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Secure Digital card

Secure Digital (SD) is a non-volatile memory card format developed by Matsushita, SanDisk, and Toshiba for use in portable devices. Today it is widely used in digital cameras, handheld computers, PDAs, Media Players, mobile phones, GPS receivers, and video game consoles. Standard SD card capacities range from 4 MB to 4 GB, and for high capacity SDHC cards from 4 GB to 32 GB as of 2008. The SDXC (eXtended Capacity), a new specification announced at the 2009 CES, will allow for 2 TB capacity cards.

The format has proven to be very popular. A change in the format, however, while allowing capacities greater than 4 GB (SDHC), has created compatibility issues with older devices that cannot read the new format. The fact that SDHC format cards have the same physical shape and form factor as the older format has caused considerable confusion for consumers.[1][2] SDHC cards require SDHC-capable device firmware generally not found with older devices.


In August 1999, Panasonic, SanDisk, 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 mm × 32 mm × 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 [2] would be widely used due to pressure from music and other media suppliers to prevent piracy.

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.

Different SD Card sizes


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

Design and implementation

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 mm × 24 mm × 2.1 mm, but as with MMCs can be as thin as 1.4 mm.
  • 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.[3]

Devices with SD slots can use the thinner MMCs, but standard SD cards will not fit into the thinner MMC slots. miniSD and microSD 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 to. If the notch is covered by a sliding write protection tab, or absent, then the card is writeable. This can be overridden if the device reading it wishes to (and supports it).

Not all devices support write protection, which is an optional feature of the SD standard.

Some SD cards have no write-protection notch,[4] 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 tab.

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File system

Like other flash card technologies, most SD cards ship preformatted with the FAT or FAT 32 file system. 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.

SD cards of size 4 GB and lower can be formatted to either FAT16 (4 GB card can be formatted to FAT16 only with 64k clusters) or FAT32 file systems. Cards 8 GB and larger can only be formatted with a file system that can handle these larger storage sizes, such as FAT32.

Defragmentation tools are used on hard disks to optimize the file system access speed. On an SD card, this is pointless, as any block can be accessed as fast as any other. Doing this will wear the card out slightly, as a limited number of writes can be made before failure.


There are different speed grades available which are measured with the same system as CD-ROMs, in multiples of 150 kB/s (1x = 150 kB/s). Basic cards transfer data up to six times (6x) the data rate of the standard CD-ROM speed (900 kB/s vs. 150 kB/s). High-speed cards are made with higher data transfer rates like 66x (10 MB/s), and high-end cards have speeds of 200x or higher. Note that maximum read speed and maximum write speed may be different, with maximum write speed typically lower than maximum read speed. Some digital cameras require high-speed cards (write speed) to record video smoothly or capture multiple still photographs in rapid succession. Higher speeds of up to 200x are defined by specification 2.0.

The following table lists some common ratings and their respective minimum transfer rates.

Rating Speed (MB/s) SDHC Class
  6x  0.9  n/a
  13x  2.0  2
  26x  4.0  4
 32x  4.8  4
 40x  6.0  6
 66x 10.0  6
100x 15.0  6
133x 20.0  6
150x 22.5  6
200x 30.0  6
266x 40.0  6
300x 45.0  6

Openness of standards

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

There are three versions of the SD specification: 1.0, 1.1 and 2.0. These were originally available only after agreeing to a non-disclosure agreement (NDA) which prohibits the development of an open source driver, a fact that generates a fair amount of consternation in the open-source and free software communities. The system was eventually reverse-engineered though, and the non-DRMed sections of the memory cards could be accessed by free software drivers.

These days however, the SD Card Association (SDA) has made access to a simplified version of the specification available under a less-restrictive licence.[5] 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).[6] Like the physical specification, most of the information had already been discovered before the public release[7] 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."[8]

For the most part, the lack of complete, open SD specifications 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.

Technical explanation

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 memory 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 (1000 USD per year plus membership at 1500 USD/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. DVD-Audio uses a very similar scheme known as Content Protection for Prerecorded Media (CPPM). This DRM has not been seen "in the wild" and few, if any, devices appear to provide support for it.

Super*Talent, a manufacturer of computer memory, has created the "Super Digital" card. They are the same in appearance and function to regular Secure Digital cards, but they lack the CPRM code commonly found in Secure Digital cards. [9]

Compared to other flash memory formats

Overall, SD is less open than CompactFlash or USB flash memory drives, which can be implemented free of charge, but require licensing fees for the 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 publicly available technical information.

Different types of MMC/SD cards

The SD card is not the only flash memory card standard ratified by the Secure Digital Card Association. 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.

These 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. However, a WiFi card for mini-SDIO is already available from Spectec.[10]

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 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 2 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 4096) 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 4096 x 512 x 512 = 1 gigabyte of storage memory. A later revision of the 1.x standard allowed a 4-bit field to indicate 1024 or 2048 bytes per block instead, yielding more than 1 gigabyte 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 1024 or 2048.

For the new SDHC high capacity card (2.0) implementation, 22 bits of the identification string are used to indicate the memory size in increments of 512 KBytes. Currently 16 of the 22 bits are allowed to be used, giving a maximum size of 32 GB. All SDHC 4-GB and larger cards must be 2.0 implementations. Two bits that were previously reserved and fixed at 0 are now used for identifying the type of card, 0=standard, 1=HC, 2=reserved, 3=reserved. Non-HC devices are not programmed to read this code and therefore cannot correctly read the identification of the card.

All SDHC readers work with standard SD cards.[11]

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 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)."[12]

SD (non-SDHC) cards 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,[13] the maximum capacity of a standard SD card is defined by (BLOCKNR x BLOCK_LEN), where BLOCKNR may be (4096 x 512) and BLOCK_LEN may be up to 2048. This allows a capacity of 4 GB. The main problem is that some of the card readers support only a block (aka. sector) size of 512 bytes, so greater than 1 GB non-SDHC cards may cause compatibility difficulties for some users.

SDHC cards with greater than 32 GB capacity

Similarly to the above, as of version 2.00 of the specification,[13] 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 2048 GB capacity. SDHC cards have fixed sector size of 512 bytes.


SDHC (Secure Digital High Capacity, SD 2.0) is an extension of the SD standard that appeared in June 2006.[14] SDHC allows standard-compliant capacities in excess of 4 GB. SDHC cards are often formatted with the FAT32 file system.[15] It uses the same physical and electrical form factor as SD, but the SD 2.0 standard in SDHC uses a different memory addressing method (sector addressing vs byte addressing), thus theoretically reaching a maximum capacity of up to 2 TB (2048 GB). Currently the SD Card association has artificially defined the maximum limit of SDHC capacity to 32 GB[16], however the SD card specification states that "SD Memory Cards with a capacity greater than 32 GB will be available with updated versions of [the specification]."[17] SDHC cards work only in SDHC compatible devices, but standard SD cards work in both SD and SDHC devices. The SDHC trademark is licensed to ensure compatibility.[18]

SD Speed Class Ratings

SDHC cards have SD Speed Class Ratings defined by the SD Association. The SD Speed Class Ratings specify the following minimum write speeds based on "the best fragmented state where no memory unit is occupied":[19]

  • Class 2: 2 MB/s - 13x
  • Class 4: 4 MB/s - 26x
  • Class 6: 6 MB/s - 40x

SDHC cards will often also advertise a maximum speed (such as 133x or 150x) in addition to this minimum Speed Class Rating. See section speeds above for a further explanation. One critical difference between the Speed Class and the maximum speed ratings is 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. "Maximum speed" ratings are quoted by the manufacturers but unverified by any independent evaluation process.

SD and SDHC compatibility issues

During early 2007, the simultaneous availability of 4 GB SD cards compliant with later revisions of version 1.x of the SD specification but incompatible with readers based on earlier revisions of the specification, and of 4 GB SDHC cards, and incompatibilities between SD and SDHC caused confusion among consumers buying memory devices. Several Manufacturers such as Sandisk, Alcotek, Toshiba and Hynix have developed SDHC cards that are now fully compatible with the SDHC SD2.0 specification.

SD and SDHC cards and devices have these compatibility issues :

  • Devices that do not specifically support SDHC do not recognize SDHC memory cards. Nonetheless, some devices require only a firmware upgrade.[20]
  • SDHC devices are backward compatible with SD memory cards.[20]
  • Some manufacturers have produced 4 GB SD cards that conform to neither the SD2.0/SDHC spec nor existing SD devices.[21]
  • File System: SD cards are typically formatted with the FAT16 file system, while SDHC cards are typically formatted as FAT32.[18] However, both types of cards can support other general-purpose file systems, such as UFS2/ext2 or the proprietary exFAT for example.


The Secure Digital Extended Capacity (SDXC) format was unveiled at CES 2009. The maximum capacity defined for SDXC cards is 2 TB (2048 GB). SDHC cards also have a maximum capacity of 2 TB based on the card data structures, but this is arbitrarily limited to 32 GB by the SD 2.0 document.

The maximum transfer rate of SDXC was announced as 104 MB/s, with plans to increase it to 300 MB/s in the future. The SDXC specification has selected Microsoft's proprietary exFAT file system as the standard for this memory card format;[22][23][24] however, as with SDHC and SD, it is possible to use another filesystem such as FAT32 or ext2.

On January 8, 2009, Panasonic announced plans for production of 64 GB SDXC cards.[25]

On March, 6, 2009, Pretec introduces world's first SDXC card [26] 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.


SDIO stands for Secure Digital Input Output.

An SDIO card is an 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 not supported in SDIO. SDIO cards can contain 8 separate logical cards, though 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.[27] This concept has proven successful and other companies started introducing similar designs branded as duo SD.
  • 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. [28]
  • 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 40/104/128, WPA-PSK, 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.[29]
  • MyMemory produces an SD card (SDIO) with Wi-Fi capability built in for 802.11b and 802.11g wireless networks, without storage capacity for Windows Mobile and Windows CE devices.
  • Gruvi - A rare type of microSD card with extra DRM features

Market penetration

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 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.

As of December 2008, nearly all leading digital camera manufacturers use SD in their product lines, including Casio, Canon, Nikon, Pentax, Kodak, Panasonic, Konica Minolta, Ricoh and Samsung.

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

Two major manufacturers, however, have stuck to their own proprietary card formats: Olympus uses xD cards, and Sony uses Memory Stick. Prior to 2007, Fujifilm also used xD cards exclusively, but has added SD functionality to all models released since then, while Olympus has released an xD-microSD adapter for their latest cameras. Even Olympus have added microSD functionality. They equipped their newer cameras with a microSD adapter.

Embedded systems

Neither SD card variant supports ATA, limiting their use as solid state disks 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.[30] In September 2008, the SD Card Association announced the Embedded SD standard to be released in November.[31]

A homebrew hardware hack has brought SD card support to the popular WRT54G router by utilising spare GPIO pins on the router's processor and the Linux kernel's MMC module. Transfer speeds of 200KiB/s can be achieved with this setup.[32]

References and Notes

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