PCI Express: The Undisputed King

The roadmap is clear, even though travel won't start for a while. PCI Express Base hardware will emerge near the end of the year, with switch-fabric shipments not expected until 2004. The Advanced Switching Standard is being incorporated into a number of PICMG (www.picmg.com) specifications, including the Advanced TCA (Telecom Com-puting Architecture) standard and the forthcoming Compact TCA standard. It has taken some time to get these standards out the door because designers wanted a lasting solution. PCI is 10 years old and still going.

PCI Express is a serial, point-to-point interface designed to replace PCI and PCI-X as the workhorse peripheral interconnect. However, the process will take years. PCI Express interfaces are compatible with PCI interfaces, allowing PCI device drivers to support PCI Express systems without modification, although a new device driver will be required to exploit any new features not found in PCI. The basic interface runs at 2.5 GHz in each direction of a full duplex interface.

The serial approach has a number of advantages. Among them are a low pin count and scalability. Adding more serial links makes higher-speed connections possible. Virtual channels allow a link to support more than one logical connection.

PCI EXPRESS BASE is the starting point. It mimics the existing PCI hierarchical bridge architecture with the host at the root. It handles anything that PCI or PCI-X systems can do now, but at a higher speed. PCI Express also offers features like hot swapping, power management, and quality-of-service support.

PCI Express Base maintains PCI's load/store interface. But it implements transactions using packets up to 128 bytes in length. The standard specifies a number of different form factors and connectors, enabling PCI Express to support a wide range of market segments. For example, a mini card and NewCard (replaces PC Card) target the laptop market.

PCI Express Base will coexist with PCI for quite some time. In fact, PCI Express/PCI bridges will be some of the first chips to be shipped. Even a system that only employs bridge chips will be more flexible than a regular PCI system. PCI Express can support more devices, and the devices can be placed farther apart than with a standard PCI system.

PCI Express support will show up in memory bridge and I/O bridge chips. The former can provide a high-speed link to graphic display adapters. Eventually, PCI Express support will be built into some processors, especially embedded microcontrollers replacing PCI interfaces.

PCI Express Advanced Switching pushes the PCI interface into the switch-fabric realm. It will compete with InfiniBand, StarFabric, and Ethernet, although all of these switch fabrics are likely to coexist. Systems that employ more than one of these switch fabrics will not be uncommon.

It's early for most designers to be using PCI Express in designs, but it's definitely time to learn more about the interface. Very soon, PCI Express will be on motherboards.