Which Capture Mode Best Supports My Project? How are Transactions Timestamped in the Data Center Software?

Question from the Customer:
I am using the Beagle USB 480 Protocol Analyze and the Data Center Software – I’m looking to improve throughput on a USB system. Looking at the data capture, I saw about 300ms delay between index 17 and 27. To make sure I understand the results, I have some questions:

  1. Index 18 is [177175 IN-NAK], - does that indicate the number of IN-NAKs is 177175? Also, during this period, the device has no available data. Is that correct?
  2. Index 18 has a timestamp - is this the first IN-NAK timestamp,?
  3. In addition, I see the timestamp of index 17 lags behind that of index 9 – what caused that to happen?

Response from Technical Support:

Thanks for your questions! For your first question, the answer is yes – the number in front IN-NAK is a count so if your capture there were 177175 IN-NAKs that occurred at Index 18.  On the second question, you are also correct.  A group of collapsed transactions are timestamped by the first transaction.

How to Interpret Timestamps in your USB Data Capture

Here’s some information about the lag time that you saw between index 17 and index 9 – whether or not the information is sequential depends on the capture and view modes that are used:

The Data Center Software, when used with the Beagle USB 480 analyzer, has two capture modes: Sequential and Aggregate.

  • Sequential – saves the information necessary to display the capture in time-ordered Packet View.
  • Aggregate – discards the information saved in Sequential Mode in order to reduce memory usage. Using this mode disables the ability to use the Packet View.

In Aggregate mode, transactions may appear out of order because the beginning of a transaction may have started before a previous transaction ended.  Because packets are grouped together based on the first packet timestamp, you may experience this overlap in timestamps.

It sounds like you most likely ran your capture in Aggregate mode.  You can check your Capture Settings to confirm.  If you ran the capture in Sequential mode, then change to the Packet View to see all packets in the time stamped order.

By default, the Data Center Software compresses individual data-less sequences to reduce memory usage. For additional information, please prefer to the Data Center Software User Manual.

The Data Center Software Provides Three Views of USB Data

You can use one of three views (Packet, Transaction, and Class) based on your system requirement. When you use the Class or Transaction view in the Data Center Software, the records may not be in time-order due to how the data is grouped for higher level parsing. You can see the records in time-order by switching to Packet view, which disables the higher level parsing. However, be aware that only captures run in sequential mode can be viewed in packet view.

To select a Capture View, use the Capture View Menu.

  • Packet – Protocol-level decoding is performed, and records are in time-order. Collapsed groups, such as SOFs and IN-NAKs are broken up as necessary to ensure records are in timestamp order. Only captures run in Sequential Mode (Section 6.2) can be viewed in Packet View
  • Transaction – Protocol-level decoding is performed. Records may not be in time-order. Collapsed groups are not broken up for time-order preservation. Since there is no time-order restriction, captures generally appear more compact in this view than in Packet View.
  • Class – Class-level parsing is performed. Records may not be in time-order. Captures generally appear high-level and compact in this view.

For your request, to view the records in in time-order we recommend using the Packet view when recording data.

Additional resources that you may find helpful include the following:

More questions? More projects? You can contact us and request a demo that applies to your application, as well as ask questions about protocol analyzers and other Total Phase products.

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