Plan For Successful USB Compliance Testing (Part I)

Figure 1: The coveted SuperSpeed USB logo

Certifying a device's implementation of a serial protocol standard is a fairly complex process involving a number of levels: electrical test, interoperability, backward compatibility, link layer, and so on. Generally, some organization oversees a given protocol, managing the revision process for the protocol itself as well as the testing process that a product must undergo. Passing the relevant compliance test suite and having a valid Trademark License Agreement on file bestows the prized right to display the protocol's logo on the product's box (Figure 1). That logo's presence tells the product's users that their device's serial interface operates within parameters set by the overseeing organization.

All of the above holds true for the Universal Serial Bus protocol. The most recent revisions of the USB specification are USB 3.0 and 3.1, both of which are overseen by USB Implementers Forum Inc. (USB-IF). USB-IF is a non-profit organization formed by the group of companies that collectively developed the USB protocol in the mid-1990s. There are now some 737 members of USB-IF; membership costs $4000 annually.

USB-IF membership has its benefits, and one of the primary ones is eligibility to participate in free quarterly Compliance Workshops, or "plugfests." That's an attractive benefit, as the alternative is to use the services of independent test houses that will take a product through the compliance process. In addition to not being free, this option comes at the cost of being somewhat less transparent. Failure and retesting might also require more fees.

Yet another means of achieving compliance with the USB standard is, of course, to do the testing in-house. Doing so can save time and money, because if you have the right test equipment on hand and the expertise to use it properly, you can go through the compliance tests in-house. Then, when you do attend a free Compliance Workshop, you can do so with a high degree of confidence that your device will pass the testing.

Figure 2: An overview of USB 3.0 compliance testing

Before even beginning the USB 3.0 (SuperSpeed) compliance testing, a device must first pass USB 2.0 (Hi-Speed) compliance testing. There are six main stages in clearing USB 3.0 compliance testing (Figure 2). The USB-IF provides a number of extremely helpful checklists to guide the test process.

Electrical testing can be accomplished using Teledyne LeCroy USB 3.0 test suites, which encompass the SDA 8 Zi-A serial data analyzer, QualiPHY automated compliance test software, the PeRT3 test system, SPARQ signal-integrity network analyzer, and Voyager protocol analyzer. This is the equipment you'll find in use at USB-IF Compliance Workshops.

Under the umbrella of electrical, or physical layer, testing, is verifying that the output from the DUT is transmitted within compliance limits to ensure sufficient signal quality. Tests include eye diagrams, characterization of the spread-spectrum clock, jitter (total, random, and deterministic), and many others. Another aspect concerns transmit equalization, de-emphasis, and preshoot. Equalization schemes make up for lossy channels. Before performing compliance testing, the channel must be emulated and the signal equalized.

Interoperability, backward compatibility, and device framework testing are within the domain of USB-IF's free USB30CV tool. A protocol analyzer such as Teledyne LeCroy's Voyager M3x USB analyzer platform is also helpful for monitoring of traffic.

In the next installment, we'll take a closer look at some aspects of physical layer and link layer compliance test.