AMS Interface · All levels

Link Training: Comparison Matrix

Comparison Matrix for Link Training.

Comparison matrix

Equalization and training policies trade BER margin against bring-up speed.

diagram

+------------------+----------------+----------------+----------------+
| Approach         | Strength       | Weakness       | Best when      |
+------------------+----------------+----------------+----------------+
| Conservative     | robust         | more overhead  | new tapeout    |
| Balanced         | practical      | needs rigor    | mature flow    |
| Aggressive       | faster         | high regressions | late stage     |
| Architectural    | durable        | slow loop      | recurring issue |
+------------------+----------------+----------------+----------------+

When to choose each approach

Choose the smallest change that proves mechanism and preserves system margins.

Interview traps

Treating all AMS problems as analog-only
Skipping package and power evidence

Evidence comparison

diagram

AMS EVIDENCE MATRIX — Link Training

+-------------------+------------------------+--------------------------+-------------------------+
| Evidence           | Tells you              | Does not prove           | Next action             |
+-------------------+------------------------+--------------------------+-------------------------+
| Waveform/log pair  | sequence behavior      | physical root cause      | correlate with signoff  |
| Signoff report     | metric movement        | contract correctness     | audit assumptions       |
| Floorplan view     | physical proximity     | dynamic mode behavior    | run stress mode         |
| Firmware trace     | state progression      | analog quality           | combine with lab data   |
| Dashboard summary  | closure status         | single-failure mechanism | inspect raw artifacts   |
+-------------------+------------------------+--------------------------+-------------------------+

AMS deep dive

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

Concept diagram

diagram

SERDES FLOW

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

Metric graph

diagram

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

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