What “Fail Safes” Should I Apply When Analyzing an Embedded USB Device?

Question from the Customer:

I am using the Beagle USB 480 Power Protocol Analyzer as a pass-through to troubleshoot an embedded USB device. The problem that I am analyzing could include hardware issues. So I can “filter out” any factors that the Beagle USB 480 analyzer may add into the system, can you tell me what occurs between the target device and host connectors?

Response from Technical Support:

Thanks for your question! The Beagle USB 480 Power analyzer is a non-intrusive, real-time data analyzer for Low/Full/High-Speed USB 2.0 data capture as well as VBUS current and voltage values. For your setup, there are some considerations to apply:

 

Beagle 480 Power Protocol Analyzer for Power Measurements and Extended Periods of Data Capture

 

Dp/Dm Lines and Impedance

The capture side of the Beagle USB 480 Power analyzer acts as a USB pass-through.

  • The Beagle USB 480 Power analyzer passes the Dp/Dm lines between the target USB HOST and USB DEVICE directly.
  • Internally, the Dp/Dm lines go to the circuits that sniff the USB data through two buffers. In these buffers, the impedance is about 200k ohms.

Cabling Effects

Due to the nature of pass-through analyzers, some power is used to drive our receiver for High-Speed USB signals. In general - cabling makes a huge difference. We recommend using the shortest possible USB certified cables. We use 30 cm certified cables for low signal environments. For information about why we recommend certified cables, take a look at Saving the World From Crummy USB Cables.

Using the VBUS Override to Ensure Capturing Data

To recognize that a host is present, the Beagle USB 480 Power analyzer must detect 5V on the VBUS.

Using our Data Center Software application, you can the override this requirement entering device({'usb.vbus_override': True}) in the Command Line window. With this override, the Beagle USB 480 Power analyzer should capture traffic regardless of whether it detects the target host or not.

For additional information, please refer to the Knowledge Base article Monitoring an Embedded USB with a Beagle USB Protocol Analyzer.

We hope this answers your question. Additional resources that you may find helpful include the following:

If you have more questions about our Beagle 480 analyzer or Total Phase products, feel free to email us at sales@totalphase.com. You can also request a demo that is specific for your application.