Self-healing or self-defeating? That’s the question circuit board manufacturers have to ask about on-board interconnects like Intel® QuickPath Interconnect (QPI) and other such buses that tout their self-healing capabilities.
Some users may like this capability because it keeps their systems running, even if the speed of the system slows down. But board manufacturers who rely solely on functional test and/or probe-based test technologies like in-circuit test (ICT) to detect shorts and opens, and other structural faults are likely sacrificing product performance and manufacturing quality by neglecting to test with boundary-scan (JTAG).
Actually, describing QPI as self-healing is misleading. When structural faults break a lane on a QPI bus, the bus doesn’t miraculously heal that broken lane so the bus trains up properly. Instead, it simply degrades its performance of the bus by shutting down several lanes – some of which may be perfectly functional – where the fault or faults are suspected. Instead of the bus being healed, self-healing simply breaks more lanes to get around the broken lanes. And when a board with a self-healed QPI bus ends up in the hands of a user, dissatisfaction and product returns can ensue as a result of poor performance, crashes, system hangs and other problems.
Functional test can’t find the structural defects on QPI because a self-healed bus will train up at slower speeds. Probe-based test equipment like ICT can’t be used on QPI because the high-speed serial channel is so sensitive, that placing any test points or test pads on the bus would distort the signaling and create signal integrity problems. As a result, ICT and flying probe testers cannot physically access QPI, which makes it impossible to detect structural faults on the bus.
A new eBook I’ve written explains this problem and how boundary-scan or JTAG testing has become the test technology of choice to solve it. The eBook is called “JTAG Diagnostics for Intel QPI Structural Defects”. You can download it today from the eResources of the ASSET web site.
