Interview Drills 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.
PROMPT
You see enumeration success, retry count, endpoint throughput on USB Link & Packet Model. Walk through root cause and fix.
STRONG ANSWER
1. Names the layer and transaction identity.
2. Explains USB combines host-driven enumeration, endpoints, packets, link states, and PHY behavior into a software-visible device model.
3. Requests USB analyzer trace, endpoint descriptor dump, link-state timeline.
4. Proposes one reduced sequence and one system regression.
WEAK ANSWER
Jumps to widening the interface, increasing FIFO depth, or blaming firmware without evidence.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".ROOT-CAUSE TREE — USB Link & Packet Model
enumeration success, retry count, endpoint throughput looks wrong
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reproducible?
/ \
no yes
| |
flaky env same first transaction every time?
/ seed / \
yes no
| |
protocol rule timing/reset/PVT
or config bug or load-dependentUSB/Ethernet/MIPI failures cross MAC counters, PCS framing, PHY adaptation, and channel SI.
HIGH-SPEED STACK
app -> MAC/framing -> PCS/encoding -> SerDes/PHY -> channel
CRC errors often mean PCS/PHY/channel, not TCP.BER vs EQ SETTING
BER
1e-3 |*
1e-6 | *
1e-9 | **** usable window
1e-12| *
+-----------------> EQ tapCRC error rate
retrain count
frame drop
lane error
BER
Ethernet link up at 100G but lossy: equalization margin on one lane narrow after package change. Digital counters were clean; PHY margin was not.
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.
Ask: what is the first transaction that deviates, and which spec rule does it test?
Connect every protocol claim to a transaction identity and measurable metric.
Store the artifact (waveform, log, counter) next to every signoff decision.
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.
Draw layers first, then place the failing transaction on the diagram.
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