Secure Digital (also known as SD) is a flash memory memory card format. SD cards are based on Toshiba's older Multi Media Card (MMC) format, but add little-used DRM encryption features and allow for faster file transfers, as well as adding a bit of thickness. Devices with SD slots can use the thinner MMC cards, but SD cards won't fit into the too-small MMC slots. Standard SD cards measure 32mm by 24mm by 2.1mm.

Typically, an SD card is used as storage media for a portable device, in a form that can easily be removed for access by a PC. For example, a digital camera would use an SD card for storing image files. With an SD reader (typically a small box that connects via USB or some other serial connection), a user could copy the pictures taken with the digital camera off to his or her computer. Modern computers, both laptops and desktops, often have SD slots built in.

SD cards are currently available in sizes up to and including 2 GB, and are used in almost every context in which memory cards are used, and in nearly every application, they are the most popular format. SD support is standard in PDAs, with Dell, PalmOne, HP, Toshiba, Sharp, and others including SD slots in all of their PDAs. Digital cameras (including Kodak's cameras) tend to support SD cards, as well, although Olympus and Fuji (with xD cards) and Sony (with Memory Sticks) favor their own proprietary formats, and many professional cameras support Compact Flash in addition to or instead of SD, to allow pro photographers to use multi-gigabyte microdrives. SD cards can also be found in flash-memory-based mp3 players, GPS trancievers, the occasional cell phone, and occasional portable game systems.

SD is the currently the most popular flash memory card format, as older competitors like Multi Media Cards and SmartMedia cards are marginalized at best and obsolete at worst. Some of the long-time competitors have found other niches, however. Compact Flash support is mostly limited to older devices and professional-quality digital cameras (which mostly include CF support to allow the use of microdrives), and Memory Sticks failed to have find any success outside of Sony's dogged support in their own devices.

The "Secure" in Secure Digital comes from their origin. To create the SD card, Toshiba added encryption hardware to the already-extant MMC card, to allay music industry concerns that MMC cards would allow for easy piracy of music. (A similar scheme is the MagicGate standard used in Memory Sticks.) In theory, this encryption would allow for DRM schemes on digital music to be easily enforced, but this capability is little used.

A recent development, pioneered by SanDisk but quickly copied by other companies, are SD cards with built-in USB plugs, to eliminate the need for an SD/USB adapter or SD slot on a PC. The high cost of these special SD cards and the ubiquity of SD readers makes it unclear how successful these SD/USB cards will be.

The signature "SD" logo was actually developed for another use entirely: it was originally used for "Super-Density Optical Disk", which was the unsuccessful Toshiba entry in the DVD format wars. This is why the "D" looks so much like half of an optical disk.

Openness of standards

Like most memory card formats, SD is covered by numerous patents and trademarks, and licensing is only available through through the Secure Digital Card Association. The SD Card Association's current licensing agreement doesn't allow for open-source SD drivers, a fact that generates a fair amount of consternation in the open-source and free software communities. The usual workaround is to develop an open-source wrapper for a closed-source SD driver available on the particular platform, but this is far from ideal.

However, all SD memory and SDIO cards are required to support the older SPI/MMC mode which supports the slightly slower 4 wire serial interface (clock, serial in, serial out, chip select) that is compatible with SPI ports on many microcontrollers. Many digital cameras, MP3 players, and other portable devices probably use MMC mode exclusively. Documentation for this mode can be purchased from the MMCA for $500; however, partial documentation for SDIO is free and there is free documentation availible for memory cards as part of some manufacturers datasheets. MMC mode does not provide access (and the free documentation does not document) the rarely used (and generally undesireable from a consumer perspective) proprietary encryption features of SD cards. There are three transfer modes supported by SD: SPI mode (separate serial in and serial out), 1 bit SD mode (separate command and data channels and a proprietary transfer format), and 4 bit SD mode (uses extra pins plus some reassigned pins) to support 4 bit wide parallel transfers. Low speed cards support 0-400K bit per second data rate and SPI and 1 bit SD transfer modes. High speed cards support 0-100 megabit per second data rate in 4 bit mode and 0-25 megabit per second in SPI and 1 bit SD modes. Royalties for SD/SDIO licenses are imposed for memory cards, and host adapters ($1000 per year plus membership at $1500/year) but SDIO cards can be made without royalties and MMC host adapters do not require a royalty. MMC cards had a 7 pin interface, SD and SDIO expanded this to 9 pins.

Compared to compact flash where the entire standard can be downloaded free of charge but you apparently have to join the Compact Flash Association at $2500/yr to use the trademarks on your product, the SD standard is less open. Compared to other small form factor standards such as xD-Picture Card and Memory Stick, the SD standard is more open and the MMC subset is much more open. USB Flash memory drives and I/O devices may also be used in similar applications; the standards can be downloaded for free but there is a $1500 one time fee (plus certification costs) for use of the certified USB logos and you must pay either the logo fee or become a member ($2500/yr) in order to get a vendor ID (not needed for host adapters). Relatively few microcontrollers have USB slave support built in and none (as of 2004) had usb master support built in. Many microcontrollers have SPI support built in. Few microcontrollers have SD card host interfaces built in and those that do typically also have compact flash and memory stick interfaces and are usually suited for usb to memory card adapters and little else. No microcontrollers with MMC/SD/SDIO slave interfaces built in were located.

See [Legitimacy of Standards] for background info.

SDIO

SD slots can actually be used for more than flash memory cards. Devices that support SDIO (typically PDAs, but occasionally laptops or cell phones) can use small devices designed for the SD form factor, like GPS receivers, WiFi or Bluetooth adapters, modems, barcode readers, FM radio tuners, RFID readers, or digital cameras. Other devices that do not appear to be realized yet are RS-232 serial adapters, TV tuners, fingerprint scanners, irda adapters, SDIO to USB host/slave adapters (would allow an SDIO equiped handheld device to use USB peripherals and/or interface to PCs), magnetic stripe readers, combination bluetooth/WiFi/GPS receivers, ethernet adapters, cellular modems (PCS/CDPD/GSM/etc), APRS/TNC adapters, etc.

Comparison of different types of MMC/SD cards

Table data compiled mostly from simplified versions of MMC and SDIO specifications and other data on SD card and MMC association web sites. Data for other card variations is interpolated.

Capacity limit in all SD/MMC formats appears to be 128GB in LBA mode (28 bit sector address).

Most, possibly all, current MMC flash memory cards support SPI mode even if not officially required as failure to do so would severely affect compatibility. All cards currently made by Sandisk, Ritek/Ridata, and Kingmax digital appear to support SPI. Also, MMC cards may be electrically identical to SD cards but in a thinner package and with a fuse blown to disable SD functionality (so no SD royalties need to be paid).

MMC defined the SPI and 1 bit MMC/SD protocols. The underlying SPI protocol has existed for years as a standard feature on many microcontrollers. From a societial perspective, the justification for a new incompatible SD/MMC protocol is questionable; the development of a new incompatible and unnecessary protocol may help trade associations collect licensing and membership fees but it raises the cost of hardware and software in many ways. The new protocol used open collector signalling to allow multiple cards on the same bus but this actually causes problems at higher clock rate. While SPI used 3 shared lines plus a separate chip select to each card, the new protocol allows up to 30 cards to be connected to the same 3 wires (with no chip select) at the expense of a much more complicated card initialization and the requirement that each card have a unique serial number for plug and pray operation; this feature is rarely used and its use is actively discouraged in new standards (which recomend a completely separate channel to each card) because of speed and power consumption issues. The quasi-proprietary 1 bit protocol was extended to support 4 bit wide (SD and MMC) and 8 bit (MMC only) transfers for more speed while much of the rest of the computer industry is moving to higher speed narrower channels; standard SPI could simply have been clocked at higher data rates (such as 133Mhz) for higher performance than offered by 4 bit SD - embedded CPUs that did not already have higher clock rates availible would not have been fast enough to handle the higher data rates anyway. The SD card association dropped support for some of the old 1 bit MMC protocol commands and added support for additional commands related to copy protection.

See Also

The SD card isn't the only flash memory card standard ratified by the Secure Digital Card Association. Other SD Card Association formats include:

• miniSD
• microSD - Basically the same as SanDisk Transflash

These smaller cards are useable in full size MMC/SD/SDIO slots with an adapter (which must route the electrical connections as well as making physical contact). It should be noted, however, that it is already difficult to create I/O devices in the SD form factor and this will be even more impractical in the smaller sizes.

SD cards are derived from the original MultiMediaCards which have separately evolved a new smaller form factor. The reduced size MMC or MMCmobile cards are roughly half the length of a standard MMC/SD card and come with a clip on passive plastic extender for use in full length slots. Full size cards can also be used in reduced size slots but they will protrude from the device. Contrast this with miniSD and microSD that require more expensive adapters with electrical connections to put a miniSD/microSD card in a full size SD slot and an adapter to use a full size card in a miniature slot would be mechanically impractical. It should also be noted that MMC is separately evolving security features that will not require royalties for host implementation. MMC has also evolved faster versions (MMCplus) that support transfer widths of 1, 4, or 8 bits (13 pins), at up to 416 megabits per second. Note that SD cards are slightly thicker than MMC cards so SD card sockets can accomodate MMC cards but MMC sockets can't accomodate SD cards.

External links

• Secure Digital Association
• MultiMedia Card Association