By Tom Anderson
Secure digital memory cards provide next-generation removable memory for portable equipment.
As the Internet continues to inspire the development of products and technology, challenges in the creation and distribution of digital data across a broad range of emerging electronic products persist. The drive toward meeting these market requirements led to the formation of the Secure Digital Association (SDA) in 1999. Currently, the SDA has more than 100 member companies, including a 19-member board of directors. Initially formed as a consortium of Panasonic, SanDisk Corp. and Toshiba Corp., the group has quickly emerged as the next industry-standard format for removable flash storage media. An important result of SDA efforts is the Secure Digital (SD) Memory Card Specification. The SD memory card, combining card and connector, addresses three of the more compelling issues facing the electronics industry: portability, security and compatibility.
Other Memory Card Types
Portability was initially addressed with removable, solid-state memory cards. These were rapidly deployed in the computer market during the early 1990s with the development of the Personal Computer Memory Card International Association's personal computer memory cards (PC cards). Input/output options and multifunction cards emerged to provide communication capabilities in a portable environment. Connectors played a key role in providing the interchangeability and high durability (up to 10,000 mating cycles) to support multi-usage requirements. The same market and technology evolution led to the release of a second generation of removable storage memory: CompactFlash memory cards (CF cards). These cards offer the same product features as PC cards but in a considerably smaller package.
Recently, multimedia memory cards (MMCs) have been developed to provide a truly miniaturized form factor. Unfortunately, MMCs were designed to fit only the first generation of simple, small portable electronic devices such as digital voice recorders, electronic books and personal digital assistants (PDAs), and thus, have smaller memory capacity and fewer features than CF cards.
They do, however, offer significant advantages over the alternative high-volume, renewable storage medium: compact discs (CDs). Although CDs offer larger memory capacity, their abundance of moving parts renders them vulnerable to shock and vibration during use, they consume considerably more energy, and they are somewhat unreliable.
Because solid-state memory cards do not share these problems, they are key in the drive to further equipment miniaturization. For example, a device using a solid-state memory card could conceivably be powered by a single AAA battery. Major trends in the developing market have demanded increased functionality and further miniaturization, which means a reduction in the size of the CF card and an increase in the memory capacity and security of the MMC.
SD Memory Cards
SD memory cards containing nonvolatile, high-data capacity memory chips (flash erasable programmable read-only memory [EPROM]) and security features that handle and protect digital files containing audio, video and still photography have emerged as a solution to these requirements. This memory system, an enhanced version of the MMC system, provides increased memory capacity and encryption technology (a key security feature vital in addressing markets requiring copyright protection) in a small, lightweight package (see Figure 1).
 Figure 1. Comparison of the four removable memory card sizes. |
The advantages of SD memory cards go well beyond security, storage capacity and compact dimensions (they are approximately the size of a postage stamp). With the ability to support audio, video, voice and still picture files, portable Internet appliances ("Post-PC" devices) can now be developed that allow end users to easily transport desktop files.
In addition, SD memory cards will enhance the emerging third-generation cellular phones (IMT-2000) and wireless application protocols being developed for improved communication and data rates. The various connector types and configurations incorporate two internal contacts, which, in conjunction with the write protect switch on the SD memory card, protect existing data from being accidentally written over. This is especially important for portable applications.
SD Memory Card Connectors
An SD memory card connector family based on fine-pitch connector technology is available. These connectors provide integral mechanical card-detection contacts for positive notification to the host at card insertion. When the SD card is inserted into the connector, the two metal contacts are brought together to complete an electrical circuit, telling the host device that the user has plugged a card into the SD slot.
Also, surface-mount technology (SMT) retention clips provide mechanical stability and positive grounding during card insertion/withdrawal. Early-mate grounding contacts are available to satisfy future electromagnetic interference/electrostatic discharge (EMI/ESD) requirements. Optional inboard and outboard SMT tail configurations are available, depending on height profiles. If the application has a critical height requirement, a low-profile connector that has outboard tails would be the best choice. If PCB real estate is more critical than height, a taller connector with inboard tails would be better. Various ejector and non-ejector options are in development. Depending on the requirements, the designer can choose how the card is removed from the device, either spring eject, push-button eject or non-eject/pulled.
Conclusion
In addition to the SD memory cards, several new enhancements are being discussed in SDA technical sessions that will promote new products and end user capabilities. Of significant importance is the SD Input/Output (SDIO) working group. This committee brings together companies representing card, host, component and software disciplines to add both wired (tethered) and wireless (Bluetooth) connectivity through the SD slot.
Increased integration of equipment is likely. Cellular phones already include headset jacks and circuitry to handle digitally stored music. The incorporation of SD memory card sockets into handsets would provide the opportunity to potentially replace hand-held CD players.
The use of SD slots will only continue to grow. Adapter cards and readers will be designed to guarantee immediate interface with existing desktop and portable electronic devices. From backward compatibility with existing MMCs to forward-looking wireless capabilities, SD technology will provide consumers with the ability to upgrade existing products and maximize the potential of future products.
TOM ANDERSON is product manager, AVX Corp., 801 17th Ave. South, P.O. Box 867, Myrtle Beach, SC 29578-0867; (843) 448-9411; Fax: (843) 448-2606; Web site: www.avxcorp.com.
SPEC SHEET
End Applications: Portable electronic equipment such as notebook computers, Internet music players, PDAs, cellular phones, GPSs, digital cameras and electronic books
Related Products: Secure digital memory cards, secure digital memory card connectors, CF cards, PC cards, MMCs, CDs
Main Point: Many challenges arise in the creation and distribution of digital data across the range of emerging electronic products. Secure digital memory cards address three of the issues: security, portability and compatibility. The advantages of these cards include the ability to support audio, video, voice and still picture files.
