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My USB Protocol Analyzer is Showing False Chirp Transactions – What Causes That?
Rena Ayeras

Software tools are available to detect, analyze and resolve problems on the USB data bus.

Image by Mohamed Hassan from Pixabay

Question from the Customer:

I am trying to understand a capture that was gathered with the Beagle USB 12 Protocol Analyzer. Can you please explain what is going on with the green text? I am not sure what Tiny J means – can you please explain that as well? Here is a snippet of the captured data from the Data Center Software:

Response from Technical Support:

Thank you for your question! The “Tiny J” is a false chirp, which indicates hardware incompatibility between the host and the device. We’ll describe what the green text represents in the Data Center Software display and provide additional information about chirps.

Information Packets about Non-Packet Bus Activity

Your question involves events that are not data packets. Instead, the event transactions represent non-packet bus activity, such as host connect, target device connect, bus reset, or bus speed events. These transactions are displayed as a text description of the event that occurred with further information available in the Info pane. These transactions appear in the Transaction window in green text. Tiny J represents a “false chirp J”.

What Causes False Chirps

The “false chirp J” is caused by the effect of a voltage divider between the device pulling up the D+ line with a 1.5K resistor and the host not driving the data line to ground with sufficiently low output resistance.

This is an artifact on the bus due to the voltage divider effect between the device’s 1.5 kohm pull-up resistor and the host’s 45 ohm termination resistor. When the device switches to high-speed operation, the Tiny J, false chirp J, will no longer be present as the device will have released its pull-up resistor. In other words, no problem has occurred that requires troubleshooting.

Why USB Devices Use Chirp

Chirping is a protocol used during reset to negotiate high-speed mode with a host or hub.

A device that is high-speed capable first connects to the host as a full-speed device. If both the host and the device are high-speed capable and negotiation is successful, they both operate in high-speed mode. For more information about USB, refer to About the USB Protocol, Common USB Bus Errors, and How to Troubleshoot Them.

Note: SuperSpeed and other higher-speed USB 3.0 devices do not require this bandwidth negotiation.

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

If you want more information, feel free to contact us with your questions, or request a demo that applies to your application.