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USB sniffer using Raspberry Pi Pico

A USB sniffer using Raspberry Pi Pico.

It implements a USB sniffer using only a single chip (RP2040 microcontroller), thanks to its Programmable IO (PIO) module, dual cores, and DMA.

Despite the hardware is very simple, it can capture packets on a USB cable with minimal interference with devices under test, similar to professional USB analyzers.

Captured packets are saved as a .pcap file and can be analyzed using Wireshark.

Implementation on a breadboard

Screenshot of Wireshark displaying captured packets Example capture file: logitech_unifying.pcap

Usage

Firmware

Firmware for Raspberry Pi Pico (.uf2 file) is available on Releases page.

Wiring

Connect USB D+ to Raspberry Pi Pico's GPIO 11, USB D- to GPIO 12, and USB GND to Pico's GND.

Capturing script

Use the Python script tools/pico_usb_sniffer.py. This scripts requires Python 3 interpreter, as well as pyserial and sliplib libraries.

usage: pico_usb_sniffer.py [-h] [-o OUTPUT] [-i [PID ...]] port

positional arguments:
  port                  name of the serial port (e.g. COM1 on Windows or /dev/ttyACM0 on Linux).

optional arguments:
  -h, --help            show this help message and exit
  -o OUTPUT, --output OUTPUT
                        path of output file. '-' indicates stdout. Default is '-'.
  -i [PID ...], --ignore-pids [PID ...]
                        Packet Identifiers (PIDs, e.g. SOF or ACK) to ignore. Case-insensitive.

Limitations

  • Quite sensitive to signal degradation. This is because it uses single-ended GPIO ports instead of a differential receiver. For robust capture, the following mitigations are useful (although you may still get corrupted packets even with these mitigations):
    • Use short cables.
    • Put a signal repeating device (e.g. an USB hub, or an isolator based on ADuM3160) between the sniffer and the device under test. Dongle-shaped one or one with very short cable is the most preferable.
  • Only Full Speed mode is supported. Low or High Speed packets on the same bus may prevent entire capturing.
    • The aforementioned USB isolator is also useful to force a High Speed capable device to use Full Speed.
  • Because it sends captured USB Full Speed packets via USB serial port (which also uses Full Speed), it cannot capture all packets when USB bandwidth is fully utilized.

Known issues

  • Sometimes, the sniffer itself (Raspberry Pi Pico) is not recognized by a PC. If the serial port of the Pico does not appear, plug the Pico into your PC again.

See also