Electronic Design

PCI: Taking The Express Or Local

PCI Express is the latest technology to capture the hearts and minds of designers, but there are advantages and perils to any choice. "PCI Express Design: A Lesson In Techno-Shock" (ED Online 10174) addresses some of the issues that board designers will have to tackle with PCI Express. It's definitely more complex than regular PCI or even PCI-X. But for many applications, it's worth the trouble. Luckily, PCI Express bridge and switch chips are out in force, and plenty of places offer help with high-speed serial designs.

There's even real activity in Advanced Switching (AS). Stargen just released its Merlin AXSys Advanced Switching Interconnect (ASI) switch chip (see "Hark The Arrival Of Advanced Switching," p. 33, ED Online 10434). AS uses the same hardware connections as PCI Express but different high-level protocols. The AXSys switch can automatically recognize and handle PCI Express connections, providing a level of backward compatibility for two very new technologies.

Right now, the best place to get familiar with PCI Express is by using existing motherboards and adapters. The PCI Express standard set is very extensive, covering everything from wire-level interconnects to cabling. Standard PCI Express connectors and board connections are out there in products like American Eltec's PC_EYE/ASYNC image capture board.

Even so, the PC motherboards where these connectors are most often found continue to decline in number, due to fewer plug-in items. Motherboards now come standard with video and network interfaces, so the PCI Express connections are hidden on the motherboard. High-end graphics, especially for gaming, are being served with x16 lane PCI Express connectors. But the x1 lane connectors easily handle most interfaces for devices. The x4 connectors on the motherboards can handle these x1 boards as well as the (almost non-existent) x2 boards.

So what's a designer to do in the embedded space? The best option right now is modules. COM Express (see "COM Express: A New Standard," ED Online 8780) incorporates both a multilane PCI Express link for video and multiple x1 lanes for off-module peripherals. This reduces the high-speed serial interface challenge to an exercise of connecting the module to a PCI Express peripheral on a motherboard/carrier board. Because much of the design complexity resides on the module end that contains the processor, developers can get to market quicker. There's also the added benefit of choosing from a number of different COM Express suppliers. Of course, it may pay to take the local and stick with PCI.

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