Worked example

Worked Example for USB Link & Packet Model focuses on enumeration success, retry count, endpoint throughput. The goal is to connect the observable symptom to protocol mechanism, ownership, and regression risk.

A product workload shows enumeration success, retry count, endpoint throughput. The first review mistake is to blame the whole interface. A better review starts by pinning one transaction, proving where protocol progress stopped, and checking whether the observed behavior is legal for USB Link & Packet Model.

Sequence under inspection

SEQUENCE — USB Link & Packet Model

  initiator            interconnect/PHY            target
      |  request (id) ------->  |                     |
      |                         |  forward ----------> |
      |                         |                     | work
      |                         |  <---- response ---- |
      |  <----- complete ------ |                     |
      |
   metric captured here: enumeration success, retry count, endpoint throughput

USB transfer = token/data/handshake

USB TRANSACTION

host: TOKEN (IN/OUT/SETUP, addr, endpoint)
        |
        v
data: DATA0/DATA1 packet
        |
        v
ack : HANDSHAKE (ACK/NAK/STALL)

NAK = "not ready, retry" (normal). STALL = "error, software must clear".

1. Capture the failing waveform and transaction log.

2. Tag the request ID, address, endpoint, or lane.

3. Find the first response, retry, stall, or missing completion.

4. Compare against USB analyzer trace, endpoint descriptor dump, link-state timeline.

5. Choose one reversible fix and write the regression list before editing RTL or firmware.

Did the fix work?

BEFORE / AFTER — USB Link & Packet Model

           failing        target
metric  |    ●              ┄┄┄┄┄┄┄
        |     \
        |      \___ ● bounded fix
        |           \
        |            ● validated
        +-------------------------------> change set
Prove the mechanism moved the metric; one good dot is not proof.

Protocol deep dive

USB/Ethernet/MIPI failures cross MAC counters, PCS framing, PHY adaptation, and channel SI.

Concept diagram

HIGH-SPEED STACK

app -> MAC/framing -> PCS/encoding -> SerDes/PHY -> channel

CRC errors often mean PCS/PHY/channel, not TCP.

Metric graph

BER vs EQ SETTING

BER
1e-3 |*
1e-6 |  *
1e-9 |     **** usable window
1e-12|          *
     +-----------------> EQ tap

Metrics and artifacts to collect

• CRC error rate

• retrain count

• frame drop

• lane error

• BER

Mini case study

Ethernet link up at 100G but lossy: equalization margin on one lane narrow after package change. Digital counters were clean; PHY margin was not.

Debug branches

• If link up but lossy, PHY margin and retrain.

• If enumeration OK but throughput low, check packet size and DMA batching.

• If MIPI frame drops, blanking budget and lane polarity.

Senior review question

Ask: what is the first transaction that deviates, and which spec rule does it test?

Key takeaways

• Connect every protocol claim to a transaction identity and measurable metric.

• Store the artifact (waveform, log, counter) next to every signoff decision.

Common pitfalls

• Debugging timeouts without finding the first bad transaction.

• Quoting peak bus width without payload efficiency and retry overhead.

• Treating VIP compliance as a substitute for system integration replay.

Narrative walkthrough

A team sees enumeration success, retry count, endpoint throughput drop 40% after a seemingly small change near USB Link & Packet Model.

They almost widen the interface. Instead they capture id=7 read burst and find W beats never matched AW len.