Link Training: Software / Programmer View

RTL / firmware view

State machine and firmware assumptions can hide first-failure evidence.

What engineers observe

• Intermittent bring-up failures

• Mode-transition instability

• Data-validity bursts

Design structure impact

• Reset choreography

• Clock/control handshakes

• Wrapper framing semantics

Toolchain interaction

• Synthesis/CDC assumptions must match analog contract

• Beware hidden mode-only paths

Mitigations

• Explicit wrappers

• Observable state machines

• Cross-team interface reviews

RTL / FW — Link Training
// mode switch sequencing changed -> re-check boundary assumptions and regressions

Layer the code touches

AMS OWNERSHIP LAYERS — Link Training

layer                owns                                failure mode
------------------   ----------------------------------  --------------------------
spec contract         clocks/resets/interfaces            hidden assumption drift
wrapper logic         synchronizers/framing/flags         silent data corruption
physical integration  floorplan/isolation/power           coupled noise and droop
signoff governance    waivers/checklists/dashboard        release with blind spots
closure               debug order + regression            fix regresses another mode

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