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PCI Express train rolls across multiple tracks

Posted: 08 Jun 2005 ?? ?Print Version ?Bookmark and Share

Keywords:pci express? silicon? server subsystems?

At its annual conference here (June 6-7), the PCI Special Interest Group detailed updates and extensions to the PCI Express specification that could drive annual silicon updates through 2007.

The group also discussed plans for using Express over cables and in specialty modules for RF and server subsystems.

The SIG outlined six new directions for Express, taking the serial technology into areas such as security and virtualization. While Express has gained huge mind share with designers, its roll out has been somewhat slower than originally anticipated, said Al Yanes, president of the PCI SIG and a senior ASIC designer at IBM Corp.

"I would have expected to see more Express parts out there by now," said Yanes, who was an early proponent of the PCI-X technology that was once a competitor to Express. He attributed the somewhat sluggish rollout to the tech industry downturn, a lack of compelling applications for the 2.5-GHz data rate of Express and the normal design challenges of leaping from MHz- to GHz-class designs.

The PCI SIG officially announced work on a Trusted Configuration Space addition to the existing Express 1.0a specification. The addition will extend to suitably modified Express I/O devices the root of trust security defined by the Trusted Computing Group.

The spec requires Express root, switch or end-device chips implementing the security scheme to support new trusted read and write commands and watch for a new security flag that triggers access to a secure execution space in memory. The spec is now in review and due to be complete later this summer as part of the Express version 1.1

"This does require hardware changes" for vendors who want to support the optional security features, said Tony Pierce, chairman of the PCI SIG.

An even bigger shift is in the works for the first half of next year when the SIG is expected to complete its version 2.0 spec which boosts data rates from 2.5 to 5 GHz. For backwards compatibility, version 2.0 lets devices automatically negotiate to either version 1.0 or 2.0 data rates. It also includes expanded error reporting and an ability to make adjustments for faults in real time.

The SIG also announced plans to extend the Express spec for virtualized I/O. The extension will allow multiple operating systems to access the same physical I/O resources either simultaneously or in serial fashion. The spec will define supersets for accessing I/O in a single or in a multihost environment.

Software virtualization is seen as a key technique for making best use of the multicore, multithreaded processors beginning to proliferate in the PC market. Advanced Micro Devices and Intel are rolling out separate techniques for virtualizing their multicore processors. With the new spec, the SIG will extend those capabilities to Express-based I/O devices.

Designers think virtualization ultimately will be applied to all PC systems - even multitasking home computers. But its first target is server blades that are evolving towards stateless collections of compute boards in a single chassis linked on an Express mezzanine bus. The virtual I/O spec will allow those compute cards to share Express, Ethernet and storage I/O resources in and outside their chassis.

The spec is still in an early stage, with the 19-company working group about to put a requirements document out for review. A completed spec is not expected until late in 2006 or early in 2007. It will also require hardware changes for chip makers who want to support its features.

New form factors

While the security, virtualization and version 2.0 specs drive hardware changes for Express devices, three other efforts will bring existing Express connections to new applications.

One of the new applications is for a version of Express that runs over an external copper cable. The spec is seen as useful for any system that wants expanded I/O in a separate chassis. That includes some PC designs that might put a monitor, graphics and I/O in a desktop system and link to a desk-side system housing the CPU and hard disk. It could also be used by a server connecting to an optional I/O subsystem. Next-generation PCMCIA cards supporting Express slots may also want to use the cable to support links to other external devices.

The SIG has yet to define the type of cable it will employ and how long the cable will reach. Because the cable will support both today's 2.5 GHz and next year's 5 GHz Express versions, distance will in part depend on jitter and EMI issues still being characterized for the high-speed version.

"I think the cable will wind up being in single-digit meters, but as always someone could come along and build a repeater," said Pierce.

He emphasized that the cabled Express link is focused on use by OEMs connecting various subsystems. It is not seen as an end user interconnect that might compete with USB or 1394 interconnects.

Separately, the SIG is defining a wireless module for notebook computers. Designed to slide behind an LCD in a notebook computer's lid, the module takes up 2.5 times more area but only half the thickness of the mini-Express cards defined for use internally in PCs.

The module will house PHY and MAC silicon and be an attach point for multiple antennae for Wi-Fi and any other radios supported by the notebooks. The standard module is expected to ease problems notebook designers have had routing antennas from chips in a motherboard through a hinge and past the display with the interference from its video signals.

The wireless module standard is expected to be in a near-complete 0.9 version by this fall, paving the way for modules that could appear in 2006-class notebooks.

Finally, the SIG showed prototypes of Express Modules designed for rack-mounted servers. The modules are essentially card cages for either one or multiple half-sized Express adapter cards designed to be easily snapped into or out of server chassis without needing to access the internals of the server.

Marvell, Mellanox and QLogic showed prototype modules, respectively, for Ethernet, Infiniband and Fibre Channel adapters supporting four to eight Express channels.

- Rick Merritt

EE Times

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