| SD card | |
| Type: | Memory card |
| Blocksize: | Variable |
| Owner: | SD Association |
| Extended From: | MultiMediaCard |
| Released: | August 1999 |
The SD card is a proprietary, non-volatile, flash memory card format developed by the SD Association (SDA). They come in three physical forms: the full-size SD, the smaller miniSD (now obsolete), and the smallest, microSD. Owing to their compact form factor, SD cards have been widely adopted in a variety of portable consumer electronics, including digital cameras, camcorders, video game consoles, mobile phones, action cameras, and camera drones.
The format was introduced in August 1999 as Secure Digital by SanDisk, Panasonic (then known as Matsushita), and Kioxia (then part of Toshiba). It was designed as a successor to the MultiMediaCard (MMC) format, introducing several enhancements including a digital rights management (DRM) feature, a more durable physical casing, and a mechanical write-protect switch. These improvements, combined with strong industry support, contributed to its widespread adoption.
To manage licensing and intellectual property rights, the founding companies established SD-3C, LLC. In January 2000, they also formed the SD Association, a non-profit organization responsible for developing the SD specifications and promoting the format.[1] As of 2023, the SDA includes approximately 1000 member companies. The association uses trademarked logos owned by SD-3C to enforce compliance with official standards and to indicate product compatibility.[2]
In 1994, SanDisk introduced the CompactFlash (CF) format, one of the first successful flash memory card types.[3] CF outpaced several competing early formats, including the Miniature Card and SmartMedia. However, the late 1990s saw a proliferation of proprietary formats such as Sony's Memory Stick and the xD-Picture Card from Olympus and Fujifilm, resulting in a fragmented memory card market.
To address these challenges, SanDisk partnered with Siemens and Nokia in 1996 to develop a new postage stamp-sized memory card called the MultiMediaCard (MMC). While technically innovative, MMC adoption was slow, and even Nokia was slow to integrate support for it into its mobile devices.
In 1999, SanDisk was approached by Panasonic (then known as Matsushita) and Kioxia (then part of Toshiba) to develop a new format as a second-generation successor to MMC.[4] The goal was to create a portable, high-performance memory card with integrated security features and broader interoperability. Concerned about losing market share to Sony's proprietary Memory Stick, Toshiba and Panasonic saw the collaboration as an opportunity to establish an open, industry-backed standard.[5]
Panasonic and Toshiba, who had previously collaborated on the Super Density Disc (a DVD precursor), reused its stylized "SD" logo for the Secure Digital (SD) card format.[6] Anticipating the growth of MP3 players, they also advocated for digital rights management (DRM) support seeking to reassure content publishers wary of piracy.[7] The DRM system adopted—Content Protection for Recordable Media (CPRM)—had been developed earlier in partnership with IBM and Intel and complied with the Secure Digital Music Initiative standard.[8] Although often cited as a factor in the format's broad industry support, CPRM was rarely implemented in practice.[9] [10] SD cards also featured a mechanical write-protect switch, and early SD slots maintained backward compatibility with MMC cards.[11] In early 2000, the first commercial SD cards offering of storage were released, with larger capacity versions following shortly after.[12] By August 2000, cards were being sold for approximately .[13] According to SanDisk, consumer adoption was accelerated by Toshiba and Panasonic's commitment to launching compatible devices in parallel with the cards.
To support standardization and interoperability, SanDisk, Toshiba, and Panasonic announced the creation of the SD Association (SDA) at the January 2000 Consumer Electronics Show (CES). Headquartered in San Ramon, California, the SDA initially included 30 member companies and has since grown to encompass around 800 organizations worldwide.[14]
At the March 2003 CeBIT trade show, SanDisk introduced and demonstrated the miniSD card format.[15] The SD Association (SDA) adopted miniSD later that year as a small-form-factor extension to the SD card standard, intended primarily for use in mobile phones. However, the format was largely phased out by 2008 following the introduction of the even smaller microSD card.[16]
The microSD format was introduced by SanDisk at CeBIT in 2004, initially under the name T-Flash, later rebranded as TransFlash or TF.[17] [18] In 2005, the SDA adopted the format under the official name microSD, though the TransFlash name remains in common use as a generic term for microSD cards.[19] [20] [21] A passive adapter allows microSD cards to be used in standard SD card slots, maintaining backward compatibility across devices.
The storage capacity of SD cards increased steadily throughout the 2010s, driven by advances in NAND flash manufacturing and interface speeds. In January 2009, the SDA introduced the Secure Digital eXtended Capacity (SDXC) format, supporting up to of storage and transfer speeds up to .[22] SDXC cards are formatted with the exFAT file system by default.
The first SDXC cards appeared in 2010, with early models offering capacities of and read/write speeds of several hundred megabits per second.[23] Consumer adoption accelerated as digital cameras, smartphones, and card readers gained SDXC compatibility.
By 2011, manufacturers offered SDXC cards in capacities, with some models supporting UHS Speed Class 10 and faster.[24] In the following years, capacity milestones were reached at regular intervals: in 2013, in 2014, in 2019, and in 2022.[25] [26]
The Secure Digital Ultra Capacity (SDUC) specification, announced in 2018, expanded maximum capacity to and increased theoretical transfer speeds to .[27] In 2024, Western Digital announced the first SDUC card, scheduled for commercial release in 2025.[28]
There are four defined SD capacity standards: Standard Capacity (SDSC), High Capacity (SDHC), Extended Capacity (SDXC), and Ultra Capacity (SDUC). In addition to specifying maximum storage limits, these standards also define preferred file systems for formatting cards.[29] [30]
| SDHC | SDXC | SDUC | |||
|---|---|---|---|---|---|
| Mark | |||||
| Max capacity | |||||
| File system | FAT12, FAT16 | FAT32 | exFAT | ||
The original Secure Digital (SD) card was introduced in 1999 as a successor to the MMC format. The name SD Standard Capacity (SDSC) was applied later to distinguish it from newer variants. Although based on the same electrical interface as MMC, the SD format introduced several enhancements aimed at improving usability, durability, and performance:
SDSC cards support capacities up to and use the FAT12 or FAT16 file system. They remain compatible with most SD-capable devices but have been largely superseded by higher-capacity formats.
Because of physical differences, full-size SD cards do not fit in slim MMC-only slots.
SD High Capacity (SDHC) was introduced in SD specification version 2.0, released in January 2006. It expands the maximum capacity to, compared to the limit of SDSC.
SDHC cards are physically identical to earlier standard-capacity SD (SDSC) cards, but differ in how they store and address data. This includes a redefinition of the Card-Specific Data (CSD) register (for details, see § Storage capacity calculations). Additionally, SDHC cards are typically preformatted with the FAT32 file system.
SDHC-compatible devices are required to support older SDSC cards. However, older SDSC devices may not recognize SDHC cards without a firmware update.[31] Older operating systems like Windows XP require patches or service packs to access SDHC cards.[32] [33] [34]
SD eXtended Capacity (SDXC) was introduced in SD specification version 3.01, released in January 2009. It expands the maximum capacity to compared to the limit of SDHC. SDXC cards are formatted with the exFAT file system, which is required by the SDXC standard.[35] [36] While Windows Vista SP1 and later and Mac OS X 10.6.5 and later support exFAT natively, support in BSD and Linux distributions was limited until Microsoft released the exFAT specification and Linux kernel 5.4 included an open-source driver.[37] [38] [39] [40]
SDXC cards can be reformatted to other file systems (e.g., ext4, UFS, VFAT or NTFS), which may improve compatibility with older devices or systems lacking exFAT support. Many SDHC-compatible hosts can use SDXC cards if reformatted to FAT32, but full compatibility is not guaranteed.[41] [42] [43]
SD Ultra Capacity (SDUC) was introduced in SD specification version 7.0, released in June 2018. It expands the maximum capacity to, compared to the limit of SDXC. Like SDXC cards, SDUC cards use the exFAT file system by default.
Bus marks indicate both the bus interface and the minimum data transfer performance of a device (as opposed to speed class ratings which indicate card performance) in terms of sustained sequential read and write speeds. These are most relevant for handling large files—such as photos and videos—where data is accessed in contiguous blocks. The SD specification has improved bus speed performance over time by increasing the clock frequency used to transfer data between the card and the host device. Regardless of the bus speed, a card may signal that it is "busy" while completing a read or write operation. Compliance with higher-speed bus standards typically reduces reliance on this "busy" signal, allowing for more efficient and continuous data transfers.
| Mark | Bus | Capacity standard | ||||||
|---|---|---|---|---|---|---|---|---|
| Speed | PCIe | Duplex | SD | SDHC | SDXC | SDUC | ||
| Default | Half | 1.01 | ||||||
| Half | 1.10 | |||||||
| UHS-I | Half | 3.01 | ||||||
| UHS-II | Full | |||||||
| Half | ||||||||
| UHS-III | Full | 6.00 | ||||||
| SD Express | 3.1x1 | |||||||
| 8.0 | ||||||||
| 4.0x2 | ||||||||
| UHS-II | UHS-III | Express | ||||||
|---|---|---|---|---|---|---|---|---|
| UHS50 | UHS104 | |||||||
| UHS-I | UHS50 | 50 | 50 | 50 | 50 | 50 | 50 | |
| UHS104 | 50 | 104 | 104 | 104 | 104 | 104 | ||
| UHS-II | 50 | 104 | 156 | 156 | 156 | 156 | ||
| 50 | 104 | 156 | 312 | 312 | 312 | |||
| UHS-III | 50 | 104 | 156 | 312 | 624 | 624 | ||
| Express | 50 | 104 | 104 | 104 | 104 | 3,938 | ||
The original SD bus interface, introduced with version 1.00 of the SD specification, supported a maximum transfer rate of . This mode is referred to as Default Speed.
With version 1.10 of the specification, the SD Association introduced High-Speed mode, which increased the maximum transfer rate to . This enhancement was designed to meet the growing performance requirements of devices such as digital cameras.
The Ultra High Speed (UHS) bus interface enables faster data transfer on SDHC, SDXC and SDUC cards.[44] [45]
UHS-compatible cards are marked with Roman numerals next to the SD logo, indicating the version of the UHS standard, and therefore the bus speeds, they support.[46] These cards offer significantly faster read and write speeds than earlier SD card types, making them well suited for high-resolution video, burst photography, and other data-intensive applications.
To achieve higher transfer speeds, UHS cards and devices use specialized electrical signaling and hardware interfaces. UHS-I cards operate at instead of the standard and use a four-bit transfer mode. UHS-II and UHS-III introduce a second row of interface pins to add a second lane of data transfer and use low-voltage differential signaling (LVDS) at to increase speed and reduce power consumption and electromagnetic interference (EMI).[47]
The following UHS speed classes are defined:
Support for the UHS-I interface was introduced in SD specification version 3.01, released in May 2010. This version added several new transfer modes: SDR50, which uses a clock with single data rate signaling to reach up to ; DDR50, a double data rate mode at that transfers data on both clock edges for up to ; and SDR104, which increases the clock speed to, enabling transfer rates up to .
In 2018, SanDisk developed a proprietary mode unofficially known as DDR200, which combines double data rate signaling with a clock to achieve read speeds of up to without requiring additional pins. Write speeds remain limited to, similar to SDR104. These higher speeds are typically used when offloading data from cards via specialized readers.[48] [49] In 2022, SanDisk introduced DDR225, further increasing performance to up to read and write. Although neither mode is officially part of the SD specification, although they have been adopted by several other manufacturers.[50] [51] In 2024 SanDisk released a 2TB microSD card that achieves 250/150 MB/s (R/W) using the faster DDR275. In the same year Silicon Motion announced the SM2709 card controller that supports the even faster DDR300 signalling. With these improvements UHS-I can now be almost as fast as UHS-II.
Support for the UHS-II interface was introduced in SD specification version 4.0, released in January 2011. It added two new transfer modes: FD156, supporting up to full-duplex, and HD312, enabling up to half-duplex. These speeds required a second row of connectors for LVDS, bringing the total to 17 for full-size cards and 16 for microSD cards.[52]
Each LVDS lane can transfer up to . In full-duplex mode, one lane is used for sending data and the other for receiving. In half-duplex mode, both lanes operate in the same direction.
While initial adoption began in cameras around 2014, wider implementation took several more years, as few applications required the extra speed provided by the interface.[53], only about 100 cameras, mostly high-end models, support UHS-II cards.[54]
Support for the UHS-III interface was introduced in SD specification version 6.0, released in February 2017. It added two new full-duplex transfer modes: FD312, offering up to, and FD624, doubling that to . UHS-III retains the same physical interface and pin layout as UHS-II for backward compatibility.[55] However,, UHS-III has seen limited adoption and is unlikely to be widely implemented, as the SDA instead prioritizes SD Express, which offers even higher transfer rates but limits backward compatibility to UHS-I speeds.[56]
SD Express was introduced in SD specification version 7.0, released in June 2018. By incorporating a single PCI Express 3.0 (PCIe) lane and supporting the NVM Express (NVMe) storage protocol, SD Express enables full-duplex transfer speeds of up to . SD Express cards support direct memory access (DMA), which can improve performance, though security researchers have warned that it may also increase the attack surface in the event of a compromised or malicious card.[57] Compatible cards must support both PCIe and NVMe, and may be formatted as SDHC, SDXC, or SDUC. For backward compatibility, SD Express cards are also required to support the High-Speed and UHS-I bus interfaces. However, because the PCIe interface reuses the second row of pins previously used by UHS-II and UHS-III, compatibility with older devices is limited to UHS-I speeds. The specification also reserves space for two additional pins for future use.[58]
In February 2019, the SD Association introduced microSD Express, along with updated visual marks to help users identify compatible cards and devices.[59] [60]
SD specification version 8.0, released in May 2020, expanded the interface to support PCIe 4.0 and introduced dual-lane configurations for full-size cards by adding a third row of electrical contacts, bringing the total to 26. This raised the theoretical maximum transfer rate to using dual-lane PCIe 4.0.[61] Due to space constraints, the microSD form factor cannot accommodate a third row of contacts and remains limited to a single PCIe lane.
Adoption has been gradual. In February 2024, Samsung began sampling its first microSD Express cards, though commercial availability remained limited.[62] Interest grew in April 2025 when Nintendo announced that the Switch 2 would support only microSD Express cards, with UHS-I card support limited to transferring media from earlier models.[63]
, only single-lane PCIe 3.1 SD Express cards are commercially available; no PCIe 4.0 or dual-lane cards have been released for general sale.
Where speed classes overlap, manufacturers often display multiple symbols on the same card to indicate compatibility with different host devices and standards.
The original speed class ratings—Class 2, 4, 6, and 10—specify minimum sustained write speeds of 2, 4, 6, and, respectively. Class 10 cards assume a non-fragmented file system and use the High Speed bus mode. These are represented by a number encircled with a "C" (e.g., C2, C4, C6 and C10).
Ultra High Speed (UHS) speed class ratings—U1 and U3—specify minimum sustained write speeds of, respectively. These classes are represented by a number inside a "U" and are designed for high-bandwidth tasks such as 4K video recording.[64]
Video speed class ratings—V6, V10, V30, V60, and V90—specify minimum sustained write speeds of, respectively.[65] [66] [67] These classes are represented by a stylized "V" followed by the number and were introduced to support high-resolution formats such as 4K and 8K and to align with the performance characteristics of multi-level cell NAND flash memory.[68] [69]
SD Express speed class ratings—E150, E300, E450, and E600—specify minimum sustained write speeds of, respectively.[70] These classes are represented by a stylized "E" followed by the number, enclosed in a rounded rectangle. They are designed for data-intensive applications such as large-scale video processing, real-time analytics, and software execution.
| Rating | Approx. (MB/s) | Comparable speed class | |
|---|---|---|---|
| 16× | 2.34 | (13×) | |
| 32× | 4.69 | (27×) | |
| 48× | 7.03 | (40×) | |
| 100× | 14.6 | (67×) |
See main article: CD and DVD writing speed.
Initially, some manufacturers used a "×" rating system based on the speed of a standard CD-ROM drive (or), but this approach was inconsistent and often unclear. It was later replaced by standardized Speed Class systems that specify guaranteed minimum write speeds.[71] [72]
Speed Class ratings guarantee minimum write performance but do not fully describe real-world speed, which can vary based on factors such as file fragmentation, write amplification due to flash memory management, controller retry operations for soft error correction and sequential vs. random write patterns.
In some cases, cards of the same speed class may perform very differently. For instance, random small-file write speeds can be significantly lower than sequential performance. A 2012 study found some Class 2 cards outperformed Class 10 cards in random writes.[73] Another test in 2014 reported a 300-fold difference in small-write performance across cards, with a Class 4 card outperforming higher-rated cards in certain use cases.[74]
| Minimum random IOPS | Minimum sustained sequential writing | ||
|---|---|---|---|
| Read | Write | ||
| 1,500 | 500 | ||
| 4,000 | 2,000 |
Application Performance Class ratings were introduced in 2016 to identify SD cards capable of reliably running and storing applications, alongside general-purpose tasks such as saving photos, videos, music, and documents.
Earlier SD card speed ratings focused on sequential read and write performance, which is important when transferring large files. However, running apps and operating systems involves frequent access to many small files—a pattern known as random access—which places different demands on storage.[75] Before the introduction of the Application Performance Classes, random access performance could vary significantly between cards and presented a limiting factor in some use cases.[76]
As SD cards saw broader use for app storage and system boot volumes—especially in mobile devices, single-board computers, and embedded systems—a new performance metric became necessary. This need became more pressing with Android's Adoptable Storage feature, which allows SD cards to function as internal (non-removable) storage on smartphones and tablets.[77]
To address this, the SD Association introduced Application Performance Classes. The first, A1, defined in SD Specification 5.1 (released November 2016), requires a minimum of 1,500 input/output operations per second (IOPS) for reading and 500 IOPS for writing, using blocks. The higher-tier A2 class, defined in Specification 6.0 (released in February 2017), raises the thresholds to 4,000 read and 2,000 write IOPS. However, achieving these speeds requires host device support for command queuing and write caching, features that allow the card to optimize the execution of multiple simultaneous tasks and temporarily store data.[78] If not properly supported, performance will fall back to A1 levels. Both A1 and A2 cards must also sustain a minimum sequential write speed of, equivalent to speed classes C10, U1 and V10.[79]
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.[80] [81]
Most full-size SD cards have a mechanical write-protect switch, a sliding tab over a notch on the left side (viewed from the top, with the beveled corner on the right), that signals to the device to treat the card as read-only. Sliding the tab up (toward the contacts) sets the card to read/write; sliding it down sets it to read-only. However, the switch position is not detected by the card's internal circuitry. Therefore, some devices ignore it, while others allow overrides.[82]
MiniSD and microSD cards lack a built-in notch but can be used with adapters that include one. Cards without a notch are always writable; cards with preloaded content have a notch but no sliding tab.
A host device can lock an SD card using a password of up to 16 bytes, typically supplied by the user.[83] 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.[84]
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".[85] 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.[86]
A smartSD memory card is a microSD card with an internal "secure element" that allows the transfer of ISO 7816 Application Protocol Data Unit commands to, for example, JavaCard applets running on the internal secure element through the SD bus.[87]
Some of the earliest versions of microSD memory cards with secure elements were developed in 2009 by DeviceFidelity, Inc.,[88] [89] a pioneer in near-field communication (NFC) and mobile payments, with the introduction of In2Pay and CredenSE products, later commercialized and certified for mobile contactless transactions by Visa in 2010.[90] DeviceFidelity also adapted the In2Pay microSD to work with the Apple iPhone using the iCaisse, and pioneered the first NFC transactions and mobile payments on an Apple device in 2010.[91] [92] [93]
Various implementations of smartSD cards have been done for payment applications and secured authentication.[94] [95] In 2012 Good Technology partnered with DeviceFidelity to use microSD cards with secure elements for mobile identity and access control.[96]
microSD cards with Secure Elements and NFC (near-field communication) support are used for mobile payments, and have been used in direct-to-consumer mobile wallets and mobile banking solutions, some of which were launched by major banks around the world, including Bank of America, US Bank and Wells Fargo,[97] [98] [99] while others were part of innovative new direct-to-consumer neobank programs such as moneto, first launched in 2012.[100] [101] [102] [103]
microSD cards with Secure Elements have also been used for secure voice encryption on mobile devices, which allows for one of the highest levels of security in person-to-person voice communications.[104] Such solutions are heavily used in intelligence and security.
In 2011, HID Global partnered with Arizona State University to launch campus access solutions for students using microSD with Secure Element and MiFare technology provided by DeviceFidelity, Inc.[105] [106] This was the first time regular mobile phones could be used to open doors without need for electronic access keys.
Vendors have sought to differentiate their products in the market through various vendor-specific features:
SDIO (Secure Digital Input Output) is an extension of the SD specification that supports input/output (I/O) devices in addition to data storage.[110] SDIO cards are physically and electrically identical to standard SD cards but require compatible host devices with appropriate drivers to utilize their I/O functions. Common examples included adapters for GPS, Wi-Fi, cameras, barcode readers, and modems.[111] SDIO was not widely adopted.
Host devices that comply with newer versions of the specification provide backward compatibility and accept older SD cards. For example, SDXC host devices accept all previous families of SD memory cards, and SDHC host devices also accept standard SD cards.
Older host devices generally do not support newer card formats, and even when they might support the bus interface used by the card, there are several factors that arise:
Due to their compact size, 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 in which the user may remove and replace cards often, such as digital cameras, camcorders and video game consoles, tend to use full-sized cards. Devices in which small size is paramount, such as mobile phones, action cameras, and camera drones, tend to use microSD cards.[112] [113]
microSD cards have been widely used in mobile phones to expand storage, particularly for photos, videos, music, documents, and other infrequently accessed files.[114]
By 2015, support for microSD cards was common in Android smartphones.[115] However, by 2025, support had declined, particularly in high-end models. This shift has been attributed to cheaper, higher-capacity internal storage with faster performance than microSD, concerns over user experience related to counterfeit cards, and the increased availability, speed, and security of cloud storage services.[116] [117]
In contrast, Apple never included microSD card slots in the iPhone, instead relying on built-in flash storage and cloud-based services.[118]
SD cards are the most widely used form of storage for digital cameras.[119]
Although many personal computers accommodate SD cards as an auxiliary storage device using a built-in slot, or can accommodate SD cards by means of a USB adapter, SD cards cannot be used as the primary hard disk through the onboard ATA controller, because none of the SD card variants support ATA signalling. Primary hard disk use requires a separate SD host controller[120] and firmware support for booting from a SD card (which is typical rather for newer systems and Tablet PCs or an SD-to-CompactFlash converter. However, on computers that support bootstrapping from a USB interface, an SD card in a USB adapter can be the boot disk, provided it contains an operating system that supports USB access once the bootstrap is complete.
In laptop and tablet computers, memory cards in an integrated memory card reader offer an ergonomic benefit over USB flash drives, as the latter sticks out of the device, and the user would need to be cautious not to bump it while transporting the device, which could damage the USB port. Memory cards have a unified shape and do not reserve a USB port when inserted into a computer's dedicated card slot.
Since late 2009, newer Apple computers with installed SD card readers have been able to boot in macOS from SD storage devices, when properly formatted to Mac OS Extended file format and the default partition table set to GUID Partition Table.
SD cards are increasing in usage and popularity among owners of vintage computers like Atari 8-bit computers. For example SIO2SD (SIO is an Atari port for connecting external devices) is used nowadays. Software for an 8-bit Atari may be included on one SD card that may have less than 4–8 GB of disk size (2019).[121]
In 2008, the SDA specified Embedded SD, "leverag[ing] well-known SD standards" to enable non-removable SD-style devices on printed circuit boards.[122] However this standard was not adopted by the market while the MMC standard became the de facto standard for embedded systems. SanDisk provides such embedded memory components under the iNAND brand.[123]
While some modern microcontrollers integrate SDIO hardware which uses the faster proprietary four-bit SD bus mode, almost all modern microcontrollers at least have SPI units that can interface to an SD card operating in the slower one-bit SPI bus mode. If not, SPI can also be emulated by bit banging (e.g. a SD card slot soldered to a Linksys WRT54G-TM router and wired to GPIO pins using DD-WRT's Linux kernel achieved only .[124]
The popular Raspberry Pi series of small single-board computers originally used full-sized SD cards for primary storage before switching to microSD for newer versions.[125] [126]
More recently (in 2020), the 8-pin interface of microSD has been adapted for use on printed circuit boards as a surface-mounted (soldered) integrated circuit chip in standard LGA8 or WSON8 packaging, measuring . Such a chip is called an . The microSD interface offers several benefits compared to more established NAND formats: the small number of pins makes it easier to route the connections (unlike eMMC and raw NAND) and the card itself handles details such as wear leveling (unlike raw NAND).[127] They can alternatively be viewed as SPI NAND or SPI Flash units (which came in a similar packaging but only supported the SPI bus) with added SD bus (1-bit and 4-bit) capability and support for the SD command set.
Prerecorded microSDs have been used to commercialize music under the brands slotMusic and slotRadio by SanDisk and MQS by Astell & Kern.
Commonly found on the market are mislabeled or counterfeit Secure Digital cards that report a fake capacity or run slower than labeled.[128] [129] [130] Software tools exist to check and detect counterfeit products,[131] [132] [133] and in some cases it is possible to repair these devices to remove the false capacity information and use its real storage limit.[134]
Detection of counterfeit cards usually involves copying files with random data to the SD card until the card's capacity is reached, and copying them back. The files that were copied back can be tested either by comparing checksums (e.g. MD5), or trying to compress them. The latter approach leverages the fact that counterfeited cards let the user read back files, which then consist of easily compressible uniform data (for example, repeating 0xFFs).
The SD card specification defines three physical sizes. The SD and SDHC families are available in all three sizes, but the SDXC and SDUC families are not available in the mini size, and the SDIO family is not available in the micro size. Smaller cards are usable in larger slots through use of a passive adapter.
The micro form factor is the smallest SD card format.[135]
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.
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 VSS (Ground) pins.
| 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 | . O | . OD | Unused (memory cards) Interrupt (SDIO cards) (negative logic) | |||
| 9 | 9 | 1 | NC | . | . | Unused | ||
| 10 | NC | . | . | Reserved | ||||
| 11 | NC | . | . | Reserved |
| MMC pin | SD pin | miniSD pin | microSD pin | Name | I/O | Logic | Description | |
|---|---|---|---|---|---|---|---|---|
| 1 | 1 | 1 | 2 | CD | I/O | . | Card detection (by host) and non-SPI mode detection (by card) | |
| 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 |
| 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:
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:
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.
All SD card families initially use a electrical interface. On command, SDHC and SDXC cards can switch to operation.
At power-up or card insertion, the voltage on pin 1 selects either the Serial Peripheral Interface (SPI) bus or the SD bus. The SD bus starts in one-bit mode, but the host device may issue a command to switch to the four-bit mode, 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.
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:
Most consumer products that take an SD card expect that it is partitioned and formatted in this way. Universal support for FAT12, FAT16, FAT16B and FAT32 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' CHKDSK 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.[137] 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 (65,536 logical sectors or more), but not more than 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 cards, but it requires the use of clusters, which are not widely supported.) FAT16B does not support cards above at all.
The SDXC specification mandates the use of Microsoft's proprietary exFAT file system, which sometimes requires appropriate drivers (e.g. exfat-utils/exfat-fuse on Linux).[138]
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 FAT12, FAT16 or FAT32.[139] 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. The SD Association provides freely downloadable SD Formatter software to overcome these problems for Windows and Mac OS X.[140]
SD/SDHC/SDXC memory cards have a "Protected Area" on the card for the SD standard's security function. Neither standard formatters nor the SD Association formatter will erase it. The SD Association suggests that devices or software which use the SD security function may format it.
The power consumption of SD cards varies by its speed mode, manufacturer and model.[141] During transfer it may be in the range of (at a supply voltage of). Specifications from TwinMOS Technologies list a maximum of during transfer. Toshiba lists . Standby current is much lower, less than for one 2006 microSD card.
Modern UHS-II cards can consume up to, if the host device supports bus speed mode SDR104 or UHS-II. Minimum power consumption in the case of a UHS-II host is .
| Bus speed mode | Max. bus speed [MB/s] | Max. clock frequency [MHz] | Signal voltage [V] | SDSC [W] | SDHC [W] | SDXC [W] | |
|---|---|---|---|---|---|---|---|
| HD312 | 312 | 52 | 0.4 | – | 2.88 | 2.88 | |
| FD156 | 156 | 52 | 0.4 | – | 2.88 | 2.88 | |
| SDR104 | 104 | 208 | 1.8 | – | 2.88 | 2.88 | |
| SDR50 | 50 | 100 | 1.8 | – | 1.44 | 1.44 | |
| DDR50 | 50 | 50 | 1.8 | – | 1.44 | 1.44 | |
| SDR25 | 25 | 50 | 1.8 | – | 0.72 | 0.72 | |
| SDR12 | 12.5 | 25 | 1.8 | – | 0.36 | 0.36 / 0.54 | |
| High Speed | 25 | 50 | 3.3 | 0.72 | 0.72 | 0.72 | |
| Default Speed | 12.5 | 25 | 3.3 | 0.33 | 0.36 | 0.36 / 0.54 |
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 and 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.
SD version 1.00 assumed per block. This permitted SDSC cards up to .
Version 1.01 let an SDSC card use a 4-bit field to indicate per block instead. Doing so enabled cards with capacity.
The format of the Card-Specific Data (CSD) register changed between version 1 (SDSC) and version 2.0 (which defines SDHC and SDXC).
In version 1 of the SD specification, capacities up to are calculated by combining fields of the CSD.
Later versions state (at Section 4.3.2) that a SDSC card shall set its READ_BL_LEN (and WRITE_BL_LEN) to indicate, so that the computation correctly reports the card's capacity, but for consistency, the host device shall not request (by CMD16) block lengths over .
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 (the C_SIZE_MULT field is removed and READ_BL_LEN is no longer used to compute capacity). The SCR register identifies the card family: SD_SPEC = 2 selects either SDHC or SDXC. SD_SPEC3 = 1 selects SDXC. Because of these redefinitions, older host devices do not correctly identify SDHC or SDXC cards nor their correct capacity.
A malfunctioning SD card can be repaired using specialized equipment, as long as the middle part, containing the flash storage, is not physically damaged. The controller can in this way be circumvented. This might be harder or even impossible in the case of monolithic card, where the controller resides on the same physical die.[142] [143]
Various passive adapters are available to allow smaller SD cards to work in larger SD card slots.
The SD format was introduced in August 1999. Like most memory card formats, SD is covered by patents and trademarks. Royalties apply to the manufacture and sale of SD cards and host adapters, with the exception of SDIO devices. As of 2025, the SD Association (SDA) charged annual membership fees of for general members and for executive members.[144]
Early versions of the SD specification were only available under a non-disclosure agreement (NDA), which restricted the development of open-source drivers. Despite these limitations, developers reverse-engineered the interface and created free software drivers for SD cards that did not use digital rights management (DRM).[145]
In 2006, the SDA began publishing a "Simplified Specification" under a less restrictive license. It includes documentation for the physical layer, SDIO, and certain extensions, allowing broader implementation without requiring an NDA or paid membership.[146] [147]
| 1.00 | 2000 | Preliminary specification | ||
| 1.01 | 2001 | Minor updates | ||
| 1.10 | 2006 | Official initial release | ||
| 2.00 | Added SDHC and Speed Classes C2, C4, and C6[148] | |||
| 3.01 | 2010 | Added SDXC, UHS-I bus and Speed Class C10/UHS Speed Class U1 | ||
| 4.10 | 2013 | Added UHS-II bus, UHS Speed Class U3, and enhanced power and function support | ||
| 5.00 | 2016 | Added Video Speed Classes V6, V10, V30, V60, and V90 | ||
| 5.10 | Added Application Performance Class A1 | |||
| 6.00 | 2017 | Added Application Performance Class A2 (with command queuing and write caching) and Card Ownership Protection | ||
| 7.10 | 2020 | Added SD Express, microSD Express, SDUC, and made CPRM optional | ||
| 8.00 | Added PCIe 4.0, added dual-lane PCIe on full-size cards | |||
| 9.00 | 2022 | Introduced new security features and enhanced write protection | ||
| 9.10 | 2023 | Added SD Express Speed Classes E150, E300, E450, and E600 |
