Question from the Customer:
We have a product with a 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. We want to monitor the USB traffic with our Beagle USB 480 Protocol Analyzer. Because the USB signal lines between the processor and peripheral device are in copper, we cannot insert the Beagle analyzer in series - instead, we want to connect the Beagle by tapping into the USB signals in parallel. Is this possible? We're pretty sure we need some series resistors - but not sure what resistance value to use and how to make that connection. Can you help us with this?
Response from Technical Support:Thanks for your question! Yes, you can definitely use your Beagle USB 480 analyzer to monitor the traffic over copper traces. Monitoring an embedded USB with a Beagle USB Protocol Analyzer is fairly simple, and we have an article addressing just that in our Knowledge Base. Here's an overview of the article:
Set UpTo monitor the D+/D- signal path of a USB bus, you don't have to "break" those lines. The VBUS, GND, D+, and D- lines can be connected to either the Type A or Type B connector on the Beagle analyzer using "T" connections, as shown below.
Figure 1: Connect a Beagle USB Protocol Analyzer to an Embedded USBThe process of using the Beagle analyzer to tap the USB lines in your embedded system depends on how accessible they are. If the USB signal lines are connected to a header or test pads, then the connection is straightforward. For your situation, you will most likely need to solder the lines from a cut-up USB cable to connect to the system. For signal integrity, keep the wires as short as possible and use twisted pairs
Regarding the use of resistors, a 20-40 ohm series resistor should be added to the D+ and D- signals between the target system and the Beagle analyzer - this prevents or minimizes signal reflections due to impedance mismatches.
VBUSTo recognize that a host is present, the Beagle analyzer will need to see 5V on VBUS. If the target system cannot provide 5V to the Beagle analyzer, it can be provided from an external source. Alternatively, this requirement can be overridden in software. You can set that up with the Data Center Software, or by customizing a program with the Beagle Software API (samples are provided that can be used as-is or modified for your specific setup). Details about API are provided in section 6 of the Beagle Protocol Analyzer User Manual.
Additional resources that you may find helpful include the following:
- Monitoring an embedded USB with a Beagle USB Protocol Analyzer
- Beagle Protocol Analyzer User Manual
- Data Center Software User Manual
- API Documentation
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.