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USB 2.0 Diagnostics and FAQ

  • No reaction after insertion: inspect VBUS, the power switch, D+/D-, node status, and Host-controller probe.
  • Repeated reconnects: inspect supply droop, cable, connector, PHY parameters, and signal integrity.
  • A high-speed device falls back to full-speed: inspect PHY, clocks, and routing. USB 2.0 high-speed still uses only D+/D-; SuperSpeed pairs belong to USB 3.x.
  • The PC cannot detect Device mode: inspect UDC binding, descriptors, gadget functions, and sys.usb.config.
  • A device disappears after suspend/resume: inspect wakeup properties, VBUS restore timing, and runtime PM.
dmesg | grep -Ei 'usb|ehci|ohci|udc|gadget|extcon'
cat /sys/kernel/debug/usb/devices
cat /sys/class/udc/*/state 2>/dev/null

General diagnostic order​

  1. Schematic and power.
  2. Node and pinctrl in the final DTB.
  3. Final .config and modules.
  4. Probe, clock, reset, and IRQ logs.
  5. Device node and sysfs/debugfs.
  6. Minimal functional test.
  7. Stress, suspend, and abnormal hot-plug tests.

Do not treat the removal of an error log as a fix. Find the first failure and validate its upstream and downstream dependencies.