usb-sniffer — wire-level capture with the ataradov hardware analyzer

Extends the debugging trio with the layer below URBs:

Skill	Answers
usbmon	what a Linux PC host exchanged (URBs)
usb-kernel-debug	why the Linux kernel acted (dmesg / dynamic debug)
target-debug	what the TinyUSB target did (device or host role)
usb-sniffer	what actually crossed D+/D- (PIDs, handshakes, resets, timing)

Reach for it when usbmon can't see (device never binds, pre-enumeration failures), can't be trusted (URB completed but did the wire really ACK?), or doesn't exist — a link where TinyUSB is the host has no usbmon on either end (an MCU host runs no kernel; a Linux gadget peer's UDC bypasses usbmon). Where a Linux PC is the host, usbmon is cheaper — no hardware, no locks.

Rig inventory — find the sniffer and what it taps

lsusb -d 6666:6620          # sniffer present? (github.com/ataradov/usb-sniffer)

The sniffer is a passive tap: host-side and device-side connectors pass through, the capture port is a separate USB device. What it taps is a cabling fact you must confirm every session, not assume: start a capture (below), provoke known control traffic to a candidate (`lsusb -v -s :

/dev/null), and see whether those requests appear on the wire. The DUT's link speed (cat /sys/bus/usb/devices//speed) picks --speed`.

The tapped board is rig hardware: hold its board lock for any session that resets or reflashes it (hil skill). The sniffer itself is not lockable and capture alone perturbs nothing.

Capture

The tool is usb_sniffer (installed in ~/.local/bin, extcap-symlinked so Wireshark's GUI also shows a "USB Sniffer" interface). Headless recipe:

timeout 15s usb_sniffer --capture --fifo /tmp/cap.pcapng --speed hs   # or fs / ls

• --speed MUST match the DUT's link speed (default is fs!). Wrong speed = no USB packets, only Syslog pseudo-packets ("Line state: SE0", "VBUS ON"). If you see only those, fix --speed before doubting the hardware.
• ALWAYS bound the capture: timeout and/or --limit N (packets). HS runs 15–20 MB/s even with --fold when any device on the bus is busy (--fold only collapses truly empty frames). Unbounded HS captures reach GB fast.
• The output is valid pcapng the moment the process dies; a plain file path works (no FIFO needed). --trigger low|high|falling|rising arms capture on the external trigger pin instead of starting immediately.
• Tool diagnostics: USB_SNIFFER_LOG=/tmp/sniffer.log usb_sniffer ...

Start the capture FIRST, then trigger the event you care about. The proven one-pass enumeration recipe (--limit makes the tool exit by itself; on a busy HS bus ~470k packets/s ≈ 20 MB/s, so 3M packets ≈ 6–7 s ≈ 120 MB — do NOT capture for 20+ s "to be safe", the raw balloons and every later tshark pass pays for it; but do NOT go below ~3M either: J-Link connect latency varies run-to-run (0.5–4 s) and a 3 s window has provably missed the ladder):

usb_sniffer --capture --fifo raw.pcapng --speed hs --fold --limit 3000000 &
sleep 1
# trigger: full ladder incl. SET_ADDRESS (needs board lock; J-Link resets the MCU):
printf 'r\ng\nqc\n' | JLinkExe -device $JLINK_DEVICE -SelectEmuBySN <probe-uid> \
  -if swd -speed 4000 -autoconnect 1 -nogui 1
wait   # tool prints "Capture limit reached" and exits

No-probe trigger alternative — kernel-side re-enumeration (may reuse the xHCI address and skip parts of the ladder; fine for descriptor reads, weak for reset timing): echo 0 | sudo tee /sys/bus/usb/devices/<port>/authorized; sleep 1; echo 1 | sudo tee ...

Reading the capture

tshark -r cap.pcapng -Y 'usb.bmRequestType'                 # the control ladder
tshark -r cap.pcapng -Y 'usb.bDescriptorType == 1' \
       -T fields -e usb.idVendor -e usb.idProduct           # VID:PID off the wire
tshark -r cap.pcapng -Y 'usbll.pid'                         # raw token/handshake level
editcap -r cap.pcapng slice.pcapng <first>-<last>           # trim huge captures

On a capture >100 MB, make exactly ONE filtered pass (the ladder filter above) to find the frame numbers of your event window, editcap -r to that window, and do all further analysis on the slice — repeated broad tshark passes over a 300 MB raw are what turn a 5-minute job into 15.

Find the DUT's wire address from the capture, not from lsusb: the SET ADDRESS request payload carries it (00 05 <addr> 00 ...), and all subsequent traffic goes to <addr>.<ep> (usbll.addr). On xHCI hosts the lsusb device number is NOT the wire address — they diverge routinely. Filter analysis to the DUT: -Y 'usbll.addr contains "4."'.

What the wire really shows (read before concluding anything)

• Downstream is broadcast. Tokens, SETUP and OUT data addressed to EVERY device on the tapped bus segment appear in the capture; upstream (DATA in response to IN) appears only from devices on the tapped branch. Lone IN→ACK pairs without DATA to some other address are normal, not corruption.
• The sniffer can capture its own upload. If its capture port shares the host controller bus with the tap, its bulk-IN polling floods the capture (easily >90% of packets) — filter it out by address; for surgically clean captures move the capture cable to a different host controller.
• Port-reset visibility depends on the tap point. Tapping the DUT's own cable: a reset reaches the sniffer PHY and you get explicit --- Bus Reset --- / Detected speed: Syslog records. Tapping a hub upstream: the hub isolates downstream port resets — no marker appears. Anchor reset timing on the hub choreography instead: SetPortFeature(PORT_RESET) to the hub's address = reset start, ClearPortFeature(C_PORT_RESET) = reset end (start the capture before triggering, or the initiating SetPortFeature is missing from the file). The DUT's silence gap corroborates, but do not read every gap as a reset — idle captures contain benign multi-ms gaps.
• FS device behind an HS hub: the upstream tap shows SPLIT transactions, not native FS packets. Tap the DUT's own cable and capture at fs for clean full-speed traffic.

Setup (one-time)

# udev: tools/88-tinyusb.rules covers 6666:6620 + blank FX2LP 04b4:8613
sudo cp tools/88-tinyusb.rules /etc/udev/rules.d/
sudo udevadm control --reload-rules && sudo udevadm trigger -s usb

# binary (ataradov repo bin/usb_sniffer_linux) + Wireshark extcap symlink (needs Wireshark >= 4.x)
cp usb_sniffer_linux ~/.local/bin/usb_sniffer && chmod +x ~/.local/bin/usb_sniffer
mkdir -p ~/.local/lib/wireshark/extcap
ln -sf ~/.local/bin/usb_sniffer ~/.local/lib/wireshark/extcap/usb_sniffer

Never run --mcu-eeprom / --fpga-flash / --fpga-erase against a working sniffer — those program NEW hardware.

Warnings

• Bound every capture (timeout / --limit) and delete or editcap-trim multi-hundred-MB raws before handing off; a forgotten capture process fills the disk at HS rates.
• The tap is passive — capturing, or unplugging the capture port, does not disturb the DUT's link. Unplugging the pass-through DOES.
• Answers must come from packet payloads (SETUP/DATA hex), not from host-side logs — that is the whole point of being on the wire; if an answer isn't in the capture, say so rather than approximating from sysfs/dmesg.
• Release the board lock and leave no capture processes running at session end (pgrep -a usb_sniffer).