I have a product with a Coldfire processor that is connected to a peripheral device via high-speed USB. The processor and the peripheral are on the same board and the USB lines are connected by copper traces. I want to monitor the USB traffic with our Beagle USB 480 Protocol Analyzer, but since the USB signal lines between the processor and peripheral device are in copper, I cannot insert the Beagle protocol in series. Instead, it looks like I have to tap into the USB signals in parallel. For signal integrity, I believe we need some series resistors, but I’m not sure what resistance values to use or where to use them.

Also, this circuit does not provide a VBUS. I understand the Beagle analyzer needs to detect 5V to recognize the embedded device – how can I work around that?

Thanks for your questions! We have a Knowledge Base article that answers both your questions with detail: Monitoring an embedded USB with a Beagle USB Protocol Analyzer.

	Figure 1: Embedded System	Figure 2: Beagle USB 480 Protocol Analyzer

Here is a summary of what you can do to monitor an embedded USB bus:

• First of all, the D+/D- signal path of a USB does not have to be broken to be monitored by a Beagle USB protocol analyzer. The VBUS, GND, D+, and D- lines can be connected to either the Type A or Type B connector on the Beagle analyzer with T connections to the traces.
• You will probably need to cut open a USB connector and connect the wires to the signal traces, or if they’re available, to a header or signal pads.
• For the signal integrity, we recommended using 20 – 40 ohm resistors to the D+ and D- signals between the target and the Beagle USB 480 analyzer.

You are correct - the Beagle USB 480 analyzer does need to detect 5V on the VBUS to recognize the host. There are two ways to work around this issue:

• Use an external power source to provide 5V to the Beagle USB 480 analyzer for the VBUS.
• Issue a command that overrides the VBUS. For details, please refer to the Command Line Option section of the Data Center Software User Manual.

Additional resources that you may find helpful include the following:

• Beagle Protocol Analyzer User Manual
• Data Center Software User Manual
• Monitoring an embedded USB with a Beagle USB Protocol Analyzer

We hope this answers your questions. If you have other questions about our protocol analyzers or other Total Phase products, feel free to email us at sales@totalphase.com, or if you already own one of our devices and have a technical question, please submit a request for technical support.