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Interface Protocols · All levels

High-Speed I/O Debug: Interview Drills

Interview Drills for High-Speed I/O Debug.

Interview drills

Interview Drills for High-Speed I/O Debug focuses on BER, link retrain count, throughput under real traffic. The goal is to connect the observable symptom to protocol mechanism, ownership, and regression risk.

diagram
PROMPT
You see BER, link retrain count, throughput under real traffic on High-Speed I/O Debug. Walk through root cause and fix.

STRONG ANSWER
1. Names the layer and transaction identity.
2. Explains debug crosses digital packet counters, PHY adaptation, board SI, firmware sequencing, and workload traffic shape.
3. Requests link monitor log, eye/margin report, packet analyzer capture.
4. Proposes one reduced sequence and one system regression.

WEAK ANSWER
Jumps to widening the interface, increasing FIFO depth, or blaming firmware without evidence.

Diagram to draw on the whiteboard

BER vs equalization

diagram
BIT ERROR RATE vs EQ SETTING

BER (log)
 1e-3 |*                         *
 1e-6 |  *                    *
 1e-9 |     *             *
1e-12 |        *  *  *  *        <- usable window
      +-------------------------> EQ / tap setting
Pick the center of the low-BER window, not the edge.

Root-cause tree to narrate

diagram
ROOT-CAUSE TREE — High-Speed I/O Debug

BER, link retrain count, throughput under real traffic looks wrong
        |
   reproducible?
     /        \
   no          yes
   |            |
 flaky env   same first transaction every time?
 / seed         /            \
              yes             no
               |               |
        protocol rule     timing/reset/PVT
        or config bug     or load-dependent

Protocol deep dive

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

Concept diagram

diagram
HIGH-SPEED STACK

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

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

Metric graph

diagram
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.

Interview whiteboard

Draw layers first, then place the failing transaction on the diagram.

Practice this lesson