Link Training: Worked Example

Worked example

Worked Example for Link Training focuses on training pass rate, time-to-L0, fallback mode frequency. The goal is to connect observed symptom to boundary mechanism, ownership, and signoff risk.

A stress workload shows training pass rate, time-to-L0, fallback mode frequency. The first review mistake is broad blame. A better review tracks one boundary transition, validates contract assumptions, and proves the first failing mechanism in Link Training.

Sequence under inspection

AMS BOUNDARY SEQUENCE — Link Training

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

metric under watch: training pass rate, time-to-L0, fallback mode frequency

Training state path

detect -> poll -> config EQ -> align -> active/L0
      ^                                  |
      +------------- retry/fallback -----+

1. Capture tagged waveform/log/report set.

2. Mark mode boundary and first failure timestamp.

3. Map to contract and owner.

4. Compare against training state trace, firmware log, protocol analyzer capture.

5. Propose one reversible fix and predefine regression matrix.

Did the fix work?

BEFORE / AFTER — Link Training

health score
  ^
  |                     --- target
  |         o regressed
  |               o before fix
  |                    o after bounded fix
  +--------------------------------------> debug iteration

Prove signal quality, timing margin, and integration safety together.

AMS deep dive

SerDes closure requires protocol, training, and SI evidence together.

Concept diagram

SERDES FLOW

training -> equalization -> lane margin -> protocol stability

Metric graph

BER VS EQ

BER
 ^
 |  high  low  high
 +-----------------> EQ setting

Reports and artifacts

• lane BER

• training state transitions

• EQ sweep report

• link retry counters

Mini case study

Link looked protocol-clean but lane margin collapsed under thermal sweep.

Debug branches

• Correlate LTSSM and lane metrics

• Check SI margins

• Review firmware timeout assumptions

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

Training protocols establish lane polarity, timing, equalization, and protocol states; digital state machines must synchronize with PHY indications and timeout policies.

Metric: training pass rate, time-to-L0, fallback mode frequency