SD card

Secure Digital

Pair of SD cards
Media type	Memory card
Capacity	Standard SD: 1 MB to 4 GB SDHC: 4 GB to 32 GB SDXC: 32 GB to 2 TiB
Developed by	SD Card Association
Dimensions	32 × 24 × 2.1 mm
Usage	Portable devices, including digital cameras and handheld computers
Extended from	MultiMediaCard (MMC)

Secure Digital (SD) is a non-volatile memory card format developed by Panasonic, SanDisk, and Toshiba for use in portable devices. It is widely used in digital cameras, digital camcorders, handheld computers, netbook computers, PDAs, media players, mobile phones, GPS receivers, and video games. Standard SD cards have a maximum of 4 GB capacity, even though the official standard allows for ≤2 GB. The capacity range for standard- and high-capacity (SDHC) cards overlap, beginning at 4 GB, but SDHC's upper limit had reached 32 GB as of mid-2009. SDXC (eXtended Capacity), a specification announced at the 2009 Consumer Electronics Show, allows for up to 2 TiB cards.

The format has proven very popular. Changes to the interface of the established format have made some older devices designed for standard SD cards (≤4 GB) unable to handle newer formats such as SDHC (≥4 GB). All SD cards have the same physical shape and form factor however, which causes confusion for many consumers.

History

Inside a 64-MB Panasonic SD Card with Samsung chip. The chip on the right is a controller.

In August 1999, SanDisk, Matsushita and Toshiba first agreed to develop and market the SD (Secure Digital) Memory Card, which was a development of the MMC. With a physical profile of 24 × 32 × 2.1 mm, the new card provided both DRM up to the SDMI standard, and a high memory density for the time.

The new format was designed to compete with Sony's Memory Stick format, which was released the previous year, featured MagicGate DRM, and was physically larger. It was mistakenly predicted that DRM features would be widely used due to pressure from music and other media suppliers to prevent piracy.

The signature SD logo was actually developed for another use entirely: it was originally used for the Super Density Disc, which was the unsuccessful Toshiba entry in the DVD format war. This is why the D resembles an optical disc.

At the 2000 CES trade show Matsushita, SanDisk, and Toshiba Corporation announced the creation of the SD Card Association to promote SD cards. It is headquartered in California and its executive membership includes some 30 world-leading high-tech companies and major content companies. Early samples of the SD Card were available in the first quarter of 2000, with production quantities of 32 and 64 megabytes available 3 months later.

In April 2006, the SDA released a detailed specification for the non-security related parts of the SD Memory Card standard. The organization also released specifications for the SDIO (Secure Digital Input Output) cards and the standard SD host controller. During the same year, specifications were finalized for the small-form-factor microSD (formerly known as TransFlash) and SDHC, with capacities in excess of 2 GB and a minimum sustained read/write speed of 2.2 MB/s.

Design and implementation

An SD card, mini SD card, and micro SD card from top to bottom.

SD cards are based on the older MultiMediaCard (MMC) format, but have a number of differences:

• The SD card is asymmetrically shaped in order not to be inserted upside down, while an MMC would go in most of the way but not make contact if inverted.
• Most SD cards are physically thicker than MMCs. SD cards generally measure 32 × 24 × 2.1 mm, but as with MMCs can be as thin as 1.4 mm if they lack a write-protect switch; such cards, called Thin SD, are described in the SD specification, but they are non-existent or rare in the market as most devices requiring a thinner card use the smaller (and thinner) versions of SD: miniSD or microSD.
• The card's electrical contacts are recessed beneath the surface of the card, protecting them from contact with a user's fingers.
• SD cards typically have transfer rates in the range of 10–20 MB/s, but this number is subject to change, due to recent improvements to the MMC standard.

Devices with SD slots can use the thinner MMCs, but standard SD cards will not fit into the thinner MMC slots. miniSD cards can be used directly in SD slots with a simple passive adapter, since the cards differ in size and shape but not electrical interface. With an active electronic adapter, SD cards can be used in CompactFlash or PC card slots. Some SD cards include a USB connector for compatibility with desktop and laptop computers, and card readers allow SD cards to be accessed via connectivity ports such as USB, FireWire, and the parallel printer port. SD cards can also be accessed via a floppy disk drive with a FlashPath adapter.

Optional write-protect tab

When looking at the card from the top (see pictures) there is one required notch on the right side (the side with the diagonal notched corner).

On the left side may be a write-protection notch. If this is present, the card cannot be written. If the notch is covered by a sliding write protection tab, or absent, then the card is writeable. Because the notch is detected only by the reader, the protection can be overridden if desired (and if supported by the reader). Not all devices support write protection, which is an optional feature of the SD standard.

Some SD cards have no write-protection notch, and it is absent completely in the MicroSD and MiniSD formats.

Some music and film media companies (e.g., Disney) have released limited catalogs of records and/or videos on SD. These usually contain DRM-encoded Windows Media files, making use of the SD format's DRM capabilities. Such media are usually permanently marked read-only by adding the notch with no tabs.

File system

Like other flash card technologies, most SD cards ship preformatted with the FAT or FAT 32 file system on top of an MBR partition scheme. The ubiquity of this file system allows the card to be accessed on virtually any host device with an SD reader. Also, standard FAT maintenance utilities (e.g., SCANDISK) can be used to repair or retrieve corrupted data. However, because the card appears as a removable hard drive to the host system, the card can be reformatted to any file system supported by the operating system. Conversely, an SD card can contain an embedded operating system (such as a Live USB) to recover a corrupted host computer by natively booting from the flash media reader, like the HP Mini NetBook, or by booting using a USB adapter.

SD cards with 4 GB and smaller capacities can be used with many systems by being formatted with FAT16 (4 GB only possible by using 64 kiB clusters, and not widely supported) or FAT32 file system (common for file systems 4 GB and larger). Cards 4 GB and larger can only be formatted with a file system that can handle these larger storage sizes, such as FAT32.

SD cards are plain block devices and do not in any way imply any specific partition layout or file system thus other partition schemes than MBR partitioning and the FAT file systems can be used. Under Unix-like operating systems such as Linux or FreeBSD, SD cards can be formatted using, for example, the UFS, EXT3 or the ReiserFS file systems; under Mac OS X, SD cards can be partitioned as GUID devices and formatted with the HFS+ file system. Under MS-Windows and some unix systems, SD cards can be formatted using the NTFS and on later versions exFAT file system. However most consumer products will expect MBR partitioning and FAT16/FAT32 filesystem.

Fragmentation may slow down the effective write speed: Defragmentation tools may be used. However, it is unnecessary to use any disk optimization tool because on an SD card the time required to access any block is the same. Defragmenting an SD card will wear the card out slightly, as the number of writes, before failure occurs, is limited (usually 100,000 times).

Transfer modes

At the physical level, SD supports at least three transfer modes:

One-bit SD mode

separate command and data channels and a proprietary transfer format.

Four-bit SD mode

uses extra pins plus some reassigned pins.

SPI mode

Serial Peripheral Interface Bus, a simpler subset of the SD protocol for use with microcontrollers.

All cards must support all three modes, except for microSD where SPI is optional. The cards must also support clock frequencies of up to 25 MHz for regular cards, and 50 MHz for high-speed cards.

Royalties for SD/SDIO licenses are imposed for manufacture and sale of memory cards and host adapters (USD$1,000/year plus membership at USD$1,500/year) but SDIO cards can be made without royalties and MMC host adapters do not require a royalty. MMCs have a seven-pin interface; SD and SDIO have expanded this to nine pins and MMC Plus expands this even further with thirteen pins.

DRM features

The digital rights management scheme embedded in the SD cards is defined as the Content Protection for Recordable Media (CPRM) by the 4C Entity and is centered around use of the Cryptomeria cipher (also known as C2). The specification is kept secret and is accessible only to licensees. This DRM has not been seen "in the wild" and few, if any, devices appear to provide support for it. DVD-Audio uses a very similar scheme known as Content Protection for Prerecorded Media (CPPM).

Super*Talent, a manufacturer of computer memory, has created the Super Digital card. They are the same in appearance and function as regular Secure Digital cards, but they lack the CPRM code commonly found in Secure Digital cards.

Compared to other flash memory formats

Overall, SD is less open than CompactFlash or USB flash memory drives; these are open standards which can be implemented free of payment for licensing, royalties, or documentation. (CompactFlash and USB flash drives may, however, require licensing fees for the use of associated logos and trademarks.)

However, SD is much more open than Memory Stick, for which no public documentation nor any documented legacy implementation is available. All SD cards (other than microSD) can, at least, be accessed freely using the well-documented SPI/MMC mode.

xD cards are simply 18-pin NAND flash chips in a special package and support the standard command set for raw NAND flash access. Although the raw hardware interface to xD cards is well understood, the layout of its memory contents — necessary for interoperability with xD card readers and digital cameras — is totally undocumented. The consortium that licenses xD cards has not released any technical information to the public.

Speeds

There are different speeds of SD card available. The official unit of measurement is the Speed Class Rating; an older unit of measurement is the × rating.

Speed Class Rating

The Speed Class Rating is the official unit of speed measurement for SD Cards, defined by the SD Association. It is equal to 8 Mbit/s, and it measures the minimum write speeds based on "the best fragmented state where no memory unit is occupied":

The following are the ratings of some currently available cards:

• Class 2: 16 Mbit/s (2 MB/s)
• Class 4: 32 Mbit/s (4 MB/s)
• Class 6: 48 Mbit/s (6 MB/s)
• Class 10: 80 Mbit/s (10 MB/s)

Even though the class ratings are defined by a governing body, like × speed ratings, class speed ratings are quoted by the manufacturers but unverified by any independent evaluation process. In applications that require sustained write throughput, such as video recording, the device may not perform satisfactorily if the SD card's class rating falls below a particular speed. For example, a camcorder that is designed to record to class 6 media may suffer dropouts or corrupted video on slower media.

Important differences between the Speed Class and the traditional CD-ROM drive speed measurement ("×" speed ratings) are:

1. the ability of the host device to query the SD card for the speed class and determine the best location to store data that meets the performance required
2. class speed defines the minimum transfer speed.

Since the class rating is readable by devices, they can issue a warning to the user if the inserted card's reported rating falls below the application's minimum requirement.

On 21 May 2009, Panasonic announced new class 10 SDHC cards, claiming that this new class is "part of SD Card Specification Ver.3.0". Toshiba also announced cards based on the new 3.0 spec As of December 2009, the SD Association's Web site does not include information on this new class or new specification.

× rating

The × rating is a unit of measurement equal to 1.2 Mbit/s. It is derived from the standard CD-ROM drive speed of 150 KB/s. Basic cards transfer data up to six times (6×) the data rate of the standard CD-ROM speed (900 KB/s vs 150 KB/s). The 2.0 specification defines speeds up to 200×, but unlike the class rating system, does not mandate that x-ratings measure the card's sustained write-speed. For most cards, the maximum read speed is typically faster than its maximum write speed, leading some manufacturers to use read-speed as the ×-rating measurement. Other vendors, such as Kingston, use write-speed.

This table lists common ratings and minimum transfer rates.

Rating	Write Speed (MByte/s)	SD Class
6×	0.9
10×	1.5
13×	2.0	2
26×	4.0	4
32×	4.8	5
40×	6.0	6
66×	10.0	10
100×	15.0	15
133×	20.0	20
150×	22.5	22
200×	30.0	30
266×	40.0	40
300×	45.0	45

Openness of standards

The inside of a Samsung 512-MB SD Card. The top chip is the SD controller and the bottom one is the NAND flash chip that actually stores the data.

Like most memory card formats, SD is covered by numerous patents (e.g., US patent 5602987) and trademarks.

Three versions of the SD specification have been set: 1.0, 1.1 and 2.0. These were originally available only after agreeing to a non-disclosure agreement (NDA) that prohibited development of an open source driver, which generated consternation in the open-source and free software communities. However, the system was eventually reverse-engineered, and the non-DRMed sections of the memory cards could be accessed by free software drivers.

Since then, the SD Card Association (SDA) has made access to a simplified version of the specification available under a less restrictive license. Although most open-source drivers were written before this, it has helped them to solve some compatibility issues.

In 2006, the SD Card Association also released a simplified version of their host controller interface specification (not to be confused with the physical specification, which covers the actual cards and their protocol). Like the physical specification, most of the information had already been discovered before the public release and at least Linux had a fully free driver for it. Still, building a chip conforming to this specification caused the One Laptop Per Child project to claim "the first truly Open Source SD implementation, with no need to obtain an SDI license or sign NDAs to create SD drivers or applications."

For the most part, the lack of a complete, open SD specification mainly affects embedded systems, since desktop users generally read SD cards via USB-based card readers. These card readers present a standard USB mass storage interface to memory cards, thus separating the operating system from the details of the underlying SD interface. However, embedded systems (such as portable music players) usually access SD cards directly, and therefore complete programming information is necessary. Desktop card readers are themselves examples of such embedded systems; the manufacturers of these readers have usually paid the SDCA for complete access to the SD specifications. Many notebook computers now include SD card readers not based on USB; device drivers for these essentially access the SD card directly, as in embedded systems.

Different types of MMC/SD cards

The SD card is not the only flash memory card standard ratified by the Secure Digital Card Association (SDCA). Other SD Card Association formats include miniSD, microSD (formerly known as TransFlash before ratification by the SD Card Association), and SDHC (Secure Digital High Capacity, for capacities above 4 GB–although, there are cards some readers cannot handle over 1 GB that are not SDHC). SDHC is not fully compatible with the format that it extends, in that SD devices that do not specifically support SDHC will not work with the newer cards.

The smaller miniSD and microSD cards are usable 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 difficult in the smaller sizes.

As SD slots still support MMCs, the separately-evolved smaller MMC variants are also compatible with SD-supporting devices. Unlike miniSD and microSD (which are sufficiently different from SD to make mechanical adapters necessary), RS-MMC slots maintain backward compatibility with full-sized MMCs, because the RS-MMCs are simply shorter MMCs. More information on these variants can be found in the article about the MultiMediaCard standard.

It is also important to note, that unlike for data storage (which typically works everywhere an SD slot is present), an SDIO device must be supported and equipped with drivers and applications for the host system and usually does not work outside of the manufacturer's scope (which means, for example, that an HP SDIO camera usually does not work with PDAs for which it is not listed as an accessory). This behavior is often not expected by end users who typically expect that only the SD slot is required. Similar compatibility issues are sometimes seen with Bluetooth devices, although to a much lesser extent thanks to standardized Bluetooth profiles.

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, MMCs may be electrically identical to SD cards but in a thinner package and with an electronic fuse blown to disable SD functionality (so no SD royalties need to be paid). Some MicroSD cards do not support SPI mode.

MMC defined the SPI and one-bit MMC/SD protocols. The underlying SPI protocol has existed for years as a standard feature on many microcontrollers. From a societal 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 signaling to allow multiple cards on the same bus but this actually causes problems at higher clock rates. While SPI used three shared lines plus a separate chip select to each card, the new protocol allows up to 30 cards to be connected to the same three 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 play operation; this feature is rarely used and its use is actively discouraged in new standards (which recommend a completely separate channel to each card) because of speed and power consumption issues. The quasi-proprietary one-bit protocol was extended to support four bit wide (SD and MMC) and eight 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 133 MHz) for higher performance than offered by four-bit SD — embedded CPUs that did not already have higher clock rates available would not have been fast enough to handle the higher data rates anyway. The SD card association dropped support for some of the old one-bit MMC protocol commands and added support for additional commands related to copy protection.

Compatibility issues with 4 GB and larger cards

Devices that use SD cards identify the card by requesting a 128-bit identification string from the card. For standard-capacity SD cards, 12 of the bits are used to identify the number of memory clusters (ranging from 1 to 4,096) and 3 of the bits are used to identify the number of blocks per cluster (which decode to 4, 8, 16, 32, 64, 128, 256, or 512 blocks per cluster).

In older 1.x implementations the standard capacity block was exactly 512 bytes. This gives 4,096 × 512 × 512 = 1 gigabyte of storage memory. A later revision of the 1.x standard allowed a 4-bit field to indicate 1,024 or 2,048 bytes per block instead, yielding up to 4 gigabytes of memory storage.

Devices designed before this change may incorrectly identify such cards, usually by misidentifying a card with lower capacity than is the case by assuming 512 bytes per block rather than 1,024 or 2,048.

For the new SDHC (2.0) implementation, 22 bits of the identification string are used to indicate the memory size in increments of 512 bytes. The SDCA currently allows only 16 of the 22 bits to be used, giving a maximum size of 32 GB. All SD cards with a capacity larger than 4 GB must use the 2.0 implementation at minimum. Two bits that were previously reserved and fixed at 0 are now used for identifying the type of card, 0: standard; 1: SDHC; 2, 3: reserved. Non-SDHC devices are not programmed to read this code and therefore cannot correctly identify SDHC or SDXC cards.

All SDHC readers work with standard SD cards.

Many older devices will not accept the 2 or 4 GB size even though it is in the revised standard. The following statement is from the SD Card Association specification:

To make 2 GByte card, the Maximum Block Length (READ_BL_LEN=WRITE_BL_LEN) shall be set to 1024 bytes. However, the Block Length, set by CMD16, shall be up to 512 bytes to keep consistency with 512 bytes Maximum Block Length cards (Less than and equal 2 Gbyte cards).

— 

Standard-SD cards (non-SDHC) with greater than 1 GB capacity

The SD Card Association's current specifications define how a standard SD (non-SDHC) card with more than 1 GB and up to 4 GB capacity should be designed. These cards should be readable in any SD 1.01 devices that take the block length data into account. Any 1 GB or lesser card should always work (so the key question is how one's reader handles block length).

According to the specification, the maximum capacity of a standard SD card is defined by (BLOCKNR × BLOCK_LEN), where BLOCKNR may be (4,096 × 512) and BLOCK_LEN may be up to 2,048. This allows a capacity of 4 GB. The main problem is that some of the card readers support only a block (or, sector) size of 512 bytes, so greater than 1 GB non-SDHC cards may cause compatibility difficulties for users of such devices.

SDHC cards with greater than 32 GB capacity

Similarly to the above, as of version 2.00 of the specification, the capacity of an SDHC card is limited to 32 GB. However, while not strictly adhering to that standard, it is in principle possible to create SDHC-like cards of up to 2 TiB capacity. SDHC cards have a fixed sector size of 512 bytes.

SDHC

8 GB SDHC cards

SDHC (Secure Digital High Capacity, SD 2.0) is an extension of the SD standard which increases card's storage capacity up to 32 GB. SDHC cards share the same physical and electrical form factor as older (SD 1.x) cards, allowing SDHC-devices to support both newer SDHC cards and older SD-cards. To increase addressable storage, SDHC uses sector addressing instead of byte addressing in the previous SD standard. Byte addressing supported card capacities up to 4 GB, whereas sector addressing can theoretically support capacities up to 2 TB (2048 GB). The current standard limits the maximum capacity of an SDHC card to 32 GB (it is expected that the SDHC specification will be revised in the future to allow card capacities greater than 32 GB). SDHC cards will not work in devices designed to the older SD 1.x specification. The SDHC trademark is licensed to ensure compatibility.

SD and SDHC compatibility issues

The SDHC specification was completed in June 2006, but by that time, non-standard high-capacity (>1GB) SD cards (based on the older 1.x specification) were already on the market. The two types of storage cards were not interchangeable, creating some confusion among customers. SD and SDHC cards and devices have these compatibility issues :

• Devices that do not specifically support SDHC do not recognize SDHC memory cards. Some devices can support SDHC through a firmware upgrade.
• SDHC devices are backward compatible with SD memory cards.
• Some manufacturers have produced 4 GB SD cards that conform to neither the SD2.0/SDHC spec nor existing SD devices.
• File System: SD cards are typically formatted with the FAT16 file system, while SDHC cards are typically formatted as FAT32. However, both types of cards can support other general-purpose file systems, such as UFS2, ext2 or the proprietary exFAT for example.
• Microsoft Windows may need a hotfix to support accessing SDHC cards.

SDXC

The Secure Digital Extended Capacity (SDXC) format was unveiled at CES 2009 (January 7-10, 2009). The maximum capacity defined for SDXC cards is 2 TB (2048 GB). The older SDHC cards also have a maximum capacity of 2 TB based on the card data structures, but this is artificially limited to 32 GB by the SD 2.0 specification. The first SDXCs being released are governed by an SD 3.0 specification (which also still specifies FAT32 and thus lower capacities), whereas higher capacity and faster SDXCs are expected to follow an SD 4.0 specification, which was due to be released in spring of 2010.

The maximum transfer rate of SDXCs which follow the SD 3.0 specification was announced as 104 MB/s (these are called UHS104 speeds), with plans that the SD 4.0 specification shall increase this to 300 MB/s. The SDcard association selected Microsoft's proprietary exFAT file system in the official SDXC specification; however, as with SD and SDHC, it is still a plain block device and thus arbitrary partitioning and other file systems can be used, such as FAT32, NTFS, ext2, etc.

On January 7, 2009, SanDisk and Sony announced the joint development of the XC variant of the competing Memory Stick format, boasting the same 2 TB maximum capacity of SDXC On January 8, 2009, Panasonic announced plans for production of 64 GB SDXC cards.

On March 6, 2009, Pretec introduced the world's first SDXC card with a capacity of 32 GB and a read/write speed of 50 MB/s. At the introduction there were no products compatible with the new memory card.

On August 3, 2009, Toshiba announced the launch of the world's first 64 GB SDXC Memory Card with a read speed of 60 MByte/s. The 64GB card (THNSU064GAA2BC) is currently sampling and will be available in the spring of 2010.

On February 8, 2010, Canon announced the launch of the new EOS Rebel T2i Digital SLR camera, the first EOS model to support SDXC memory cards.

The first integrated SDXC card readers are available from JMicron and are expected to be used in laptops in 2010.

SD and SDXC compatibility issues

SD and SDXC cards and devices have these compatibility issues:

• SDHC devices will only support the SDXC cards which use UHS104 speeds; SDHC devices will not recognize the SDXC cards which use the faster (SD 4.0), final specification of SDXC.
• SDXC devices are backward compatible with SD and SDHC memory cards.
• Linux (with a proprietary driver) and Microsoft Windows 7/Vista SP1+ are the only operating systems to currently support SDXC.

Secure Digital Input Output (SDIO)

A camera that uses the SDIO interface to connect to some HP iPAQ devices.

An SDIO card is a combination of an SD card and an I/O device. This kind of combination is increasingly found in portable electronics devices.

Devices that support SDIO (typically PDAs like the Palm Treo, but occasionally laptops or mobile phones) can use small devices designed for the SD form factor, like GPS receivers, Wi-Fi or Bluetooth adapters, modems, Ethernet adapters, barcode readers, IrDA adapters, FM radio tuners, TV tuners, RFID readers, digital cameras, or other mass storage media such as hard drives.

A number of other devices have been proposed but not yet implemented, including RS-232 serial adapters, fingerprint scanners, SDIO to USB host/slave adapters (which would allow an SDIO-equipped handheld device to use USB peripherals and/or interface to PCs), magnetic strip readers, combination Bluetooth/Wi-Fi/GPS transceivers, cellular modems (PCS, CDPD, GSM, etc.), and APRS/TNC adapters.

SDIO cards are fully compatible with SD Memory Card host controller (including mechanical, electrical, power, signaling and software). When an SDIO card is inserted into a non SDIO-aware host, it will cause no physical damage or disruption to device or host controller. SPI bus topology is mandatory for SDIO, unlike SD Memory. Most of the SD Memory commands are supported in SDIO. SDIO cards can contain 8 separate logical cards, although at the moment this is at most a memory and IO function. SD slots will take SD cards only. SDIO slots will take SD cards and SDIO cards.

SD cards with extra features

Various manufacturers have tried to make their SD cards stand out from the crowd in different ways

• SD Plus - A type of SD card made by Sandisk that has an integrated USB connector so it can be plugged directly into a USB port without needing any special card reader. This concept has proven successful and other companies started introducing similar designs branded as duo SD or 3 Way in the case of A-DATA's MicroSDHC to SDHC and USB all-in-one product, which was available in 2008 only.
• Capacity Display - In 2006, A-DATA announced an SD card with its own digital display that would show how much free space is left on the card.
• Eye-Fi, Inc. - Produces an SD card with Wi-Fi capability built in for 802.11g, 802.11b, and backwards-compatible 802.11n wireless networks and supporting static WEP 40; 104; and 128, WPA-PSK, and WPA2-PSK security standards. The card works with any digital camera with an SD slot and can transmit captured images over a wireless network. When not in range of a wireless network connection, the card makes use of its 2 GB capacity (EYE-FI-2 GB model) until the images can be transferred. Some models geotag their pictures.
• Gruvi - A rare type of microSD card with extra DRM features

Market penetration

A camcorder with a 4 GB SD card

Secure Digital cards are ubiquitous in consumer electronic devices, and have become the dominant means of storing several gigabytes of data in a small size.

Devices such as netbooks, digital cameras, camcorders, PDAs, mobile phones, video game consoles and digital audio players as well as many others use them.

Smaller devices tend to use MicroSD, or MiniSD rather than full sized SD cards.

SD cards are not generally used in mass produced devices where only a small amount of storage is needed due to economic reasons, or where a very large amount of storage is required.

Digital cameras

SD/MMC cards have replaced Toshiba's SmartMedia as the dominant memory card format used in digital cameras. In 2001, SmartMedia had achieved nearly 50% use, but by 2005 SD/MMC had achieved over 40% of the digital camera market and SmartMedia's share had plummeted, with cards not being easily available in 2007.

At this time all the leading digital camera manufacturers use SD in their product lines, including Canon, Casio, Fujifilm, Kodak, Nikon, Olympus, Panasonic, Pentax, Ricoh, Samsung and Sony. Previously, Olympus and Fujifilm used xD cards exclusively, while Sony only used Memory Stick. As of January 2010, they have added SD functionality to all models released since then.

Some prosumer and professional camera models continue to offer CompactFlash, either on a second card slot or as the only storage, as it has historically offered a better price/capacity ratio and faster transfer rates.

Embedded systems

Unlike CompactFlash, none of the SD card variants supports ATA signaling, limiting their use as solid state drives unless a separate converter chip is used. Although embedded systems exist that use SD cards as their main storage mechanism, a special SD controller chip is often used. In September 2008, the SD Card Association announced the Embedded SD standard to be released in November.

A homebrew hardware hack has brought SD card support to the popular Linksys WRT54G router by utilizing spare GPIO pins on the router's processor and the Linux kernel's MMC module. Transfer speeds of 200 KB/s can be achieved with this setup.

See also

• Comparison of memory cards
• SD Card Association
• P2 card
• Memory Stick
• Serial Peripheral Interface Bus (SPI)

External links

• SD Association
• Maxim Semiconductor app note describing SD with source code
• The Joint Electron Devices Engineering Council
• SanDisk memory card device compatibility lookup
• SD/SDHC controller for mobile devices from Cypress Semiconductor Corp
• SD card controller, Open Cores
• mmca.org, MultiMedia Card Association
• cs.ucr.edu (v1.9)
• sdcard.org, SD Specifications Part 1 Physical Layer Specification, version 2.00 (pdf)
• sdcard.org, Simplified SDIO Card Specification
• k9spud.com, Interfacing dsPIC30F4013 to SD Cards
• hjreggel.net, Comparison of practically all Memory CARDS
• elm-chan.org, How to Use an MMC