SD card

"SDHC" redirects here. For the gene, see SDHC (gene).

SD, SDHC, SDXC

SD (top), miniSD, microSD cards
Media type	Memory card
Capacity	SDSC (SD): 1 MB to 2 GB,   some 4 GB available. SDHC: 4 GB to 32 GB SDXC: ≥32 GB to 2 TB
Developed by	SD Card Association
Dimensions	Standard: Template:Convert/3 Mini: Template:Convert/3 Micro: Template:Convert/3
Weight	Standard: ~2 g Mini: ~0.8 g Micro: ~0.25 g
Usage	Portable devices, including digital cameras and handheld computers
Extended from	MultiMediaCard (MMC)

Secure Digital or (SD) is a non-volatile memory card format for use in portable devices, such as mobile phones, digital cameras, GPS navigation devices, and tablet computers.

The Secure Digital standard was introduced in 1999 as an evolutionary improvement over MultiMediaCards (MMC). The Secure Digital standard is maintained by the SD Card Association (SDA). SD technologies have been implemented in more than 400 brands across dozens of product categories and more than 8,000 models.

The Secure Digital format includes four card families available in three different form factors. The four families are the original Standard-Capacity (SDSC), the High-Capacity (SDHC), the eXtended-Capacity (SDXC), and the SDIO, which combines input/output functions with data storage. The three form factors are the original size, the "mini" size, and the "micro" size (see illustration). There are many combinations of form factors and device families.

Electrically passive adapters allow the use of a smaller card in a host device built to hold a larger card.

Host devices that comply with newer versions of the specification provide backward compatibility and accept older SD cards, but there are several factors that can prevent the use of a newer SD card:

• A newer card may offer greater capacity than the host device can handle.
• A newer card may use a file system the host device cannot navigate.
• Use of an SDIO card requires the host device be designed for the input/output functions the card provides.
• The organization of the card was changed starting with the SDHC family.
• Some vendors produced SDSC cards above 1 GB before the SDA had standardized a method of doing so.

The SDA uses several trademarked logos to enforce compliance with its specifications and assure users of compatibility.

Types of cards

The SDA has extended the SD specification in various ways:

• It defined electrically identical cards in smaller sizes: miniSD and microSD (originally named TransFlash or TF). Smaller cards are usable in larger slots through use of a passive adapter. By comparison, Reduced Size MultiMediaCards (RS-MMCs) are simply shorter MMCs and can be used in MMC slots by use of a physical extender.
• It defined higher-capacity cards, some with faster speeds and added capabilities: SDHC (Secure Digital High Capacity) and SDXC (Secure Digital eXtended Capacity). These cards redefine the interface so that they cannot be used in older host devices.
• It defined an SDIO card family that provides input-output functions and may also provide memory functions. These cards are only fully functional in host devices designed to support their input-output functions.

Physical size

Size comparison of families: SD (blue), miniSD (green), microSD (red)

The SD card specification defines three physical sizes. The SD and SDHC families are available in all three sizes, but the SDXC family is not available in the mini size, and the SDIO family is not available in the micro size.

Standard size

• SD (SDSC), SDHC, SDXC, SDIO
• Template:Convert/3
• Template:Convert/3 (as thin as MMC) for Thin SD (rare)

Mini size

• miniSD, miniSDHC, miniSDIO
• Template:Convert/3

Micro size

The micro form factor is the smallest SD card format.

• microSD, microSDHC, microSDXC
• Template:Convert/3

SDHC

Official SDHC logo

The Secure Digital High Capacity (SDHC) format, defined in Version 2.0 of the SD specification, supports cards with capacities up to 32 GB. The SDHC trademark is licensed to ensure compatibility.

SDHC cards are physically and electrically identical to standard-capacity SD cards (SDSC). The major compatibility issues between SDHC and SDSC cards are the redefinition of the Card-Specific Data (CSD) register in Version 2.0 (see below), and the fact that SDHC cards are shipped preformatted with the FAT32 file system.

Host devices that accept SDHC cards are required to accept SDSC cards. However, host devices designed for SDSC do not recognize SDHC or SDXC memory cards, although some devices can do so through a firmware upgrade. Older operating systems require patches to support SDHC. For instance, Microsoft Windows XP before SP3 requires a patch to support access to SDHC cards. Windows Vista SP1 also requires a later service pack.

SDXC

Official SDXC logo

Sandisk Ultra microSDXC 64GB card (UHS-I speed)

The Secure Digital extended Capacity (SDXC) format supports cards up to 2 TB (2048 GB), compared to a limit of 32 GB for SDHC cards in the SD 2.0 specification.

History

SDXC was announced at Consumer Electronics Show (CES) 2009 (January 7–10, 2009). At the same show, SanDisk and Sony also announced a comparable Memory Stick XC variant with the same 2 TB maximum as SDXC, and Panasonic announced plans to produce 64 GB SDXC cards.

On March 6, 2009, Pretec introduced the first SDXC card, a 32 GB card with a read/write speed of 400 Mbit/s. But it was not until early 2010 that compatible host devices came onto the market, including Sony's Handycam HDR-CX55V camcorder, Canon's EOS 550D (also known as Rebel T2i) Digital SLR camera, a USB card reader from Panasonic, and an integrated SDXC card reader from JMicron. The earliest laptops to integrate SDXC card readers relied on a USB 2.0 bus, which does not have the bandwidth to support SDXC at full speed.

Also in early 2010, commercial SDXC cards appeared from Toshiba (64 GB), Panasonic (64 GB and 48 GB), and SanDisk (64 GB). In early 2011, Centon Electronics, Inc. (64 GB and 128 GB) and Lexar (128 GB) began shipping SDXC cards rated at Speed Class 10. Pretec offered cards from 8 GB to 128 GB rated at Speed Class 16.

In September 2011, SanDisk released a 64 GB microSDXC card. Kingmax released a comparable product in 2011.

In late 2012, Lexar released the first 256 GB SDXC card, based on 20 nm NAND flash technology.

Compatibility with SDHC

SDXC host devices accept all previous families of SD memory cards. Conversely, SDHC host devices accept SDXC cards that follow Version 3.0, since the interface is identical, but the following issues may affect usability:

• SDXC cards are pre-formatted with Microsoft's proprietary and patented exFAT file system, which the host device might not support. Since Microsoft does not publish the specifications of exFAT and its use requires a non-free license, many alternative or older operating systems do not support exFAT for technical or legal reasons. The use of exFAT on some SDXC cards may render SDXC unsuitable as a universal exchange medium, as an SDXC card that uses exFAT would not be usable in all host devices. Since the FAT32 file system supports volumes up to the SDXC's maximum theoretical capacity of 2 TB as well, if the product using the card accepts file systems other than exFAT on SDXC cards, a user could reformat an SDXC card to use FAT32 for greater portability between computers (see below). FAT32-formatted SDXC cards can be used in a host device built for SDHC if the host device can handle volumes larger than 32 GB.
• SDHC host devices do not test the new capability bits defined for SDXC 4.0 cards and thus cannot use the new features of SDXC, such as transfer speeds above UHS104 (104 MB/s).

Host operating system support

Microsoft Windows versions that support SDXC are: Windows RT, Windows 8, Windows 7, Windows Vista SP1+, Windows XP SP2 or SP3 with KB955704, Windows Server 2008 SP1+, Windows Server 2003 SP2 or SP3 with KB955704, and Windows CE 6.0 and higher.

Apple Mac OS X versions that support SDXC cards and exFAT are Mac OS X Snow Leopard 10.6.5 or later including OS X Lion 10.7.

BSD and Linux systems that support SDHC cards also support SDXC cards that contain a compatible file system, but they normally do not support the proprietary exFAT file system that is installed on SDXC cards, because of patent issues. However, there is an open source implementation available as a FUSE module. The user may reformat the card to contain a different file system (see below).

SDIO

Camera using the SDIO interface to connect to some HP iPAQ devices

A SDIO (Secure Digital Input Output) card is an extension of the SD specification to cover I/O functions. Host devices that support SDIO (typically PDAs like the Palm Treo, but occasionally laptops or mobile phones) can use the SD slot to support GPS receivers, modems, barcode readers, FM radio tuners, TV tuners, RFID readers, digital cameras, and interfaces to Wi-Fi, Bluetooth, Ethernet, and IrDA. Many other SDIO devices have been proposed, but it is now more common for I/O devices to connect using the USB interface.

SDIO cards support most of the memory commands of SD cards. SDIO cards can be structured as 8 logical cards, although currently, the typical way that an SDIO card uses this capability is to structure itself as one I/O card and one memory card.

Host support for SDIO

The SDIO and SD interfaces are mechanically and electrically identical. Host devices built for SDIO cards generally accept SD memory cards without I/O functions. However, the reverse is not true, because host devices need suitable drivers and applications to support the card's I/O functions. For example, an HP SDIO camera usually does not work with PDAs that do not list it as an accessory. Inserting an SDIO card into any SD slot causes no physical damage nor disruption to the host device, but users may be frustrated that the SDIO card does not function fully when inserted into a seemingly compatible slot. (Bluetooth devices exhibit comparable compatibility issues, although to a lesser extent thanks to standardized Bluetooth profiles.)

Features

This microSDHC card holds 8 billion bytes. It rests on a section of a magnetic-core memory (used until the 1970s) that holds eight bytes using 64 cores. (This SD card's capacity is one billion times larger)

Card security

Cards can protect their contents from erasure or modification, prevent access by non-authorised users, and protect copyrighted content using Digital Rights Management (DRM).

Commands to disable writes

The host device can command the SD card to become read-only (to reject subsequent commands to write information to it). There are both reversible and irreversible host commands that achieve this.

Write-protect notch

The user can designate most full-size SD cards as read-only by use of a sliding tab that covers a notch in the card. (The miniSD and microSD formats do not support a write protection notch.)

When looking at the SD card from the top, the right side (the side with the beveled corner) must be notched.

On the left side there may be a write-protection notch. If the notch is omitted, the card can be read and written. If the card is notched, it is read-only. If the card has a notch and a sliding tab which covers the notch, the user can slide the tab upward (toward the contacts) to declare the card read/write, or downward to declare it read-only.

The presence of a notch, and the presence and position of a tab, have no effect on the SD card's operation. A host device that supports write protection should refuse to write to an SD card that is designated read-only in this way. Some host devices do not support write protection, which is an optional feature of the SD specification. Drivers and devices that do obey a read-only indication may give the user a way to override it.

Cards sold with content which must not be altered are permanently marked read-only by having a notch and no sliding tab.

Card password

MicroSD to SD adapter (left), microSD to miniSD adapter (middle), microSD card (right)

A host device can lock an SD card using a password of up to 16 bytes, typically supplied by the user. A locked card interacts normally with the host device except that it rejects commands to read and write data. A locked card can be unlocked only by providing the same password. The host device can, after supplying the old password, specify a new password or disable locking. Without the password (typically, in the case that the user forgets the password), the host device can command the card to erase all the data on the card for future re-use (except card data under DRM), but there is no way to gain access to the existing data.

DRM copy-protection

All cards incorporate DRM copy-protection. Roughly 10% of the storage capacity of an SD card is a "Protected Area" not available to the user, but is used by the on-card processor to verify the identity of an application program that it then allows to read protected content. The card prohibits other accesses, such as users trying to make copies of protected files. (Reformatting an SD card may erase this Protected Area; see Risks of reformatting below.)

The DRM scheme embedded in the SD cards is the Content Protection for Recordable Media (CPRM or CPPM) specification of the 4C Entity, which features the Cryptomeria cipher (also termed C2). The specification is kept secret and is accessible only to licensees. The scheme has not been broken or cracked; however, this feature of SD cards is rarely used to protect content. DVD-Audio uses the same DRM scheme.

Windows Media files can be DRM-encoded so as to make use of the SD card's DRM abilities.

Windows Phone 7 devices use SD cards designed for access only by the phone manufacturer or mobile provider. An SD card inserted into the phone underneath the battery compartment becomes locked "to the phone with an automatically generated key" so that "the SD card cannot be read by another phone, device, or PC". Symbian devices, however, are some of the few that can perform the necessary low-level format operations on locked SD cards. It is therefore possible to use a device such as the Nokia N8 to reformat the card for subsequent use in other devices.

A Super Digital product formerly made by Super Talent was essentially the SD card without the DRM features.

Vendor enhancements

SD cards with dual-interface SD and USB connections

SanDisk cards in various colors

Vendors have sought to differentiate their products in the market through various vendor-specific features:

• Integrated Wi-Fi — Several companies produce SD cards with built-in Wi-Fi transceivers supporting static security (WEP 40; 104; and 128, WPA-PSK, and WPA2-PSK). The card lets any digital camera with an SD slot transmit captured images over a wireless network, or store the images on the card's memory until it is in range of a wireless network. Examples include: Eye-Fi / SanDisk, Transcend, Toshiba, PQI Air Card and ez Share. Some models geotag their pictures.
• Pre-loaded content — In 2006, SanDisk announced Gruvi, a microSD card with extra digital rights management (DRM) features, which they intended as a medium for publishing content. SanDisk again announced pre-loaded cards in 2008, under the slotMusic name, this time not using any of the DRM capabilities of the SD card. In 2011, SanDisk offered various collections of 1000 songs on a single slotMusic card for about $40, now restricted to compatible devices and without the ability to copy the files.
• Integrated USB connector — The SanDisk SD Plus product can be plugged directly into a USB port without needing a USB card reader. Other companies introduced comparable products, such as the Duo SD product of OCZ Technology and the 3 Way (microSDHC, SDHC, and USB) product of A-DATA, which was available in 2008 only.
• Different colors — SanDisk has used various colors of plastic or adhesive label, including a "gaming" line in translucent plastic colors that indicated the card's capacity.
• Integrated display — In 2006, A-DATA announced a Super Info SD card with a digital display that provided a two-character label and showed the amount of unused memory on the card.

Speeds

An SD card's speed is measured by how quickly information can be read from, or written to, the card. In applications that require sustained write throughput, such as video recording, the device might not perform satisfactorily if the SD card's class rating falls below a particular speed. For example, a high-definition camcorder may require a card of not less than Class 6, suffering dropouts or corrupted video if a slower card is used. Digital cameras with slow cards may take a noticeable time after taking a photograph before being ready for the next, while the camera writes the first picture.

A card's speed depends on many factors, including:

• The frequency of soft errors that the card's controller must re-try
• The fact that, on most cards, writing data requires the controller to read and erase a larger region, then rewrite that entire region with the desired part changed
• File fragmentation: where there is not sufficient space for a file to be recorded in a contiguous region, it is split into non-contiguous fragments. This does not cause rotational or head-movement delays as with electromechanical hard drives, but may decrease speed; for instance, by requiring additional reads and computation to determine where on the card the file's next fragment is stored.

With early SD cards the speed was specified as a "times" ("×") rating, which compared the average speed of reading data to that of the original CD-ROM drive. This was superseded by the Speed Class Rating, which guarantees a minimum rate at which data can be written to the card.

The newer families of SD card improve card speed by increasing the bus rate (the frequency of the clock signal that strobes information into and out of the card). Whatever the bus rate, the card can signal to the host that it is "busy" until a read or a write operation is complete. Compliance with a higher speed rating is a guarantee that the card limits its use of the "busy" indication.

Speed Class Rating

Class	Minimum performance
Class 2	2 MByte/sec
Class 4	4 MByte/sec
Class 6	6 MByte/sec
Class 10	10 MByte/sec
UHS Speed Grade 1	10 MByte/sec

The SD Association defines standard speed classes indicating minimum performance to record video. Both read and write speeds must exceed the specified value. These are defined in terms of suitability for different applications:

• Class 2 for SD video recording
• Class 4 and 6 for high-definition video (HD) to Full HD video recording,
• Class 10 for Full HD video recording and consecutive recording of HD stills
• UHS Speed Grade 1 for real-time broadcasts and large HD video files

The specification defines these classes in terms of performance curves that translate into the following minimum read-write performance levels on an empty card:

Speed Classes 2, 4, and 6 assert that the card supports the respective number of megabytes per second as a minimum sustained write speed for a card in a fragmented state. Class 10 asserts that the card supports 10 MByte/s as a minimum non-fragmented sequential write speed. By comparison, the older "×" rating measured maximum speed under ideal conditions, and was vague as to whether this was read speed or write speed.

The host device can read a card's speed class, unlike the earlier "×" speed ratings. A device can warn the user if the card reports a speed class that falls below an application's minimum need.

The speed class rating does not totally characterize card performance. Different cards of the same class may vary considerably while meeting class specifications. In addition, speed may vary markedly between writing a large amount of data to a single file (sequential access, as when a digital camera records large photographs or videos) and writing a large number of small files (a random-access use common in smartphones). One study found that, in this random-access use, some Class 2 cards achieved a write speed of 1.38Mbit/s, while all cards tested of Class 6 or greater (and some of lower Classes; lower Class does not necessarily mean better small-file performance), including those from major manufacturers, were over 100 times slower.

“×” rating

Rating	Approx. (MB/s)	Comparable Speed Class
16×	2.34	(13×)
32×	4.69	(27×)
48×	7.03	(40×)
100×	14.6	(67×)

Main article: X (writing speed)

The “×” rating, made obsolete by speed classes, is a multiple of the standard CD-ROM drive speed of 150 kB/s (approximately 1.23 Mbit/s). Basic cards transfer data at up to six times (6×) the CD-ROM speed; that is, 900 kB/s or 7.37 Mbit/s. The 2.0 specification defines speeds up to 200×, but is not as specific as Speed Classes are on how to measure speed. Manufacturers may report best-case speeds and may report the card's fastest read speed, which is typically faster than the write speed. Some vendors, including Transcend and Kingston report their cards' write speed. When a card lists both a speed class and an “×” rating, the latter may be assumed a read speed only.

This table shows the approximate relationship between certain “×” ratings and the newer Speed Class designations:

Ultra-High Speed

The Ultra-High Speed (UHS) bus is available on some SDHC and SDXC cards. The following ultra-high speeds are specified:

• UHS-I cards, specified in SD Version 3.01, support a clock frequency of 100 MHz (a quadrupling of the original "Default Speed"), which in four-bit transfer mode could transfer 50 MB/s. UHS-I cards declared as UHS104 also support a clock frequency of 208 MHz, which could transfer 104 MB/s. UHS-I is the fastest class for which products are currently available.
• Double data rate operation at 50 MHz (DDR50) is also specified in Version 3.01, and is mandatory for microSDHC and microSDXC cards labeled as UHS-I. In this mode, four bits are transferred when the clock signal rises and another four bits when it falls, transferring an entire byte on each full clock cycle.
• UHS-II cards, defined in Version 4.0, further raise the data transfer rate to a theoretical maximum of 312 MB/s using additional row of pins.

UHS memory cards work best with UHS host devices. The combination lets the user record HD resolution videos with tapeless camcorders while performing other functions. It is also suitable for real-time broadcasts and capturing large HD videos.

Cards that comply with UHS show Roman numerals 'I' or 'II' on the label, and report this capability to the host device. Use of UHS requires that the host device command the card to drop from 3.3-volt to 1.8-volt operation and select the 4-bit transfer mode.

UHS cards can use UHS Speed Grade rating, which is comparable to speed class. Two grades are currently defined: 0 for read/write performance of less than 10 MB/s (no symbol), and 1 for performance of more than 10 MB/s (1 inside 'U'). Manufacturers can also display standard speed class symbol alongside UHS speed grade.

Markets

Secure Digital cards are used in many consumer electronic devices, and have become a widespread means of storing several gigabytes of data in a small size. Devices where the user may remove and replace cards often, such as digital cameras, camcorders, and video game consoles, tend to use full-sized cards. Devices where small size is paramount, such as mobile phones, tend to use microSD cards. SD cards are not the most economical solution in devices that need only a small amount of non-volatile memory, such as station presets in small radios. They may also not present the best choice for applications where higher storage capacities or speeds are a requirement as provided by other flash card standards such as CompactFlash.

Many personal computers of all types and personal digital assistants (PDAs) use SD cards, either through built-in slots or through an active electronic adaptor. Adaptors exist for the PC card, ExpressBus, USB, FireWire, and the parallel printer port. Active adaptors also let SD cards be used in devices designed for other formats, such as CompactFlash. The FlashPath adaptor lets SD cards be used in a floppy disk drive.

Digital cameras

A camcorder with a 4 GB SDHC card

SD/MMC cards 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 by 2007.

At this time all the leading digital camera manufacturers used SD in their consumer product lines, including Canon, Casio, Fujifilm, Kodak, Leica, Nikon, Olympus, Panasonic, Pentax, Ricoh, Samsung, and Sony. Formerly, Olympus and Fujifilm used XD-Picture Cards (xD cards) exclusively, while Sony only used Memory Stick; by early 2010 all three supported SD.

Some prosumer and professional digital cameras continued to offer CompactFlash (CF), either on a second card slot or as the only storage, as CF supports much higher maximum capacities and faster transfer speeds and historically was cheaper for the same capacity.

Secure Digital memory cards can be used in Sony XDCAM EX camcorders with an adapter.

Personal computers

USB-based universal Memory card reader

A shield (daughterboard) that gives Arduino prototyping microprocessors access to SD cards.

Although many personal computers accommodate SD cards as an auxiliary storage device through a built-in slot or a USB adaptor, SD cards cannot be used as the primary hard disk through the onboard ATA controller because none of the SD card variants supports ATA signalling. This use requires a separate SD controller chip or a SD-to-CompactFlash converter. However, on computers that support bootstrapping from a USB interface, an SD card in a USB adaptor can be the primary hard disk, provided it contains an operating system that supports USB access once the bootstrap is complete.

Since late 2009, newer Apple computers with installed SD card readers have been able to boot in Mac OS X from SD storage devices, when properly formatted to Mac OS Extended file format and the default partition table set to GUID. (See Other file systems below.)

Problems with using an SD card as the primary hard disk include the speed of the card, and that intensive writes to the card reduce its rated lifespan.

Embedded systems

In 2008, the SDA specified Embedded SD, "leverag[ing] well-known SD standards" to enable non-removable SD-style devices on printed circuit boards. SanDisk provides such memory components under the iNAND brand.

Most modern microcontrollers have built-in SPI logic that can interface to an SD card operating in its SPI mode, providing non-volatile storage. Even if a microcontroller lacks the SPI feature, the feature can be emulated by bit banging. For example, a home-brew hack combines spare General Purpose Input/Output (GPIO) pins of the processor of the Linksys WRT54G router with MMC support code from the Linux kernel. This technique can achieve throughput of up to 1.6 Mbit/s.

Fakes

Images of genuine, questionable, and fake/counterfeit microSD (Secure Digital) cards before and after decapsulation. Details at source.

Commonly found on the market are mislabeled or counterfeit Secure Digital cards that report a fake capacity or run slower than labeled.

History

In 1999, SanDisk, Matsushita, and Toshiba agreed to develop and market the Secure Digital (SD) Memory Card, which was a development of the MultiMediaCard (MMC). The new card provided both digital rights management (DRM) up to the Secure Digital Music Initiative (SDMI) standard, and a high memory density for the time.

The new format was designed to compete with the Memory Stick, a DRM product that Sony released the prior year. It was mistakenly predicted that DRM features would be widely used due to pressure from music and other media suppliers to prevent piracy.

The trademarked SD logo was originally developed 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 Consumer Electronics Show (CES) trade show, the three companies announced the creation of the SD Card Association (SDA) to promote SD cards. The SD Card Association's headquarters are in San Ramon, California, United States and it comprises some 30 product manufacturers that make interoperable memory cards and devices. Early samples of the SD Card were available in the first quarter of 2000, with production quantities of 32 and 64 MB cards available 3 months later.

Relation to MMC

Size comparison of various flash cards: SD, CompactFlash, MMC, xD

The SD cards changed the MMC design in several ways:

• Asymmetrical slots in the sides of the SD card prevent inserting it upside down, while an MMC goes in most of the way but makes no contact if inverted.
• Most SD cards are 2.1 mm (0.083 inches) thick, compared to 1.4 mm (0.055 inches) for MMCs. The SD specification defines a card called Thin SD with a thickness of 1.4 mm, but they are rare, as the SDA went on to define even smaller form factors.
• The card's electrical contacts are recessed beneath the surface of the card, protecting them from contact with a user's fingers.
• The SD specification envisioned capacities and transfer rates exceeding those of MMC, and these have both grown over time. For a comparison table, see below.

Full-sized SD cards do not fit in the slimmer MMC slots, and there are other issues that affect the ability to use one format in a host device designed for the other.

Follow-on products

In March 2003, SanDisk Corporation announced the introduction of the miniSD and demonstrated it at CeBIT 2003. The SDA adopted the miniSD card in 2003 as a small form factor extension to the SD card standard. While the new cards were designed especially for mobile phones, they are usually packaged with a miniSD adapter that provides compatibility with a standard SD memory card slot.

At CTIA Wireless 2005, the SDA announced the small microSD form factor (and SDHC, with capacities in excess of 2 GB and a minimum sustained read and write speed of 17.6 Mbit/s). SanDisk had conceived microSD when its CTO and the CTO of Motorola concluded that current memory cards were too large for mobile phones. The card was originally called T-Flash, but just before product launch, T-Mobile sent a cease and desist order to SanDisk claiming that T-Mobile owned the trademark on T-(anything), and the name was changed to TransFlash. TransFlash and microSD cards are the same; each can be used in devices made for the other. SanDisk induced the SDA to administer the microSD standard. The SDA approved the final microSD specification on July 13, 2005. Initially, microSD cards were available in capacities of 32, 64, and 128 MB.

In April 2006, the SDA released a detailed specification for the non-security related parts of the SD memory card standard and for the Secure Digital Input Output (SDIO) cards and the standard SD host controller.

In September, 2006, SanDisk announced the 4 GB miniSDHC. Like the SD and SDHC, the miniSDHC card has the same form factor as the older miniSD card but the HC card requires HC support built into the host device. Devices that support miniSDHC work with miniSD and miniSDHC, but devices without specific support for miniSDHC work only with the older miniSD card.

In January 2009, the SDA announced the SDXC family, which supports cards up to 2 TB and speeds up to 300 MByte/s.

Openness of the standard

Dismantled microSD to SD adapter showing the passive connection from the microSD card slot on the bottom to the SD pins on the top

Like most memory card formats, SD is covered by numerous patents and trademarks. Royalties for SD card licences are imposed for manufacture and sale of memory cards and host adapters (US$1,000/year plus membership at US$1,500/year), but SDIO cards can be made without royalties.

Early versions of the SD specification were available only after agreeing to a non-disclosure agreement (NDA) that prohibited development of an open source driver. However, the system was eventually reverse-engineered, and free software drivers provided access to SD cards that did not use DRM. Since then, the (SDA) has provided a simplified version of the specification under a less restrictive license. Although most open-source drivers were written before this, it has helped to solve compatibility issues.

In 2006 the SDA released a simplified version of the specification of the host controller interface (as opposed to the specification of SD cards) and later also for the physical layer, ASSD extensions, SDIO, and SDIO Bluetooth Type-A, under a disclaimers agreement. Again, most of the information had already been discovered and 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."

The proprietary nature of the complete SD specification affects embedded systems, laptop computers, and some desktop computers; many desktop computers do not have card slots, instead using USB-based card readers if necessary. 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 gain direct access to SD cards and thus need complete programming information. Desktop card readers are themselves embedded systems; their manufacturers have usually paid the SDA for complete access to the SD specifications. Many notebook computers now include SD card readers not based on USB; device drivers for these essentially gain direct access to the SD card, as do embedded systems.

Technical details

Transfer modes

Cards may support various combinations of the following bus types and transfer modes. The SPI bus mode and one-bit SD bus mode are mandatory for all SD families, as explained in the next section. Once the host device and the SD card negotiate a bus interface mode, the usage of the numbered pins is the same for all card sizes.

• SPI bus mode: Serial Peripheral Interface Bus is primarily used by embedded microcontrollers. This bus type supports only a 3.3-volt interface.
• One-bit SD bus mode: Separate command and data channels and a proprietary transfer format.
• Four-bit SD bus mode: Uses extra pins plus some reassigned pins. UHS-I and UHS-II requires this bus type.

The physical interface comprises 9 pins, except that the miniSD card adds two unconnected pins in the center and the microSD card omits one of the two V_SS (Ground) pins.

Official pin numbers for each card type (top to bottom): MMC, SD, miniSD, microSD. This shows the evolution from the older MMC, on which SD is based.

SPI Bus Mode

MMC Pin	SD Pin	miniSD Pin	microSD Pin	Name	I/O	Logic	Description
1	1	1	2	nCS	I	PP	SPI Card Select [CS] (Negative Logic)
2	2	2	3	DI	I	PP	SPI Serial Data In [MOSI]
3	3	3		VSS	S	S	Ground
4	4	4	4	VDD	S	S	Power
5	5	5	5	CLK	I	PP	SPI Serial Clock [SCLK]
6	6	6	6	VSS	S	S	Ground
7	7	7	7	DO	O	PP	SPI Serial Data Out [MISO]
	8	8	8	NC nIRQ	. O	. OD	Unused (memory cards) Interrupt (SDIO cards) (Negative Logic)
	9	9	1	NC	.	.	Unused
		10		NC	.	.	Reserved
		11		NC	.	.	Reserved

One-Bit SD Bus Mode

MMC Pin	SD Pin	miniSD Pin	microSD Pin	Name	I/O	Logic	Description
1	1	1	2	NC	.	.	Unused
2	2	2	3	CMD	I/O	PP, OD	Command, Response
3	3	3		VSS	S	S	Ground
4	4	4	4	VDD	S	S	Power
5	5	5	5	CLK	I	PP	Serial Clock
6	6	6	6	VSS	S	S	Ground
7	7	7	7	DAT0	I/O	PP	SD Serial Data 0
	8	8	8	NC nIRQ	. O	. OD	Unused (memory cards) Interrupt (SDIO cards) (Negative Logic)
	9	9	1	NC	.	.	Unused
		10		NC	.	.	Reserved
		11		NC	.	.	Reserved

Four-Bit SD Bus Mode

MMC Pin	SD Pin	miniSD Pin	microSD Pin	Name	I/O	Logic	Description
.	1	1	2	DAT3	I/O	PP	SD Serial Data 3
.	2	2	3	CMD	I/O	PP, OD	Command, Response
.	3	3		VSS	S	S	Ground
.	4	4	4	VDD	S	S	Power
.	5	5	5	CLK	I	PP	Serial Clock
.	6	6	6	VSS	S	S	Ground
.	7	7	7	DAT0	I/O	PP	SD Serial Data 0
	8	8	8	DAT1 nIRQ	I/O O	PP OD	SD Serial Data 1 (memory cards) Interrupt Period (SDIO cards share pin via protocol)
	9	9	1	DAT2	I/O	PP	SD Serial Data 2
		10		NC	.	.	Reserved
		11		NC	.	.	Reserved

Notes:

1. Direction is relative to card. I = Input, O = Output.
2. PP = Push-Pull logic, OD = Open-Drain logic.
3. S = Power Supply, NC = Not Connected (or logical high).

Interface

Inside a 512 MB SD card: NAND flash chip that holds the data (bottom) and SD controller (top)

Inside a 2 GB SD card: two NAND flash chips (top and middle), SD controller chip (bottom)

Inside a 16 GB SDHC card

Command interface

SD cards and host devices initially communicate through a synchronous one-bit interface, where the host device provides a clock signal that strobes single bits in and out of the SD card. The host device thereby sends 48-bit commands and receives responses. The card can signal that a response will be delayed, but the host device can abort the dialogue.

Through issuing various commands, the host device can:

• Determine the type, memory capacity, and capabilities of the SD card
• Command the card to use a different voltage, different clock speed, or advanced electrical interface
• Prepare the card to receive a block to write to the flash memory, or read and reply with the contents of a specified block.

The command interface is an extension of the MultiMediaCard (MMC) interface. SD cards dropped support for some of the commands in the MMC protocol, but added commands related to copy protection. By using only commands supported by both standards until determining the type of card inserted, a host device can accommodate both SD and MMC cards.

Electrical interface

All SD card families initially use a 3.3-volt electrical interface. On command, SDHC and SDXC cards switch to 1.8-volt operation.

At initial power-up or card insertion, the host device selects either the Serial Peripheral Interface (SPI) bus or the one-bit SD bus by the voltage level present on Pin 1. Thereafter, the host device may issue a command to switch to the four-bit SD bus interface, if the SD card supports it. For various card types, support for the four-bit SD bus is either optional or mandatory.

After determining that the SD card supports it, the host device can also command the SD card to switch to a higher transfer speed. Until determining the card's capabilities, the host device should not use a clock speed faster than 400 kHz. SD cards other than SDIO (see below) have a "Default Speed" clock rate of 25 MHz. The host device is not required to use the maximum clock speed that the card supports. It may operate at less than the maximum clock speed to conserve power. Between commands, the host device can stop the clock entirely.

SDIO cards

The SDIO family comprises Low-Speed and Full-Speed cards. Both types of SDIO cards support SPI and one-bit SD bus types. Low-Speed SDIO cards are allowed to also support the four-bit SD bus; Full-Speed SDIO cards are required to support the four-bit SD bus. To use a SDIO card as a "combo card" (for both memory and I/O), the host device must first select four-bit SD bus operation. Two other unique features of Low-Speed SDIO are a maximum clock rate of 400 kHz for all communications, and the use of Pin 8 as "interrupt" to try to initiate dialogue with the host device.

Ganging cards together

The one-bit SD protocol was derived from the MMC protocol, which envisioned the ability to put up to 3 cards on a bus of common signal lines. The cards use open collector interfaces, where a card may pull a line to the low voltage level; the line is at the high voltage level (because of a pull-up resistor) if no card pulls it low. Though the cards shared clock and signal lines, each card had its own chip select line to sense that the host device had selected it.

The SD protocol envisioned the ability to gang 30 cards together without separate chip select lines. The host device would broadcast commands to all cards and identify the card to respond to the command using its unique serial number.

In practice, cards are rarely ganged together because open-collector operation has problems at high speeds and increases power consumption. Newer versions of the SD specification recommend separate lines to each card.

Achieving higher card speeds

The SD specification defines four-bit-wide transfers. (The MMC specification supports this and also defines an eight-bit-wide mode.) Transferring several bits on each clock pulse improves the card speed. Advanced SD families have also improved speed by offering faster clock frequencies and double data rate (explained here).

File system

Like other types of flash memory card, an SD card of any SD family is a block-addressable storage device, in which the host device can read or write fixed-size blocks by specifying their block number.

MBR and FAT

Most SD cards ship preformatted with one or more MBR partitions, where the first or only partition contains a file system. This lets them operate like the hard disk of a personal computer. Per the SD card specification, an SD card is formatted with MBR and the following file system:

• For SDSC cards:
  • Capacity of less than 32680 logical sectors (smaller than 16 MB): FAT12 with partition type 01h and BPB 3.0 or EBPB 4.1
  • Capacity of 32680 to 65535 logical sectors (between 16 MB and 32 MB): FAT16 with partition type 04h and BPB 3.0 or EBPB 4.1
  • Capacity of at least 65536 logical sectors (larger than 32 MB): FAT16B with partition type 06h and EBPB 4.1
• For SDHC cards:
  • Capacity of less than 16450560 logical sectors (smaller than 7.8 GB): FAT32 with partition type 0Bh and EBPB 7.1
  • Capacity of at least 16450560 logical sectors (larger than 7.8 GB): FAT32X with partition type 0Ch and EBPB 7.1
• For SDXC cards: exFAT with partition type 07h

Most consumer products that take an SD card expect that it is partitioned and formatted in this way. Universal support for FAT12, FAT16, FAT16B, FAT32 and FAT32X allows the use of SDSC and SDHC cards on most host computers with a compatible SD reader, to present the user with the familiar method of named files in a hierarchical directory tree.

On such SD cards, standard utility programs such as Mac OS X's "Disk Utility" or Windows' SCANDISK can be used to repair a corrupted filing system and sometimes recover deleted files. Defragmentation tools for FAT file systems may be used on such cards. The resulting consolidation of files may provide a marginal improvement in the time required to read or write the file, but not an improvement comparable to defragmentation of hard drives, where storing a file in multiple fragments requires additional physical, and relatively slow, movement of a drive head. Moreover, defragmentation performs writes to the SD card that count against the card's rated lifespan. The write endurance of the physical memory is discussed in the article on flash memory; newer technology to increase the storage capacity of a card provides worse write endurance.

When reformatting an SD card with a capacity of at least 32 MB (65536 logical sectors or more), but not more than 2 GB, FAT16B with partition type 06h and EBPB 4.1 is recommended if the card is for a consumer device. (FAT16B is also an option for 4 GB cards, but it requires the use of 64 kiB clusters, which are not widely supported.) FAT16B does not support cards above 4 GB at all.

The SDXC specification mandates the use of Microsoft's proprietary exFAT file system, which is supported only by some proprietary operating systems.

Other file systems

Because the host views the SD card as a block storage device, the card does not require MBR partitions or any specific file system. The card can be reformatted to use any file system the operating system supports. For example:

• Under Windows, SD cards can be formatted using NTFS and, on later versions, exFAT.
• Under Mac OS X, SD cards can be partitioned as GUID devices and formatted with the HFS Plus file system or still use exFAT.
• Under Unix-like operating systems such as Linux or FreeBSD, SD cards can be formatted using the UFS, Ext2, Ext3, Ext4, btrfs, HFS Plus, or ReiserFS file system.

Any recent version of the above can format SD cards using the UDF file system.

Additionally, an SD card called Live SD can contain an embedded operating system (such as Live USB). Computers that can bootstrap from an SD card (either using a USB adapter or inserted into the computer's flash media reader) instead of the hard disk drive may thereby be able to recover from a corrupted hard disk drive. A Live SD can be write-locked to preserve the system's integrity.

Risks of reformatting

Reformatting an SD card with a different file system, or even with the same one, may make the card slower, or shorten its lifespan. Some cards use wear leveling, in which frequently modified blocks are mapped to different portions of memory at different times, and some wear-leveling algorithms are designed for the access patterns typical of the file allocation table on a FAT12, FAT16 or FAT32 device. In addition, the preformatted file system may use a cluster size that matches the erase region of the physical memory on the card; reformatting may change the cluster size and make writes less efficient.

SD/SDHC/SDXC memory cards have a "Protected Area" on the card for the SD standard's security function; a standard formatter may erase it, causing problems if security is used. The SD Association provides free SD Formatter software to overcome these problems. The SD Formatter does not format the "Protected Area", and the Association recommends the use of appropriate application software or SD-compatible device that provides SD security function to format the "Protected Area" in the memory card.

Power consumption

The power consumption of microSD cards varies by manufacturer and model. During transfer it may be in the range of 66–330 mW (20–100 mA at a supply voltage of 3.3 V). Specifications from TwinMos technologies list a maximum of 149 mW (45 mA) during transfer. Toshiba lists 264–330 mW (80–100 mA). Standby current is much lower, less than 0.2 mA for one 2006 Micro SD card. If there is data transfer for significant periods, battery life may be reduced noticeably (smartphones typically have batteries of capacity from 1000 mAh to 2000 mAh).

Storage capacity and incompatibilities

All SD cards let the host device determine how much information the card can hold, and the specification of each SD family gives the host device a guarantee of the maximum capacity a compliant card reports.

By the time the Version 2.0 (SDHC) specification was completed in June 2006, vendors had already devised 2 GB and 4 GB SD cards, either as specified in Version 1.01, or by creatively reading Version 1.00. The resulting cards do not work correctly in some host devices.

SDSC cards above 1 GB

4 GB SDSC card

A host device can ask any inserted SD card for its 128-bit identification string (the Card-Specific Data or CSD). In standard-capacity cards (SDSC), 12 bits identify the number of memory clusters (ranging from 1 to 4,096) and 3 bits identify the number of blocks per cluster (which decode to 4, 8, 16, 32, 64, 128, 256, or 512 blocks per cluster). The host device multiplies these figures (as shown in the following section) with the number of bytes per block to determine the card's capacity in bytes.

SD version 1.00 assumed 512 bytes per block. This permitted SDSC cards up to 4,096 × 512 × 512 = 1 GB, for which there are no known incompatibilities.

Version 1.01 let an SDSC card use a 4-bit field to indicate 1,024 or 2,048 bytes per block instead. Doing so enabled cards with 2 GB and 4 GB capacity, such as the Transcend 4 GB SD card and the Memorette 4 GB SD card.

Early SDSC host devices that assume 512-byte blocks therefore do not fully support the insertion of 2 GB or 4 GB cards. In some cases, the host device can read data that happens to reside in the first 1 GB of the card. If the assumption is made in the driver software, success may be version-dependent. In addition, any host device might not support a 4 GB SDSC card, since the specification lets it assume that 2 GB is the maximum for these cards.

Storage capacity calculations

The format of the Card-Specific Data (CSD) register changed between version 1 (SDSC) and version 2.0 (which defines SDHC and SDXC).

Version 1

In Version 1 of the SD specification, capacities up to 2 GB are calculated by combining fields of the CSD as follows:

Capacity=(C_SIZE+1) × 2(C_SIZE_MULT + 2 + READ_BL_LEN)
Where: 0 <= C_SIZE <= 4095,
0 <= C_SIZE_MULT <= 7,
READ_BL_LEN is 9 (for 512 bytes/sector) or 10 (for 1024 bytes/sector)

Later versions state (at Section 4.3.2) that a 2 GB SDSC card shall set its READ_BL_LEN (and WRITE_BL_LEN) to indicate 1024 bytes, so that the above computation correctly reports the card's capacity; but that, for consistency, the host device shall not request (by CMD16) block lengths over 512 bytes.

Versions 2 and 3

In the definition of SDHC cards in Version 2.0, the C_SIZE portion of the CSD is 22 bits and it indicates the memory size in multiples of 512 KB (the C_SIZE_MULT field is removed and READ_BL_LEN is no longer used to compute capacity.) Two bits that were formerly reserved now identify the card family: 0 is SDSC; 1 is SDHC or SDXC; 2 and 3 are reserved. Because of these redefinitions, older host devices do not correctly identify SDHC or SDXC cards nor their correct capacity.

• SDHC cards are restricted to reporting a capacity not over 32 GiB.
• SDXC cards are allowed to use all 22 bits of the C_SIZE field. An SDHC card that did so (reported C_SIZE > 65375 to indicate a capacity of over 32 GiB) would violate the specification. A host device that relied on C_SIZE rather than the specification to determine the card's maximum capacity might support such a card, but the card might fail in other SDHC-compatible host devices.

Capacity is calculated thus:

Capacity=(C_SIZE+1)*524288
where for SDHC  4112<=C_SIZE<=65375    (approx. 2 GB) < capacity < 32 GiB
for SDXC 65535<=C_SIZE 32 GiB <= capacity <= 2 TiB max.

Capacities above 4 GB can only be achieved by following Version 2.0 or later versions. In addition, capacities equal to 4 GB must also do so to guarantee compatibility.

Comparison to other flash memory formats

Overall, SD is less open than CompactFlash or USB flash memory drives. Those open standards can be implemented without paying for licensing, royalties, or documentation. (CompactFlash and USB flash drives may require licensing fees for the use of the SDA's trademarked logos.)

However, SD is much more open than Memory Stick, for which no public documentation nor any documented legacy implementation is available. All SD cards can be accessed freely using the well-documented SPI bus.

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.

See also

• Comparison of memory cards
• Serial Peripheral Interface Bus (SPI)
• Flash memory
• Universal Flash Storage

External links

Organizations

• Official website, SD Card Association.

Specifications

• SD Simplified Specifications, SD card, Free

Software

• SD Formatter for SD / SDHC / SDXC cards (Windows and Mac), SD card.

Interfacing

• Interfacing to SD cards, Elm chan.
• Interfacing AVR (Arduino) to SD cards, C source code, Dharmani tech.
• Interfacing ARM to SD cards, C source code, DE: Uni KL.
• Interfacing MSP430 to SD cards, C source code (PDF), Michigan State University.
• Interfacing MAXQ2000 to SD cards, good technical descriptions, C source code, Maxim IC.
• SD card controller, Verilog source code, Open cores.