SD card — BrandWiki

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]

History

Origins and standardization

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.[3]

To address these challenges, SanDisk partnered with Siemens and Nokia in 1996 to develop a new postage stamp-sized memory card called the MultiMediaCard

Capacity standards

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]

SD (SDSC)

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 2 GB 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.

Bus marks

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.

Default Speed

The original SD bus interface, introduced with version 1.00 of the SD specification, supported a maximum transfer rate of 12.5 MB/s. This mode is referred to as Default Speed.

High Speed

With version 1.10 of the specification, the SD Association introduced High-Speed mode, which increased the maximum transfer rate to 25 MB/s. This enhancement was designed to meet the growing performance requirements of devices such as digital cameras.

UHS (Ultra High Speed)

The Ultra High Speed (UHS) bus interface enables faster data transfer on SDHC, SDXC and SDUC cards.

Card speed class ratings

Speed Class ratings were introduced to indicate the minimum data transfer performance of an SD card (as opposed to bus speed rating, which indicates device performance) in terms of sustained sequential write performance. This performance is important when transferring large files, especially during tasks like video recording, which requires consistent throughput to avoid dropped frames.[48]

Where speed classes overlap, manufacturers often display multiple symbols on the same card to indicate compatibility with different host devices and standards.

Original speed class (C)

The original speed class ratings—Class 2, 4, 6, and 10—specify minimum sustained write speeds of 2, 4, 6, and 10 MB/s, 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).

UHS speed class (U)

Ultra High Speed (UHS) speed class ratings—U1 and U3—specify minimum sustained write speeds of 10 MB/s, respectively. These classes are represented by a number inside a "U" and are designed for high-bandwidth tasks such as 4K video recording.[67]

Performance ratings

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.[79] 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][77][80]

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.[79]

Features

Card security

Commands to disable writes

The host device can command the SD card to become read-only (to reject subsequent commands to write information to it). There are both reversible and irreversible host commands that achieve this.[84][85]

Write-protect notch

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.[86]

Markets

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.[117][118]

Mobile phones

microSD cards have been widely used in mobile phones to expand storage, particularly for photos, videos, music, documents, and other infrequently accessed files.[119]

By 2015, support for microSD cards was common in Android smartphones.[120] However, by 2025, support had declined, particularly in high-end models.

Technical details

Physical size

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.

Standard

SD (SDSC), SDHC, SDXC, SDIO, SDUC
32 x
32 x (as thin as MMC) for Thin SD (rare)

MiniSD

miniSD, miniSDHC, miniSDIO
21.5 x

microSD

Storage capacity and compatibilities

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

By the time the version 2.0 (SDHC) specification was completed in June 2006, vendors had already devised 2 GB and 4 GB SD cards, either as specified in Version 1.01, or by creatively reading Version 1.00. The resulting cards do not work correctly in some host devices.[150][151]

SDSC cards above 1 GB

SD version 1.00 assumed 512 bytes per block. This permitted SDSC cards up to 1 GB.

Version 1.01 let an SDSC card use a 4-bit field to indicate 1,024 bytes per block instead. Doing so enabled cards with 2 GB capacity.

Storage capacity calculations

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

Data recovery

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.[152][153]

Adapters

Various passive adapters are available to allow smaller SD cards to work in larger SD card slots.

Openness of specification

The SD format was introduced in August 1999.[5] 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 US$2500 2025 for general members and US$4500 2025 for executive members.[154]

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).[155]

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.[156]

Revisions

External links

References

Matsushita Electric, SanDisk and Toshiba to Form SD Association to Promote Next Generation SD Memory Card Toshiba, 2015-03-30, retrieved 2016-02-23^
Using SD Memory Cards is Easy SD Association, 2010-06-22, retrieved 2014-01-02^
Ronni Shendar. The Invention of the SD Card: When Tiny Storage Met Tech Giants

SD Express

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 985 MB/s. 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.[59] 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.[60]

In February 2019, the SD Association introduced microSD Express, along with updated visual marks to help users identify compatible cards and devices.[61][62]

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 3,938 MB/s using dual-lane PCIe 4.0.[63] 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.[64] 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.[65]

As of June 2025, 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.[55]

smartSD cards

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.[91]

Some of the earliest versions of microSD memory cards with secure elements were developed in 2009 by DeviceFidelity, Inc.,[92][93] 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.[94] 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.[95][96][97]

Various implementations of smartSD cards have been done for payment applications and secured authentication.[98][99] In 2012 Good Technology partnered with DeviceFidelity to use microSD cards with secure elements for mobile identity and access control.[100]

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,[101][102][103] while others were part of innovative new direct-to-consumer neobank programs such as moneto, first launched in 2012.[104][105][106][107]

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.[108] 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.[109][110] This was the first time regular mobile phones could be used to open doors without need for electronic access keys.

Interface

Command interface

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

Through issuing various commands, the host device can:

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.

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

Electrical interface

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

MBR and FAT

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

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,[142] 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.[143] 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.

Risks of reformatting

Reformatting an SD card with a different file system, or even with the same one, may make the card slower, or shorten its lifespan. Some cards use wear leveling, in which frequently modified blocks are mapped to different portions of memory at different times, and some wear-leveling algorithms are designed for the access patterns typical of FAT12, FAT16 or FAT32.[145] 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.[146]

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.[146]

weight
class	skin-invert
type	Memory card
blocksize	Variable
owner	SD Association
dimensions	Standard: 32 * 1612.8 mm3 Mini: 21.5 * 602 mm3 Micro: 15 * 165 mm3

History

Origins and standardization

Capacity standards

SD (SDSC)

Bus marks

Default Speed

High Speed

UHS (Ultra High Speed)

Card speed class ratings

Original speed class (C)

UHS speed class (U)

Performance ratings

Features

Card security

Commands to disable writes

Write-protect notch

Markets

Mobile phones

Technical details

Physical size

Standard

MiniSD

microSD

Storage capacity and compatibilities

SDSC cards above 1 GB

Storage capacity calculations

Data recovery

Adapters

Openness of specification

Revisions

See also

External links

References

Smaller formats

Increasing storage density

SDHC

SDXC

SDUC

UHS-I

UHS-II

UHS-III

UHS-I

UHS-II

UHS-III

SD Express

Video speed class (V)

SD Express Speed Class (E)

"×" rating

Real-world performance

Card password

Commands to disable writes

Write-protect notch

Card password

smartSD cards

Vendor enhancements

SDIO cards

Compatibility

Digital cameras

Personal computers

Embedded systems

Music distribution

Counterfeits

Standard

MiniSD

microSD

Transfer modes

Interface

Command interface

Electrical interface

MBR and FAT

Risks of reformatting

Command interface

Electrical interface

MBR and FAT

Risks of reformatting

Power consumption

Version 1

Versions 2 and 3

Version 1

Versions 2 and 3