Sun advances work on novel chip interconnect

SAN JOSE, Calif. — Sun Microsystems will demonstrate at Hot Chips Tuesday (Aug 21) it has solved one of the key problems of its Proximity technology. The novel chip interconnect someday could enable a new level of ultra-dense, high speed computer and communications systems.

Much more work remains before the computer maker is ready to talk about products based on its technology. However, Proximity holds promise for becoming an alternative to optical communications at the board level.

Sun will show at Hot Chips a four-port, 10 Gbit/second Ethernet switch based on a new package that uses Proximity to link seven chips in a multi-chip module. The system itself is not novel because chip makers build single 10 Gbit Ethernet chips today supporting up to 24 ports.

What the prototype demonstrates is that Sun has found a low-cost way to make and align the capacitive coupling pads that send signals between chips at a Gbit/second. That has been one of the big hurdles in bringing the Proximity technology to market.

"The real challenge was in the mechanics, and we now have a package for Proximity," said Hans Eberle, a distinguished engineer at Sun Labs overseeing the project.

The module puts three Gbit Ethernet ASICs face up in a row, linked by three Proximity bridge chips laid in an overlapping fashion on top of them. Each pair of ASICs sports 72 Proximity channels.

Sun is not disclosing yet the process it used to make the pads in the 180nm ASIC fabricated at TSMC or the package it uses to align them. However, Eberle said the techniques use existing high-volume manufacturing processes and could scale to chips sporting 256 channels or more.

The prototype is Sun's first system-level demonstration of Proximity and the first to use connections between more than two chips. Unlike some multi-chip modules today, Sun's package allows components to be reused if one die has a failure.

The Sun module essentially forms a single-stage, four-port crossbar switch. By scaling the number of chips and interconnect channels, Sun believes it ultimately could develop single-stage crossbar switches that support 256 or even 1,000 ports. Conventional approaches to such large scale designs require many more chips arranged in multi-stage hierarchies that are more expensive, require more power and would have half the bandwidth and three to five times the latency of a Proximity design.

Scaling up from 72 channels to 256 channels between chips is a likely next step for Sun's researchers as they push the potential of Proximity toward interesting commercial capabilities. "We think we can get to designs with hundreds of thousands of Proximity channels and multiple Terabits/second of bandwidth using this technology," Eberle said.

Keeping power in check in such high-channel designs will be part of the job for a next-generation prototype. "We haven't tried to optimize power yet," Eberle said.

"We are still a few years away from a product. There's quite a lot of work to do such as testing the technology under stress," he added.

Sun has several projects based on Proximity in the works, including ones focused on its use for linking microprocessors. The technology was initially developed by researchers including graphics guru Ivan Sutherland as part of a large supercomputer project called High Productivity Computing Systems (HPCS) run by the Defense Advanced Research Projects Agency.

In November 2006, DARPA selected proposals from Cray and IBM, rejecting Sun's bid. But Sun has maintained work on the interconnect and other parts of its HPCS project.