + package + timing -> release decision

Metric graph

RISK DASHBOARD

open blockers █████
waivers       ███

Reports and artifacts

• supply ripple spectrum

• IR around analog islands

• package PI summary

• release dashboard

Mini case study

IR hotspot at analog island caused jitter spikes only under burst traffic.

Debug branches

• Correlate droop and jitter

• Check package return path

• Validate mode-specific activity profile

Senior review question

Ask: what boundary condition proves this topic is actually closed?

Key takeaways

• State boundary, mode, and evidence tag with every claim.

• Always align analog, digital, and physical owners before signoff decisions.

Common pitfalls

• Fixing averages while tails still fail.

• Skipping package/supply evidence in jitter or SerDes issues.

• Shipping with waivers that lack owner and expiration criteria.

Principal AMS review addendum

IR drop localized near analog islands distorts bias points and clock quality; strap topology and domain partitioning define resilience under burst load.

Metric: local droop at analog islands, guardband consumption, reset excursions