Secure Digital

Secure Digital
SD cards
SD (top), miniSD, microSD cards
Media type Memory card
Capacity SDSC (SD): 1 MB to 2 GB although 4 GB cards are available
SDHC: 4 GB to 32 GB
SDXC: >32 GB to 2 TB
Developed by SD Card Association
Dimensions Standard: 32×24×2.1 mm
Mini: 21.5×20×1.4 mm
Micro: 15×11×1.0 mm
Weight Standard: ~2.0 g
Mini: ~1.0 g
Micro: ~0.5 g
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 the SD Card Association (SDA) for use in portable devices. The SD technology is used by more than 400 brands across dozens of product categories and more than 8,000 models, and is considered the de-facto industry standard.[1]

Secure Digital devices comprise cards of the same physical dimensions but different communication protocols. The Standard-Capacity (SDSC) card family (the original SD card), stores up to 2 GB (but see below).[2] The High-Capacity (SDHC) card family has a capacity of 4 GB to 32 GB.[3] eXtended-Capacity (SDXC) card family has a capacity starting above 32 GB with a maximum of 2 TB.[4][5] The SDIO family implements input/output functions rather than just data storage.

The current Secure Digital specifications now also comprise three different form factors: the original size, the "mini" size, and the "micro" size (see illustration). Electrically passive adaptors allow the use of a smaller card in a host device built to hold a larger card. There are many combinations of form factors and device families. SD, miniSD, and microSD are members of the SD family. SDHC, miniSDHC, and microSDHC are members of the SDHC family. SDXC and microSDXC are members of the SDXC family. SDIO and miniSDIO are members of the SDIO family.

The newer (higher-capacity) communication protocols provide backward compatibility, but older host devices do not recognize the newer card families. The existence of cards of different families but with the same physical size has confused customers.[6] (Most incompatibility issues could be resolved by updating the firmware of the host device, but vendors of host devices are more likely to support newer card families by releasing an entirely new product.) In addition, each family specification guarantees a maximum device capacity that the host device is required to support. For example, the SDSC specification set an upper limit on number of blocks that suggested a 2 GB maximum. Some SD cards provided double this amount (though 4 GB would now be achieved using SDHC) by cleverly doubling the block size. Though these 4 GB devices were technically SD-compliant, some SD host devices did not support them.



8-GB microSDHC card atop 8 bytes of magnetic-core memory; 1 core is 1 bit

In 1999, SanDisk, Matsushita, and Toshiba first agreed to develop and market the SD (Secure Digital) Memory Card, which was a development of the MMC (MultiMediaCard). With a physical profile of 32×24×2.1 mm, 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 Sony's Memory Stick format, which was released the prior year, and featured MagicGate DRM. It was mistakenly predicted that DRM features[7] 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 Consumer Electronics Show (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 MB cards available 3 months later.

In March 2003, SanDisk Corporation announced the introduction of the miniSD and demonstrated it at CeBIT 2003.[8] The miniSD card was adopted in 2003 by the SD Association as a small form factor extension to the SD card standard. While the new cards were designed especially for use in mobile phones, they are usually packaged with a miniSD adapter which enables compatibility with all devices equipped with a standard SD memory card slot.

In April 2006, the SD Association released a detailed specification for the non-security related parts of the SD memory card standard. The organization also released specifications for the Secure Digital Input Output (SDIO) cards and the standard SD host controller. During the same year, specifications were finalized for the small-form-factor microSD (formerly named TransFlash) and SDHC, with capacities in excess of 2 GB and a minimum sustained read and write speed of 17.6 Mbit/s.

In September, 2006, SanDisk announced the 4 GB miniSDHC.[9] 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 will work with miniSD and miniSDHC, but devices without specific support for miniSDHC will work only with the older miniSD card.

In January 2009, the SD Association announced the SDXC family that will support cards up to 2 TB memory size and speeds up to 300 Mbyte/s.[10]


The microSD format was created by SanDisk. The concept was the brainchild of the CTOs of SanDisk and of Motorola, who believed current memory card formats were too large for phones. It was originally called T-Flash, but just before product launch, T-Mobile sent a cease and desist order to SanDisk claiming they own the trademark on T-(anything) and the name was then changed to TransFlash.[11] After pressure from Toshiba and Panasonic, SanDisk's partners in the 3C licensing group of the SDA, SanDisk negotiated to have the new format become a standard administered by the SD Card Association (SDA). The SDA announced the microSD format at CTIA Wireless 2005 on March 14, 2005, and approval of the final microSD specification on July 13, 2005. At launch, the microSD format was available in capacities of 32, 64, and 128 MB. TransFlash and microSD cards are the same; each can be used in devices made for the other.

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 to prevent inserting it upside down, while an MMC will go in most of the way but make no contact if inverted.
  • SD cards are thicker than MMCs. SD cards generally measure 32 × 24 × 2.1 mm, but as with MMCs can be as slim 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 slimmer card use the smaller 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 80–160 Mbit/s, but this is likely to grow, due to recent improvements to the MMC standard.[12]

Devices with SD slots can use the slimmer MMCs, but standard SD cards will not fit into the slimmer 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.

Physical size

Size comparison of families: SD, miniSD, microSD

Each SD card family is available in up to 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
  • 32 mm × 24 mm × 2.1 mm; the MMC and rare thin SD cards are 1.4 mm thick.
Mini size
  • miniSD, miniSDHC, miniSDIO
  • 21.5 mm × 20 mm × 1.4 mm
Micro size
  • microSD, microSDHC, microSDXC
  • 15 mm × 11 mm × 1.0 mm

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 cut-off corner.

On the left side there 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,[13] 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 abilities.[citation needed] Such media are usually permanently marked read-only by adding the notch with no tabs.

File system

USB-based SD card reader
Transcend Micro SD card being used in a Kingston Micro SD Reader device

Like other flash memory card technologies, most SD cards ship preformatted with a file system on top of an MBR partition scheme. SD cards are typically formatted as FAT16, SDHC cards as FAT32, SDXC cards as exFAT. The ubiquity of FAT16 and FAT32 allows those cards 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, and some utilities can recover deleted files, providing that they have not been overwritten. 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 called Live SD can contain an embedded operating system (such as a Live USB) to recover a corrupted host computer by booting with a USB adapter or natively from the flash media reader. A Live SD can be write-locked to preserve the system's integrity.

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 bigger). Cards 4 GB and bigger can only be formatted with a file system that can handle these 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; partition schemes other 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 Plus file system. Under 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, or exFAT filesystem.

Defragmentation tools may be used on SD cards. The resulting consolidation of files may provide a marginal improvement in the time required to read or write the file,[14] but not an improvement comparable to defragmentation of hard drives, where storing a file in multiple fragments may involve a time penalty to move between physical areas of the drive. 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 currently provides worse write endurance.

Transfer modes

SD cards with dual-interface SD and USB connections

Depending upon the ability of a specific SD card, it may support various combinations of the following bus types and transfer modes. The SPI bus and one-bit SD bus are mandatory for all SD families. See the electrical interface section for a more detailed description.

  • SPI: Serial Peripheral Interface Bus is primarily used by embedded microcontrollers. This bus type supports only 3.3 volt power and communications.
  • One-bit SD: Separate command and data channels and a proprietary transfer format.
  • Four-bit SD: Uses extra pins plus some reassigned pins. UHS-I and UHS-II requires this bus type but after the card is reconfigured to communicate at 1.8 volts.
    • UHS-I: The Ultra High Speed mode is found exclusively on SDXC and SDHC products.[15] SDXC or SDHC products with the UHS-I symbol can support data transfer speeds up to 104 MB/s. UHS-I quadruples the extant top speed of 25 MB/s. UHS bus interfaces are backward compatible. SDXC UHS-I and SDHC UHS-I memory cards can achieve best performance when paired with a UHS-I device and are designed to allow consumers to record HD resolution videos to tapeless camcorders, plus perform other simultaneous recording functions.
    • UHS-II: Available exclusively on SDXC and SDHC products. The standard raises the data transfer speed to a theoretical maximum of 312 MB/s.[16]
  • USB: Some SD cards may have an additional USB connector. Though not part of the SD electrical specification, this concept still meets the SD physical size specification.

Electrical interface

SD card pin assignment

All SD card families must be able to run at 3.3 volt with a 3.3 volt logic electrical interface, though SDHC and SDXC family cards can step down to 1.8 volt after receiving specific commands.[17]

All SD card families must support two signaling bus types at power-up: Serial Peripheral Interface Bus (SPI) bus and one-bit SD bus. Depending upon the card type, an additional four-bit SD bus type may be either optional or mandatory for the card.[17]

The host has the choice of selecting either SPI or SD bus type. After power-up of an SD card, the host will "tell" the card which bus type it wants to use for communications, either SPI or one-bit SD signaling. In one-bit SD bus mode, the host can send additional commands to change over to four-bit SD bus, 1.8 voltage, and higher transfer speeds.[17]

There is a special family of SD cards, called SDIO, of which there are two types: Low-Speed and Full-Speed. Both types of SDIO cards support SPI and one-bit SD bus types, but support for the four-bit SD bus type varies depending on the card. For Low-Speed SDIO cards, the four-bit SD bus is optional. For Full-Speed SDIO cards, the four-bit SD bus ability must exist. If a SDIO card is a "combo card", which means that it has memory and I/O, then it must be configured for four-bit SD bus before using both memory and I/O. The Low-Speed SDIO cards have an additional unique limit of a maximum clock rate of 400 kHz for all communications. SDIO cards support another unique feature, which is an optional interrupt output pin.[18]

The signaling buses supports various clock rates, including a stopped clock. After power-up, the host must communicate with the SD card up to a maximum clock rate called the Default Speed (DS), which is a bus clock up to 400 KHz for Low-Speed SDIO cards, or up to 25 MHz for all other types of SD cards. Since Low-Speed SDIO cards are limited to 400 kHz, it is recommended this be the initial maximum clock rate until additional commands can be sent to determine the exact "flavor" of SD card.[17]

SD cards may support even higher clock rate modes, which require added configuring before use, which are called: High Speed (HS), UHS-I, UHS-II. The High Speed (HS) mode supports a bus clock up to 50 MHz and supported by most SD cards. The UHS-I and UHS-II clock modes are available only by SDHC and SDXC family cards, and require a four-bit SD bus communicating at 1.8 volts.[17]

Though an SD card may be able to communicate at higher clock rates, the host does not need to communicate at high speeds. For low-power uses, it may be desirable to run at slower speeds to use less power.[17]

Pin Name I/O Logic Description
1 nCS I PP Card Select (Neg True)
2 DI I PP Data In
3 VSS S S Ground
4 VDD S S Power
5 CLK I PP Clock
6 VSS S S Ground
7 DO O PP Data Out
8 NC
NC (Memory Cards)
Interrupt (SDIO Cards)
9 NC . . NC
One-Bit SD Bus
Pin Name I/O Logic Description
1 NC . . NC
2 CMD I/O PP,OD Command, Response
3 VSS S S Ground
4 VDD S S Power
5 CLK I PP Clock
6 VSS S S Ground
7 DAT0 I/O PP Data 0
8 NC
NC (Memory Cards)
Interrupt (SDIO Cards)
9 NC . . NC
Four-Bit SD Bus
Pin Name I/O Logic Description
1 DAT3 I/O PP Data 3
2 CMD I/O PP,OD Command, Response
3 VSS S S Ground
4 VDD S S Power
5 CLK I PP Clock
6 VSS S S Ground
7 DAT0 I/O PP Data 0
8 DAT1
Data 1. SDIO Cards share
with Interrupt Period
9 DAT2 I/O PP Data 2


  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).

DRM features

All SD cards incorporate a digital rights management (DRM) scheme. Roughly 10% of the storage capacity of an SD card is not available to the user, but is used by the on-card processor to verify the identity of an application program that it will then allow to read protected content. The card prohibits other accesses, such as users trying to make copies of protected files.

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 hacked, but this feature of SD cards is rarely actually used to protect content. DVD-Audio uses the same DRM scheme.

Windows Phone 7 devices use SD cards designed to be accessed 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".[19] Symbian devices, however, are some of the very few which 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.[20]

The Super Digital cards manufactured by Super*Talent are the same in appearance and function as Secure Digital cards, but they lack the CPRM feature of Secure Digital cards.[21]

Power use

The power consumption of microSD cards varies by manufacturer, but appears to 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.[22] Toshiba, on the other hand, lists 264-330 mW (80-100 mA).[23]


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

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. The Class number represents a multiple of 8 Mbit/s (1 MB/s), the least sustained write speeds for a card in a fragmented state (Class 2, 4, 6) or the minimum non-fragmented sequential write speed (Class 10).[17]

These are the ratings of all currently available cards:[14][24]

Class Speed
SDHC Speed Class 2.svg Class 2 2 MB/s
SDHC Speed Class 4.svg Class 4 4 MB/s
SDHC Speed Class 6.svg Class 6 6 MB/s
SDHC Speed Class 10.svg Class 10 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 and not verified 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. On slower class cards, digital cameras may experience a lag of several seconds between photo-taking, while the camera writes the picture to the card.

Important differences between the Speed Class and the traditional CD-ROM drive speed measurement ("×" speed ratings) are that speed class:[25]

  1. may be queried by the host device;
  2. 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 an application's minimum need.[25]

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".[26] Toshiba also announced cards based on the new 3.0 spec.[27]

On 1 June 2010, Pretec announced the new Class-16 HD-video grade SDXC 64 GB card at Computex Taipei 2010.[28]

× rating

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

The × rating is equal to 1.2 Mbit/s. It is derived from the standard CD-ROM drive speed of 1.2 Mbit/s (approximately 150 kB/s). Basic cards transfer data up to six times (6×) the data rate of the standard CD-ROM speed (7.2 Mbit/s vs 1.2 Mbit/s). The 2.0 specification defines speeds up to 200×, but unlike the class rating system, does not mandate that ×-ratings measure the card's least sustained write-speed. So, typically, manufacturers provide ×-ratings based on maximum read/write speeds. Furthermore, for most cards, the fastest read speed is typically swifter than its fastest write speed, leading some manufacturers to use read-speed as the ×-rating measurement. Other vendors, such as Transcend and Kingston, use write-speed.[29]

This table lists common ratings, the minimum transfer rates, and the corresponding Speed Class.

Rating Read Speed
Read Speed
Write Speed
Write Speed
7.2 0.9
10× 12.0 1.5
13× 16.0 2.0 16.0 2.0 2
26× 33.0 4.0 32.0 4.0 4
32× 38.4 4.8 40.0 5.0
40× 48.0 6.0 48.0 6.0 6
66× 80.0 10.0 80.0 10.0 10
100× 120.0 15.0 120.0 15.0
133× 160.0 20.0 160.0 20.0
150× 180.0 22.5 180.0 22.5
200× 240.0 30.0 240.0 30.0
266× 320.0 40.0 320.0 40.0
300× 360.0 45.0 360.0 45.0
400× 480.0 60.0 480.0 60.0
600× 720.0 90.0 720.0 90.0

UHS Speed Class

Requiring a new bus, the UHS Speed Class is intended for use in applications like real-time broadcasts and capturing large HD videos. The only currently available UHS SD cards are UHS Speed class 1.[30]

Types of cards

microSD to SD adapter (left), microSD to miniSD adapter (middle), microSD card (right)
microSD card is inserted into an SD adapter, which is inserted into an SD to USB reader

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 named TransFlash before ratification by the SD Card Association), and SDHC (Secure Digital High Capacity, for capacities above 4 GB–although, there are some card readers that 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 and make physical contact). However, 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.[citation needed]

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 devices (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 needed. 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 Serial Peripheral Interface Bus (SPI) mode even if not officially needed, 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).

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. The new protocol used open collector signaling to allow multiple cards on the same bus, but this 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 far more complex card initializing and a need 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 use 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.

Storage capacity

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,[31] 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.

Compatibility issues with 4 GB and larger cards

4 GB standard SD card, not SDHC

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, theoretically yielding up to 4 gigabytes of memory storage. However the specification specifies 2 gigabytes as the maximum capacity.

Host devices designed before this change may incorrectly identify such cards, usually by misidentifying a card with less 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, 32 bits of the identification string are used to indicate the memory size in increments of 512 bytes. The SDCA currently allows only 26 of the 32 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 formerly reserved and fixed at 0, now called the "CSD Structure", are being used for identifying the type of card, 0 is standard capacity; 1 is high (SDHC) and extended (SDXC) capacity; 2 and 3 are reserved. Older host devices are not aware of this new field thus cannot correctly identify SDHC or SDXC cards.

All SDHC readers are able to use standard SD cards,[32] and all SDXC readers are able to use SD and SDHC cards.

Many older devices will not accept the 2 GB size even though it is in the revised standard. Some manufacturers have ignored the maximum size specification and manufactured cards with a 4 GB which have similar compatibility issues and also may not work in newer devices supporting the 2 GB size.[citation needed] 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).

Storage capacity calculations

SD cards contain a Card-Specific Data (CSD) register which holds the card's capacity, among many other things. The format changed considerably between version 1.0 (SD) and version 2.0 (SDHC, SDXC), this is the cause of the incompatibility between SD and newer cards. CSD v2.0 expanded the C_SIZE register, removed the C_SIZE_MULT register, and no longer uses READ_BL_LEN for capacity calculation.[34] Capacity is calculated thus:

CSD Version 1.0:

Capacity=(C_SIZE+1)<<(C_SIZE_MULT+2)<<READ_BL_LEN 2 GiB max.
Where 0<=C_SIZE<=4095, 0<=C_SIZE_MULT<=7, READ_BL_LEN==9 || READ_BL_LEN==10

CSD Version 2.0:

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

SDHC cards with greater than 32 GB capacity

Similarly to the above, as of version 2.00 of the specification,[31] 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 TB capacity. Some SDHC devices will accept SDXC cards with capacities greater than 32 GB and recognize the full capacity, however compatibility is not guaranteed in all cases. SDHC cards have a fixed sector size of 512 bytes.


8 GB SDHC Card, top and bottom

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 as in the prior 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[35] (it is expected that the SDHC specification will be revised in the future to allow card capacities greater than 32 GB).[25] SDHC cards will not work in devices designed to the older SD 1.x specification. The SDHC trademark is licensed to ensure compatibility.[36]

SD and SDHC compatibility issues

The SDHC specification was completed in June 2006,[37] but by that time, non-standard high-capacity (>1 GB) 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.[38] Some devices can support SDHC through a firmware upgrade.[39]
  • SDHC devices are backward compatible with SD memory cards.[39]
  • Some manufacturers have produced 4 GB SD cards that conform to neither the SD2.0/SDHC spec nor existing SD devices.[40]
  • File System: SD cards are typically formatted with the FAT16 file system, while SDHC cards are typically formatted as FAT32.[36] However, both types of cards can support other general-purpose file systems, such as UFS2, ext2 or the proprietary exFAT for example. Changing the filesystem used, or even reformatting may cause performance or life-span problems, as the card controllers frequently optimise a small area of the device for the access patterns typical of a FAT16, or FAT32 file allocation table (FAT) area.[41]
  • Microsoft Windows may need a hotfix to support accessing SDHC cards.[42][43][44]


The Secure Digital Extended Capacity (SDXC) format was unveiled at Consumer Electronics Show (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 formally announced in January 2011.[45]

The maximum transfer rate of SDXCs which follow the SD 3.0 specification was announced as 832 Mbit/s (these are called UHS104 speeds),[45] with plans that the SD 4.0 specification shall increase this to 2.4 Gbit/s.

The SDcard association selected Microsoft's proprietary exFAT file system in the official SDXC specification;[46][47][48] 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 btrFS, ext4, HFS Plus, NTFS, UFS, 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.[49]

On January 8, 2009, Panasonic announced plans for production of 64 GB SDXC cards.[50]

On March 6, 2009, Pretec introduced the world's first SDXC card[51] with a capacity of 32 GB and a read/write speed of 400 Mbit/s. At the introduction, there were no products compatible with the new memory card.

On August 3, 2009, Toshiba announced it will launch the world's first 64 GB SDXC Memory Card[52] with a read speed of 480 Mbit/s. The 64 GB card (THNSU064GAA2BC) was planned to be available in the spring of 2010.[53][54] Toshiba card was available from April 13.

On January 6, 2010, Panasonic announced its first SDXC cards with 64 GB and 48 GB to be available in February[55] (RP-SDW64GE1K and RP-SDW48GE1K).

On January 6, 2010, Sony announced the launch of Handycam HDR-CX55V with SDXC support.

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.[56]

On February 19, 2010, Panasonic launched in Japan World's first available for consumers SDXC memory cards with 64 GB and 48 GB (RP-SDW64GE1K and RP-SDW48GE1K) together with USB card readers compatible with SDXC format.[clarification needed]

On February 22, 2010, SanDisk launched its 64 GB SanDisk Ultra SDXC card.[57]

The first integrated SDXC card readers are available from JMicron[58] and are expected to be used in laptops in 2010.[59]

In January, 2011, Centon Electronics, Inc. began shipping its 64 GB and 128 GB Class 10 SDXC card.

In March, 2011, Lexar began shipping its 128 GB Class 10 SDXC card.[60]

In September 2011, a 64 GB microSDXC card was released by SanDisk [61]

SDHC and SDXC compatibility issues

In the 3.0 specification, the electronic interface of SDHC and SDXC cards is the same. This means that SDHC hosts that have drivers that recognize the newly used capability bits, and have operating system software that understands the exFAT filesystem, are compatible with SDXC cards. The decision to label cards with a capacity greater than 32 GB as SDXC and to use a different filesystem is due solely to the limitations in creating larger filesystems in certain versions of Microsoft Windows.[citation needed] Other operating system kernels, such as Linux, make no distinction between SDHC and SDXC cards, as long as the card contains a compatible filesystem.

SDHC and SDXC cards and hosts have these compatibility issues:

  • Existing SDHC hosts will only support the SDXC cards at up to UHS104 speeds.[45]
  • SDXC hosts are backward compatible with SD and SDHC memory cards.[62]
  • The operating systems that support SDXC, as of 2011, are: Linux (with a proprietary driver for the exFAT filesystem),[63] Microsoft Windows 7, Windows Vista SP1+,[62] Windows XP SP2 or SP3 with KB955704,[64] Windows Server 2008 SP1+, Windows Server 2003 SP2 or SP3 with KB955704, Windows CE 6+, Mac OS X 10.6.5 or later.[65]


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

A SDIO (Secure Digital Input Output) card is a combination of an SD card and an I/O device. This kind of combination is increasingly found in portable electronics devices.[citation needed]

Hosts that support SDIO (typically PDAs like the Palm Treo, but occasionally laptops or mobile phones) can use small hosts 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 the 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. Most of the SD Memory commands are supported in SDIO. SDIO cards can contain 8 separate logical cards, although currently, 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.[66] 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.[67][68]
  • Eye-Fi, Inc. - Produces an SD card with Wi-Fi ability built in for 802.11g, 802.11b, and backward 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.[69] Some models geotag their pictures.
  • Gruvi - A rare type of microSD card with extra DRM features

Pre-loaded content

Towards the end of the first decade of the 2000s many manufacturers saw the need to distinguish their SD cards from one another. One idea was to introduce pre-loaded content onto new SD cards.

SanDisk introduced their SlotMusic which enabled users to buy digital music files already loaded onto their cards.[70]

Market penetration

A camcorder with a 4 GB SDHC 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, personal digital assistant (PDAs), mobile phones, video game consoles, digital audio players, and 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 needed.

Digital cameras

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, with cards not being easily available in 2007.

At this time all the leading digital camera manufacturers use 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; however as of January 2010, all three support SD.

Some prosumer and professional digital 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.

Secure Digital memory cards can be used in Sony XDCAM EX camcorders via the MEAD-SD01 adapter.[71]

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.[72] In September 2008, the SD Card Association announced the Embedded SD standard to be released in November.[73]

Most modern microcontrollers have built-in SPI peripheral, thus it is easy to interface a SD card to them. Even if a processor does not have the SPI peripheral, the SPI is relatively easy to emulate using the bit banging method. 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.[74] Microcontrollers can likewise use SD cards as non-volatile storage. This technique can achieve throughput of up to 1.6 Mbit/s.

Openness of standards

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

Like most memory card formats, SD is covered by numerous patents 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.[75] 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) and later also for physical layer, ASSD extensions, SDIO and SDIO Bluetooth Type-A specifications under a disclaimers agreement.[76] Like the physical specification, most of the information had already been discovered before the public release[77] 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."[78]

For the most part, the lack of a complete, open SD specification mainly affects embedded systems and laptop 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.

Royalties for SD card licences 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.

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 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.

Comparison of technical features of MMC and SD card variantsv · d · e
Type MMC RS-MMC MMC Plus SecureMMC SD SDIO miniSD microSD
SD Socket Yes Mechanical adapter Yes Yes Yes Yes Electromechanical adapter Electromechanical adapter
Pins 7 7 13 7 9 9 11 8
Form factor shallow shallow/narrow shallow shallow deep (some) deep narrow/slim/shallow narrow/slim/extra shallow
Breadth 24 mm 24 mm 24 mm 24 mm 24 mm 24 mm 20 mm 11 mm
Width 32 mm 18 mm 32 mm 32 mm 32 mm 32 mm+ 21.5 mm 15 mm
Depth 1.4 mm 1.4 mm 1.4 mm 1.4 mm 2.1 mm (some) 2.1 mm 1.4 mm 1 mm
SPI mode Optional Optional Optional Yes Yes Yes Yes Yes
1-bit mode Yes Yes Yes Yes Yes Yes Yes Yes
4-bit mode No No Yes  ? Optional Optional Optional Optional
8-bit mode No No Yes No No No No No
Interrupts No No No No No Optional No No
Max clock rate 20 MHz 20 MHz 52 MHz 20 MHz? 208 MHz 50 MHz 208 MHz 208 MHz
Max transfer 20 Mbit/s 20 Mbit/s 416 Mbit/s 20 Mbit/s? 832 Mbit/s 200 Mbit/s 832 Mbit/s 832 Mbit/s
Max SPI transfer 20 Mbit/s 20 Mbit/s 52 Mbit/s 20 Mbit/s 50 Mbit/s 50 Mbit/s 50 Mbit/s 50 Mbit/s
DRM No No No Yes Yes N/A Yes Yes
User encrypt No No No Yes No No No No
Simplified spec Yes Yes No Not yet? Yes Yes No No
Membership cost JEDEC: $4400/yr, optional SD Card Association: $2000/yr, general; $4500/yr, executive
Specification cost Free  ? Simplified spec: free
Full spec: free to members, $1000/yr to R&D non-members
Host license No No No No Yes $1000/yr
Card royalties Yes Yes Yes Yes Yes Yes +$1000/yr Yes Yes
Open source compatible Yes Yes Yes? Yes? Yes Yes Yes Yes
Nominal operating voltage 3.3V 1.8V/3.3V 1.8V/3.3V[79][80] 1.8V/3.3V 3.3V 3.3V 3.3V 3.3V
Type MMC RS-MMC MMC Plus SecureMMC SD SDIO miniSD microSD

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

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

See also


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  2. ^ "SD Card - SD Card Association". 
  3. ^ SDHC Card - SD Card Association
  4. ^ SDXC Card - SD Card Association
  5. ^ Before Using your SDXC Memory Card - SD Card Association
  6. ^ "Using SD Memory Cards is Easy" (video on YouTube) - SD Card Association
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  10. ^ SD Association - Press Release - Announced SDXC.
  11. ^ PhoneScoop - Sandisk T-Flash announcement.
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  21. ^ "Super Talent Technology - DDR and DDR2 Memory". Retrieved 2010-08-22. 
  22. ^
  23. ^
  24. ^ What are the applications for Speed Class? -[dead link] Archive copy at the Wayback Machine
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  28. ^ "Press Release 2010 Vol. 8". Pretec. 2010-06-01. Retrieved 2010-08-22. 
  29. ^ "Kingston Technology Company - Flash Memory Cards and X-Speed Ratings". Retrieved 2010-08-22. 
  30. ^ "SD Speed Class/UHS Speed Class". Retrieved 11 May 2011. 
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  37. ^ A look into how SDHC will affect the future Nand Flash market. DRAMeXchange, December 2006
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  40. ^ Techgage review, including an OCZ 4 GBan OCZ 4 GB SD (non-SDHC) card
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  43. ^ "Microsoft Support 939772 - Some Secure Digital (SD) cards may not be recognized in Windows Vista". 2008-05-15. Retrieved 2010-08-22. 
  44. ^ "Microsoft Support 949126 - A Secure Digital High Capacity (SDHC) card is not recognized on a Windows Vista Service Pack 1-based computer". 2008-02-21. Retrieved 2010-08-22. 
  45. ^ a b c "SD Association Triples Speeds with UHS-II". 5 January 2011. Retrieved 2011-08-09. 
  46. ^ "SDXC memory cards promise 2TB of storage, 300MBps transfer". Retrieved 2010-08-22. 
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  48. ^ "SDXC signals new generation of removable memory with up to 2 terabytes of storage" (PDF). Retrieved 2010-08-22. 
  49. ^ "SanDisk and Sony to expand Memory Stick Pro and Memory Stick Micro formats". Retrieved 2010-08-22. 
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  53. ^ Ng, Jansen (2009-12-22). "Toshiba Sampling First SDXC Flash Memory Cards". DailyTech. Retrieved 2009-12-22. 
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  64. ^
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  66. ^
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  68. ^ OhGizmo! » Archive » Company Announces SD Card With Embedded Capacity Display
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  70. ^ AudioHolics
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