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Signs You Need to Check VBUS for the Root of Your USB Problem
Jessica Hopkins

When your USB system isn’t working as expected and you’re experiencing system failure or other unexplainable issues, it can be tough to pinpoint the root cause of the bug. While there are many possible reasons for abnormal behavior, one issue in particular may be overlooked, and that is having a bug on the USB VBUS. Before diving into different signs that can be a result of VBUS issues, we’ll provide a brief explanation of VBUS itself and its role in USB.

Understanding VBUS

USB devices are connected by a cable connection. This cable includes multiple different wires that are used to transmit data and power between devices. The VBUS wire is used to implement the electrical component within a USB device, and it carries a nominal 5V supply. Having a stable VBUS connection is very important to the operation of the USB devices. Without it, abnormal device behavior can occur.

Signs to Look For with VBUS Connection Issues

What are the signs that you need to check the USB VBUS for the root cause of your USB bug?

1. Your entire USB system shuts off during operation or testing:

It is possible that the USB system is shutting down during your operation or testing because there is an overdraw of current from the device. The USB specification mentions that if a device draws too much current, it is required to shut down.

The USB 2.0 specification has specific upper limits on current consumption:

  • No more than one-unit load, which is 100 mA, after connect and before enumeration
  • No more than five-unit loads, which is 500 mA, after enumeration
USB 3.0 devices and 3.0 capable ports on hosts offer higher current budgets:

  • No more than one-unit load, which is 150 mA, after connect and before enumeration
  • No more than six-unit loads, which is 900 mA, after enumeration

The Beagle USB 480 Power Protocol Analyzer will be able to detect these spikes in current and provide a visual representation of this event that is directly correlated to the data trace capture. This will allow users to pinpoint when and where the high current spike took place for easier debugging.

2. Your device is not detected or enumerated by the Operating System on the host (device perspective):

This could be caused if the connected device exceeds the maximum amount of current on connect or after the enumeration process on VBUS. This could end up triggering the 'over-current' protection circuit on the host and cause the device to be electrically disconnected, and it can become an endless loop where enumeration never happens or completes.

The device in this case could be violating the upper limit current consumption; using a Beagle USB 480 Power Protocol Analyzer will be very helpful in monitoring the current from when the device is connected through and after enumeration.

3. Your device is not detected or enumerated by the Operating System on the host (host perspective):

Another possible scenario is a misbehaving USB port on a host. It is possible that the device could be functioning as normal, but the host port is malfunctioning. It might not be supplying the right voltage levels on the VBUS because of damaged circuitry internally. This issue can also be detected by monitoring voltage on VBUS using the Beagle USB 480 Power Protocol Analyzer.

4. Your device works as expected on a laptop computer as long as the computer is plugged in to an AC power source. The same device does not work with the same laptop computer when it is operating on a battery charge:

Sometimes, when laptop computers are connected to an AC power source, the current levels on a USB port can be higher than normal. If there is a non-compliant device plugged in that violates the maximum draw of current defined in the specification, the host can end up supplying higher power levels and the system will function routinely. However, when the laptop is solely running on battery, power becomes critical, and in this case, the host will detect the overdraw in current and electrically disconnect the device.

5. A device works as expected with a laptop computer as long as it is connected through a self-powered hub. The same device does not work when it is connected directly to the same laptop computer:

This is similar to the above condition, except in this case, the self-powered hub is providing higher power levels and is tolerating the misbehaving device, whereas the computer does not.

Troubleshooting VBUS with the Beagle USB 480 Power Protocol Analyzer

All of these VBUS issues can be detected with the Beagle USB 480 Power Protocol Analyzer. This tool makes it easy to measure current and voltage from USB connections by providing a real-time graph of VBUS current and voltage values that is also a bi-directional correlation of the USB data. By selecting either a specific USB transaction in the data capture or a point on the Current/Voltage Monitor graph, users can easily diagnose the cause of VBUS bugs by allowing them to pinpoint the exact moment the bug took place.

 Monitoring VBUS current and voltiage

There are two editions of the Beagle USB 480 Power Protocol Analyzer. Both editions enable VBUS current and voltage measurement within our industry-leading Data Center Software. They both provide an extra-large hardware buffer and one-click correlation of voltage and current measurement to protocol-level activity. The Ultimate edition provides enhanced USB 2.0 advanced triggering, which allows users to create complex, multi-state matches for USB 2.0. This is an optional upgrade for the Standard edition.

Advanced Trigger set up with Data Control Center Software

For more information about the Beagle USB 480 Power Protocol Analyzer, please see the following

If you have questions, please contact sales@totalphase.com. You can also request a demo for your specific application.

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