Figure 1: To accommodate all 16-bit PC cards, Cardbus sockets must include an additional row of contacts to boost the interface from 16 to 32 bits and enable higher speed operation.

The connector choices include existing PCMCIA and Cardbus connectors, as well as new CompactFlash (CF) and Miniature Card connectors.

These formats suit personal digital assistants (PDAs), digital cameras, digital audio recorders, hand-held data- collection scanners, advanced cellular phones, set-top boxes (for transporting data, audio, video or images, including software and games), and any other products that need to permit quick memory changes. In many cases, users want to transfer the contents of the memory modules to desktop computers. These applications will require connectors for both the portable product and a 11 reader" on the PC. For other applications that do not need frequent memory changes, like laptop computers, designers can take advantage of SO DIMM connectors.

Catching Up with the PC and Cardbus Cards
Widely adopted and well understood, the PC card has established itself as the basic laptop medium for memory, 1/0 cards (e.g., for CD-ROM drives), modems, modular network interfaces, security monitors and add-on disk drives.

PC cards were originally intended to enable the addition of memory and mass storage to portable products. The interface has been used to accommodate many different types of memory cards, including DRAM, SRAM, one-time programmable and mask ROM, flash, EEPROM and EPROM. However, PC cards are less important for memory applications than they once were due to cheaper memory ICs, the high value of each PC card slot for other uses, and better interface alternatives. The low price of DRAM has made it easy to include more memory inside a laptop from the vendor, making it less necessary to add DRAM via a PC card. Sockets such as the SO DIMM also make it easier to upgrade memory inside the box without taking up precious space. Further, users are reluctant to pull out a modem or Ethernet PC card to add memory. Specialized memory, such as flash modules, is an exception.

The PC card provides a 16-bit interface that operates at speeds as high as 25 MHz. The Cardbus card offers an upgrade to 32 bits of address and data, and 33 MHz operation. A Cardbus card can also operate as a bus master, which offloads the host processor and can therefore improve overall throughput. Another difference between the PC and Cardbus cards is that the latter only runs at 3.3 V. Thus, all Cardbus cards use the PC card low voltage key.

Except for the ability to support 5 V operation, Cardbus sockets must accommodate all 16-bit PC cards. Physically, this requirement means that connectors must include an additional row of contacts to boost the interface from 16 to 32 bits (Figure 1). Additional ground connections enablehigher speed operation.

Figure 2. Applications using the 50-Pin CompactFlash header and socket (left) can be designed in a smaller form factor than those using PCMCIA or Cardbus sockets.

Because CF logical interface is based on the longstanding ATA standard, compatibility is not a problem for a wide range of devices. In fact, CF cards include an intelligent microcontroller with an ATA/IDE interface. Other types of flash modules provide the memory device only, forcing the host product to manage the interface. This difference can be a big advantage for CF because products that use other types of cards will need updated software drivets to implement enhancements - a difficult task for a product such as a camera. As a result, CF has gained important wins in the digital camera market. Additionally, manufacturers are working on a new generation of cellular phones that receive and store faxes and voice messages on the cards. These same capabilities will also be available in advanced pagers, which together with cell phones will create a huge market for CF connectors.

Measuring 43 x 36 x 3.3 mm, and weighing about half an ounce, a CF card is less than half the size of a Type II PC card and one-fourth the volume. The cards have only 50 pins, compared to 68 for standard PC cards. Despite the differences in size and pin count, CF cards can plug into existing PCMCIA ports by using a passive 68-pin Type II adapter card. These adapters meet all PCMCIA electrical and mechanical interface specifications. Because CF works with both 5 and 3.3 V power, it should work with the newer 3.3 V only Cardbus interface.

Applications designed expressly for CF can be designed in a smaller form factor than those using PCMCIA or Cardbus sockets by using the 50-pin CF header and socket (Figure 2). Similar to standard PCMCIA sockets, CF sockets are available with or without ejectors, and the headers come with either single-row or straddle-pin alignments. Because of the similarity to PCMCIA, one company*, whose PCMCIA connector became the adopted standard by JEIDA, was able to develop reliable connector designs for CE.

Figure 3. Miniature Card connectors with spring metal contacts ensure that they have the long-term reliability for use in consumer products.

The card design has been patented by Duel Systems, now a division of Methode Electronics. Other vendors, including Intel, AMD, Sharp and Fujitsu, license the use of the design. The format is controlled by the Miniature Card Implementers Forum.

At a size of 38 x 33 x 3.5 mm, the Miniature Card is slightly smaller than a CF card and much smaller than a PC card. A Miniature Card is about one-fourth the size of a PC card, yet utilizes a 60-contact interface, which is close to the 68 pins used by PC cards.

Another difference between the Miniature Card and CF/PC cards ties in the way the cards are inserted into the sockets. With both the CF and PC cards, users slide the cards into the sockets until the pins are seated. To insert a Miniature Card, users tilt out the socket's hatch, drop in the card, and close the hatch. When the hatch pivots down, connection is made between the two rows of contacts on the socket and the bottom edge of the card.

Miniature Card sockets originally relied on elastomeric connectors. While these connectors are reliable for a limited number of insertions and removals, consumer products need high cycle life for long-term reliability. Miniature Card connectors with spring metal contacts were developed to meet this need (Figure 3). The closing action of the socket's hatch wipes the card's contacts against those of the socket for a reliable electrical connection. This design exhibits a reliability in office environments of 10,000 cycles, which is enough for years of service.

As with CF, a Miniature Card can plug into Type II PCMCIA sockets by using an adapter. Because the two logical interfaces are similar, the adapter mostly has to route signals to their appropriate pins. However, the reset signal must be inverted.

In addition to flash memory, the Miniature Card can accommodate DRAM, SRAM and ROM, while a CF card is limited to flash memory. Miniature Card storage capacities currently vary from 2 to 8 MB. This memory is a linear array that can accommodate whatever file structure designers want to use for a product, and software can execute directly from the card. Miniature Card proponents see this versatility as an advantage over CF's ATA interface.

The CF card has about a two-year head start on the Miniature Card, so the latter cannot claim the same number of impressive design wins. However, the Miniature Card can claim endorsements from 38 leading consumer product manufacturers. It has also been specified by the International Voice Association as a medium to ensure that digital audio recording devices are fully compatible with one another and can utilize the same transportable medium for all products.

Conclusion
Whether the CF or Miniature Card is the best choice depends on the nature of the application. Many observers expect that both formats will succeed for different types of products. Meanwhile, the Cardbus card will carry forward the legacy of the PC card. With reliable connectors available for each of these formats, designers can base their choices on the size, cost and functional characteristics.

* Fujitsu Components America Inc.

Bob Thornton is product marketing manager for connectors, Fujitsu Components America Inc., 250 E. Caribbean Dr., Sunnyvale, Calif. 94089; (800) 380-0059.

Reprinted with permission of Connector Specifier Magazine. Copyright 1997 by IHS Publishing Group. All rights reserved.