PCI Express may alter PC interconnect landscape

As engineers start preparations for the arrival of PCI Express (formerly 3GIO), predictions vary on the extent of change that the fast serial interconnect will bring to the architectural landscape of the PC.

While most see PCI Express as a straight PCI replacement, they also voice concerns about a fragmenting market and additional design costs. But a few say it may enable a repartitioning of the PC, possibly creating silicon opportunities?albeit largely for existing players.

PCI Express is expected to hit the PC market as a 2.5Gbps interface that will initially be used on chipsets, graphics processors and Gigabit Ethernet controllers. Samples of chips that use the new link are expected to appear late next year.

The initial expectations of many systems and chip makers are that the link will add throughput speed and development cost to their products, but some are beginning to dig deeper.

"If the industry can standardize on one high-speed interconnect, there are lots of interesting things you can do in repartitioning the PC," said Tony Tamasi, general manager of desktop graphics processors at Nvidia Corp.

Tamasi said PCI Express could enable a significant upgrade for his company's south bridge chip, which is part of the nForce chip set with integrated graphics capabilities now using the 800MBps HyperTransport link defined by Advanced Micro Devices Inc. The link could provide the plumbing for NVIDIA to integrate Gigabit Ethernet, USB 2.0 and 1394 interconnects into a future version of that chip, which the company calls its media and communications controller.

Others envision an even more radical move, in which the traditional PC north and south bridge chips are eliminated to create a system that has the PCI Express switch at its center. "A pure-switched architecture has a lot of advantages. There's a lot of architectural discussions about that now," said a systems designer who asked not to be named.

"I believe this technology allows a variety of partitioning options. Different companies and market segments will adopt different partitionings," said Ajay Bhatt, who directs the research lab at Intel Corp. Bhatt said, however, that he thinks PCI Express switches will see limited use in workstations and servers, and that they are not likely to find a spot in mainstream PCs.

Wherever the PCI Express switches are placed, they should have peer-to-peer communications capabilities, said Roger Tipley, chairman of the PCI Special Interest Group. That could enable devices to go around the host so that, for instance, video could be streamed directly off a disk to a display.

A spokesman for Intel Capital said that venture group has seen startups that plan to use PCI Express but no companies are basing their business around the interconnect. The group welcomes such pitches, but expects most PCI Express products will come from existing companies with PCI parts, he said.

Evolutionary rollout

PCI Express will be deployed in an evolutionary way in first-generation systems in 2004, linking graphics and Gigabit Ethernet to new chip sets, said Bob Gregory, Intel's top spokesman for the interconnect.

"You will see an evolution from first-generation platforms that add the capability to second-generation [ones] that optimize around it to third-generation systems that fully embrace it. The technology will support different ways to repartition the platform. Everyone in the industry is starting to work through how you might partition a second-generation platform," Gregory said.

Intel is expected to use PCI Express to link its traditional north and south bridge chips in its next-generation chip sets. Bhatt's group, which designed PCI Express, also designed the proprietary Hub Link Intel uses to connect those bridges today. (North bridge chips contain memory controllers and link to a processor; south bridge parts typically aggregate connections to I/O devices.)

Some said PCI Express could be used to move graphics and frame buffers off the PC motherboard and into notebook docking stations or displays. But Nvidia's Tamasi expressed skepticism about such a move.

"It might happen. I'd love to sell chips in more places, but it would be a pretty fundamental shift in how the PC is put together," Tamasi said.

Dean McCarron, analyst with Mercury Research, said he doubts PCI Express will spark major architectural changes in the PC. That's because the existing bridge system is already cost-effective and flexible enough for designers. "I don't think you will see anything remarkably different, because what drives the PC architecture is not new technology, but cost," he said.

Indeed, some see the advent of new interconnects like PCI Express and HyperTransport from AMD as necessary but painful transitions that require additional engineering resources without a guarantee of additional revenue.

"The market is fragmenting and that makes it much more difficult," said Fred Leung, associate vice president for sales and marketing at chip set maker Acer Laboratories Inc. "PCI Express to me is just like the migration from VL bus to PCI. It's something we have to do. It doesn't change our mode of operation at all."

Acer Labs has kept a lid on its costs by moving more of its design to China, where it now has 180 engineers. Leung hopes Acer Labs can craft one chip set that supports PCI Express and HyperTransport to further reduce design costs.

The additional design costs for PCI Express "will be painful," said Nvidia's Tamasi, in part because the link is incompatible with the Accelerated Graphics Port used to link graphics to Intel-based chip sets. Nvidia expects to support both links on products for an uncertain time, doubling the circuit and physical design, validation and qualification times for the I/O portions of its products.

Designers reticent

While graphics will still embrace PCI Express to get much-desired bandwidth boost, server designers are more reticent. The server group at Compaq?now part of Hewlett-Packard?has stated it plans to eschew PCI Express for PCI-X on all its servers.

"We don't see PCI Express getting into servers," said Raju Vegesna, founder of ServerWorks Inc., a server chip set supplier that counts the former Compaq group as one of its biggest customers. "I'm sure some people will have it, but no one has convinced me of the benefits of PCI Express for servers."

Intel's Bhatt countered that PCI Express brings isochronous capabilities, error-logging and -reporting and an ability to stretch traces 20 inches?about twice the length of PCI lines. "In terms of board routing and layout, this is much more friendly," he said.

While the two sides battle it out, the Arapaho Working Group that spearheaded the definition of PCI Express is quietly setting up a committee to define interoperability between PCI-X and PCI Express.

? Rick Merritt

EE Times