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Testing SuperSpeed USB 3.1 (Part 1)

29 Jan 2015  | Randy White

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The software receiver used to embed the channel is also used to open the eye. The Gen1 reference receiver used a passive model of poles and zeros. The Gen2 reference receiver now uses an AC gain parameter with an active model that should improve the eye margin. This, in addition to a 1-tap decision feedback equaliser, is required to overcome the worst-case channel that is used for compliance testing.

Receiver testing
The role of receiver testing is to validate that the receiver is capable of properly recovering a worst-case signal. Beyond validating functionality, a jitter-tolerance test is required for certification. Receiver testing requires a transmitter source to generate an impaired pattern that can be sent to the receiver. The receiver is then put into a mode that echoes or loops back the signal. The loopback method must be far end retimed, thereby testing both the clock data recovery as well as the signal conditioning blocks such as equalisation. The loopback signal is then compared to the actual generated signal to make sure there are no errors. Idle symbols like SKPs are not compared during the BER test.

One of the most important parts of receiver testing is link training to ensure the receiver equaliser is tuned properly for a given channel. Gen2 adds three new sub-states in the polling state machine to allow a port to identify itself as 10 Gbit/s capable and to synchronise with another link partner. To test a receiver, you need to enter loopback mode after full link training. One of the challenging parts of link training with test equipment will be to ensure proper stimulus and response for loopback entry. The loopback process, which includes a full link handshake, not only allows for PHY synchronisation but for proper equaliser adaptation as well. In fact, the USB 3.1 specification was recently updated to double the length of the TSEQ (training sequence equaliser) training pattern to allow a receiver more time to test many coefficient settings.

In looking at the jitter tolerance parameters required for Gen1 and Gen2 in table 2, a few points stand out. The corner frequency has moved from 4.9MHz to 7.5MHz. Rj (random jitter) is much lower than Gen1. In fact, the original USB 3.1 spec specified Rj at about 1.3 ps rms but now this has been lowered to 1 ps to accommodate for higher Dj (deterministic jitter) in the channel. Finally, a calibrated receiver setup for Gen1 used 3 dB de-emphasis but with Gen2, this requires a 3-tap emphasis setup.

Table 2: Receiver test parameters for USB 3.1 Gen 2 are highlighted in red.

Just because receiver testing is a go/no go test, don't be fooled by its apparent simplicity. One of the more time consuming parts of testing is calibration, which in this case involves calibrating the generator to proper minimum swing along with the prescribed amount of jitter. Then you need to calibrate all the stress together with the long channel included. Because of the symmetrical nature of the Type-C connector, the setup is the same for host and device calibration and testing (more on this in part 2).

Calibration is much easier and more accurate with software automation. There are three parts to this setup. First is the stress generator. This is the BERT which is the source as well as the error detector for the BER test. A real-time oscilloscope is used to capture the clock and data patterns. Finally compliance software is used to verify each part of the stress recipe.

This may sound strange, but I actually recommend that you go through the manual calibration process. While performing the calibration can be painfully slow, it will help you see how important each step is in the process. It also allows you trust the automation because you'll know what to expect.

With higher speeds, a new connector and much improved power delivery capability, the excitement around USB 3.1 is already starting to grow. There is much work to be done of course to create a robust ecosystem, but USB 3.1 is already shaping up to be one of the most important versions of USB yet. In my next post I'll be taking a look at testing challenges posed by the new connector type. In the meantime, add your comments, thoughts and questions below.

About the author
Randy White contributed this article.

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