Electronic Design

The Eyes Have It In Serial-Link Testing

Many engineers are getting onboard with third-generation serial-link technology in their next system designs, and for good reason. These intra-system communication schemes move a lot of data in a hurry. For example, the Serial Advanced Technology Attachment (SATA) 3.0 standard jumps to 6 Gbits/s, a transfer rate made necessary by the proliferation of flash-memory drives and the lightning-fast read speeds they deliver.

But adoption of any new standard-based protocol for in-system busing is going to mean rethinking your compliance-test strategy. I recently had a chance to catch up on third-generation serial-link test with Chris Loberg, a senior technical marketing manager at Tektronix, who highlighted some of the issues facing test engineers when it comes to gaining standard certification for systems incorporating these emerging standards.

The classic tool for evaluating the quality of serial-link functionality is to observe the eye pattern on a scope display. “The biggest difference in third-generation serial-link technologies as compared to the previous generation is the stress that is being put on the eye,” says Loberg. This stress stems from two primary factors. For one, the complexity of the signaling, with its long strings of zeroes or ones, is more apt to cause baseline wander. For the other, clock rates continue to rise, which translates into tighter margins. As a result, every sequence generates intersymbol interference, periodic and random jitter/noise, and crosstalk.

Loberg’s experience with qualification and design labs working with these latest serial-link standards shows that test engineers are looking for more insight into receiver testing. Traditionally, receiver testing has centered on bit-error-rate testers (BERTs), which would simply provide a ratio of errors to transmitted bits. That was generally good enough a few years ago but not any longer.

“Now, with designers trying to be more competitive with their receiver designs, test has to be a lot more flexible,” says Loberg. This means going beyond the basic drill of running the BERT overnight and getting a number. “People are testing a variety of parameters in the lab, using different taps, FFT (fast Fourier transform) filters, and other variable elements, and wanting to evaluate the output with a more insightful tool than a BERT.”

Thus, debugging a third-generation serial communication link requires equipment that can go further than a simple oscilloscope or BERT in terms of insight into what’s really happening with the signal at the receive end of the link. There’s also a need for an arbitrary waveform generator that can provide nuances such as noise.

The right set of tools can make the difference between getting a simple pass/fail answer and getting significant information that can help you improve that eye diagram, opening up the eye so there’s sufficient margin to overcome extraneous interference and noise.

Stay tuned for more on third-generation serial-link testing in the form of a Technology Report in Electronic Design’s Sept. 9 issue. In the meantime, let’s hear from readers: What kinds of challenges are you facing in debugging third-generation serial links? Are the tools you’re using adequate for the task, or is there something missing? Are test and measurement vendors like Tektronix providing the right tools for the job? Leave a comment below, and let’s get the dialog going.


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