Electronic Design

ExpressCard: Changing The Face Of Computing

PCI Express and USB 2.0 bring speed and simplicity to a new, removable device standard.

It's small. It's removable. It uses standard interfaces. And, it's going to change the way people work with computers. It's ExpressCard, the latest standard from the Personal Computer Memory Card International Association (PCMCIA).

ExpressCard is designed to replace the aging PC Card standard and its expensive interface. Instead, ExpressCards can use a half-duplex, 480-Mbit/s Hi-Speed USB 2.0 interface or the faster full-duplex, single-lane, 2.5-Gbit/s PCI Express interface. An optional, half-duplex 100-kbit/s SMBus for system management may be useful for some devices.

A compliant ExpressCard host must support the USB and PCI Express interfaces. An ExpressCard can support one or more interfaces, but typically only one interface will be used. An exception might be a port expander that brings the PCI Express and USB interfaces outside the PC or embedded device. It's possible for a host to support only one of the interfaces, but it would limit the number of supported ExpressCards based on the ExpressCard requirements. Still, this has obvious advantages for lower-end microcontroller-based applications where the MCU has a USB host interface but not a PCI Express interface.

ExpressCard provides more functionality than PC Card. Yet the use of serial buses means a single, inline 26-pin connector handles the interface. USB needs only two pins, while PCI Express uses four. There are two clock pins. Most of the remaining pins are used for power, ground, and specialized functions such as sleep support, card detect, and reset. The 26-pin connector also makes it easier to route the signals for this smaller connector while still providing higher throughput. PCI Express and USB already have hot-swap capabilities, so ExpressCard gets this feature for free.

ExpressCards come in two form factors. The ExpressCard/34 is 34 mm wide. The larger ExpressCard/54 is 54 mm wide. It suits compact, rotating magnetic media devices currently implemented in the PCMCIA form factor. An ExpressCard/54 socket will accept either card. The recommended configuration for laptops is one of each, but actual implementations can have any combination. In fact, it's possible to build an entire PC system using multiple ExpressCards.

The ExpressCard standard is significant because it uses interfaces that are or will be standard on every PC and many embedded systems. The interface support, then, will already be part of a system design. Supporting an ExpressCard is simply a matter of adding a connector—a big change from PCMCIA cards.

Software is the other plus. USB and PCI Express drivers for a device will be the same for devices using existing USB or PCI Express connections.

THE MOST IMPORTANT INNOVATION
ExpressCard may seem like an incremental improvement, but it could be the most important innovation of the year. PC Card interfaces have primarily been limited to laptops. CompactFlash has the edge on handhelds, and most PCs still have floppy drives. ExpressCard will be the one standard that ties everything together. As a removable storage interface, flash-based ExpressCard storage devices will quickly replace current USB flash devices.

The one missing link is an even more compact version of ExpressCard/34 for handhelds like cameras, phones, and PDAs. The approach offers significant advantages over all current alternatives and would eliminate the need for the plethora of memory card devices and incompatible CompactFlash devices. A single standard would do wonders for the consumer market.

Personal Computer Memory Card International Association (PCMCIA)
www.expresscard.org

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