SD card

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Secure Digital
(SD, SDHC, SDXC, SDUC)
SD-Logo.svg
SD Cards.svg
From top to bottom: SD, miniSD, microSD
Media type Memory card
Capacity
  • SD: Up to 2 GB
  • SDHC: over 2 GB to 32 GB
  • SDXC: over 32 GB to 2 TB
  • SDUC: over 2 TB to 128 TB
Block sizeVariable
Read mechanism
  • Standard: 12.5 MB/s
  • High-speed: 25 MB/s
  • UHS-I: 50 MB/s or 104 MB/s
  • UHS-II: 156 MB/s full-duplex, or 312 MB/s half-duplex
  • UHS-III: 312 MB/s full-duplex, or 624 MB/s half-duplex
  • Express: ≥ 985 MB/s full-duplex
Write mechanismVariable
Standard SD Standard
Developed by SD Association
Dimensions
  • Standard:
  • 32.0×24.0×2.1 mm (1.260×0.945×0.083 in)
    1,612.8 mm3 (0.09842 cu in)
  • Mini:
  • 21.5×20.0×1.4 mm (0.846×0.787×0.055 in)
    602 mm3 (0.0367 cu in)
  • Micro:
  • 15.0×11.0×1.0 mm (0.591×0.433×0.039 in)
    165 mm3 (0.0101 cu in)
Weight
  • Standard: ~2 g
  • Mini: ~800 mg
  • Micro: ~250 mg
UsagePortable devices, such as digital cameras and mobile phones (including most smartphones)
Extended from MultiMediaCard
ReleasedAugust 1999

Secure Digital, officially abbreviated as SD, is a proprietary, non-volatile, flash memory card format the SD Association (SDA) developed for use in portable devices.

Contents

Because of their small physical dimensions, SD cards became widely used in many consumer electronic devices, such as digital cameras, camcorders, video game consoles, mobile phones, action cameras such as the GoPro Hero series, and camera drones. [1] [2]

The standard was introduced in August 1999 by SanDisk, Panasonic (Matsushita) and Toshiba as an improvement on MultiMediaCards (MMCs). [3] SDs have become an industry standard. The three companies formed SD-3C, LLC, a company that licenses and enforces intellectual property (IP) rights associated with SD memory cards and SD host-and-ancillary products. [4]

In January 2000, the companies formed the SD Association (SDA), a non-profit organization to create and promote SD Card standards. [5] As of 2023, the SDA has approximately 1,000 member companies. It uses several SD-3C-owned trademarked logos to enforce compliance with its specifications and denote compatibility. [6]

History

1999–2005: Creation and introduction of smaller formats

In 1999, SanDisk, Panasonic (Matsushita) and Toshiba agreed to develop and market the Secure Digital (SD) memory card. [7] The card was derived from the MultiMediaCard (MMC) [8] and provided digital rights management (DRM) based on the Secure Digital Music Initiative (SDMI) standard and a high memory density ("data/bits per physical space"), i.e. a large quantity of data could be stored in a small physical space.[ citation needed ]

SD was designed to compete with the Memory Stick, a flash storage format with DRM Sony had released the year before. Toshiba hoped the SD card’s DRM would encourage music suppliers concerned about piracy to use SD cards. [9]

The trademarked SD logo was originally developed for the Super Density Disc, which was the unsuccessful Toshiba entry in the DVD format war. For this reason, the letter "D" is styled to resemble an optical disc. [10]

At the 2000 Consumer Electronics Show (CES), the three companies announced the creation of the SD Association (SDA) to promote SD cards. The SD Association, which was headquartered in San Ramon, California, United States, then had 30 member companies and product manufacturers that made interoperable memory cards and devices. Early samples of the SD card [11] became available in the first quarter of 2000, and production quantities of 32 and 64 megabyte (MB) [a] cards became available three months later.[ citation needed ] The first 64 MB cards were offered for sale for 200 USD. [12] SD was envisioned as a single memory card format for several kinds of electronic devices, that could also function as an expansion slot for adding new capabilities for a device. [13] The first 256 MB and 512 MB SD cards were announced in 2001. [14]

miniSD

MiniSD memory card including adapter MiniSD memory card including adapter.jpg
MiniSD memory card including adapter

At March 2003 CeBIT, SanDisk Corporation introduced, announced and demonstrated the miniSD form factor. [15] The SDA adopted the miniSD card in 2003 as a small-form-factor extension to the SD card standard. While the new cards were designed for mobile phones, they were usually packaged with a miniSD adapter that provided compatibility with a standard SD memory card slot.[ citation needed ]

microSD

microSD card inserted on a smartphone Samsung Galaxy A02s microSD slot.JPG
microSD card inserted on a smartphone

MicroSD form-factor memory cards were introduced in 2004 by SanDisk at CeBIT [16] and originally called T-Flash, [17] and later TransFlash, [18] commonly abbreviated to "TF". T-Flash was renamed microSD in 2005 when it was adopted by the SDA. [19] TransFlash and microSD cards are functionally identical, allowing either to operate in devices made for the other. [20] A passive adapter allows the use of microSD and TransFlash cards in SD card slots. [20] [21]

2006–2008: SDHC and SDIO

This microSDHC card holds 8 billion bytes. Beneath it is a section of a magnetic-core memory (used until the 1970s) that holds eight bytes using 64 cores. The card covers approximately 20 bits ( .mw-parser-output .frac{white-space:nowrap}.mw-parser-output .frac .num,.mw-parser-output .frac .den{font-size:80%;line-height:0;vertical-align:super}.mw-parser-output .frac .den{vertical-align:sub}.mw-parser-output .sr-only{border:0;clip:rect(0,0,0,0);clip-path:polygon(0px 0px,0px 0px,0px 0px);height:1px;margin:-1px;overflow:hidden;padding:0;position:absolute;width:1px} 2+1/2 bytes). 8 bytes vs. 8Gbytes.jpg
This microSDHC card holds 8 billion bytes. Beneath it is a section of a magnetic-core memory (used until the 1970s) that holds eight bytes using 64 cores. The card covers approximately 20 bits (2+1⁄2 bytes).

In September 2006, SanDisk announced the 4 GB miniSDHC. [22] 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.[ citation needed ] Devices that support miniSDHC work with miniSD and miniSDHC, but devices without specific support for miniSDHC work only with the older miniSD card. Since 2008, miniSD cards are no longer produced, due to market domination of the even smaller microSD cards.[ citation needed ]

2009–2019: SDXC

Macro shot of a microSDXC memory card with eight gold plated electrical contacts MicroSDXC.64GB.P1127589.jpg
Macro shot of a microSDXC memory card with eight gold plated electrical contacts

The storage density of memory cards increased significantly[ quantify ] throughout the 2010s, allowing the earliest devices to offer support for the SD:XC standard, such as the Samsung Galaxy S III and Samsung Galaxy Note II mobile phones, to expand their available storage to several hundreds of gigabytes.

In January 2009, the SDA announced the SDXC family, which supports cards up to 2 TB [b] and speeds up to 300 MB/s. [23] SDXC cards are formatted with the exFAT file system by default. [24] SDXC was announced at the Consumer Electronics Show (CES) 2009 (January 7–10). At the same show, SanDisk and Sony also announced a comparable Memory Stick XC variant with the same 2 TB [b] maximum as SDXC, [25] and Panasonic announced plans to produce 64 GB SDXC cards. [26] On March 6, Pretec introduced the first SDXC card, [27] a 32 GB card with a read/write speed of 400 Mbit/s. But only early in 2010 did compatible host devices come onto the market, including Sony's Handycam HDR-CX55V camcorder, Canon's EOS 550D (also known as Rebel T2i) Digital SLR camera, [28] a USB card reader from Panasonic, and an integrated SDXC card reader from JMicron. [29] The earliest laptops to integrate SDXC card readers relied on a USB 2.0 bus, which does not have the bandwidth to support SDXC at full speed. [30]

In early 2010, commercial SDXC cards appeared from Toshiba (64 GB), [31] [32] Panasonic (64 GB and 48 GB), [33] and SanDisk (64 GB). [34]

In early 2011, Centon Electronics, Inc. (64 GB and 128 GB) and Lexar (128 GB) began shipping SDXC cards rated at Speed Class 10. [35] Pretec offered cards from 8 GB to 128 GB rated at Speed Class 16. [36] In September 2011, SanDisk released a 64 GB microSDXC card. [37] Kingmax released a comparable product in 2011. [38]

In April 2012, Panasonic introduced MicroP2 card format for professional video applications. The cards are essentially full-size SDHC or SDXC UHS-II cards, rated at UHS Speed Class U1. [39] [40] An adapter allows MicroP2 cards to work in current P2 card equipment. [41]

Panasonic MicroP2 cards shipped in March 2013 and were the first UHS-II compliant products on market; initial offer includes a 32 GB SDHC card and a 64 GB SDXC card. [39] [42] Later that year, Lexar released the first 256 GB SDXC card, based on 20 nm NAND flash technology. [43]

In February 2014, SanDisk introduced the first 128 GB microSDXC card, [44] which was followed by a 200 GB microSDXC card in March 2015. [45] September 2014 saw SanDisk announce the first 512 GB SDXC card. [46]

Samsung announced the world's first EVO Plus 256 GB microSDXC card in May 2016, [47] and in September 2016 Western Digital (SanDisk) announced that a prototype of the first 1 TB [c] SDXC card would be demonstrated at Photokina. [48]

In August 2017, SanDisk launched a 400 GB microSDXC card. [49]

In January 2018, Integral Memory unveiled its 512 GB microSDXC card. [50] In May 2018, PNY launched a 512 GB microSDXC card. In June 2018 Kingston announced its Canvas series of microSD cards which were capable of capacities up to 512 GB, [d] in three variations, Select, Go! and React. [51]

In February 2019, Micron and SanDisk unveiled their microSDXC cards of 1 TB capacity. [52]

2019–present: SDUC

The Secure Digital Ultra Capacity (SDUC) format supports cards up to 128 TB [b] and offers speeds up to 985 MB/s.

In April 2024, Western Digital (SanDisk) revealed the world's first 4 TB SD card at NAB 2024, which will make use of the SDUC format. It is set to release in 2025. [53]

Capacity

Secure Digital includes five card families available in three form factors. The five families are the original standard capacity (SDSC), high capacity (SDHC), extended capacity (SDXC), ultra capacity (SDUC) and SDIO, which combines input/output functions with data storage. [54] [55] [56]

Comparison of SD card capacity standards [57]
SD (SDSC)SDHCSDXCSDUC
Logo SD-Logo.svg SDHC-Logo.svg SDXC-Logo.svg SDUC.svg
Year1999200620092018
CapacityMin—>2 GB>32 GB>2 TB
Max2 GB32 GB2 TB128 TB
Typical FS FAT12/FAT16 FAT32 exFAT

SD (SDSC)

Secure Digital Standard Capacity (SD) logo. The specification defines cards with a capacity of up to 2 GB. SD-Logo.svg
Secure Digital Standard Capacity (SD) logo. The specification defines cards with a capacity of up to 2 GB.

The second-generation Secure Digital (SDSC or Secure Digital Standard Capacity) card was developed to improve on the MultiMediaCard (MMC) standard, which continued to evolve, but in a different direction. Secure Digital changed the MMC design in several ways:

Full-size SD cards do not fit into the slimmer MMC slots, and other issues also affect the ability to use one format in a host device designed for the other.[ citation needed ]

SDHC

Secure Digital High Capacity (SDHC) logo. The specification defines cards with a capacity of more than 2 GB up to 32 GB. SDHC-Logo.svg
Secure Digital High Capacity (SDHC) logo. The specification defines cards with a capacity of more than 2 GB up to 32 GB.

The Secure Digital High Capacity (SDHC) format, announced in January 2006 and defined in version 2.0 of the SD specification, supports cards with capacities up to 32 GB. [d] [54] The SDHC trademark is licensed to ensure compatibility. [59]

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

Version 2.0 also introduces a high-speed bus mode for both SDSC and SDHC cards, which doubles the original Standard Speed clock to produce 25  MB/s. [60]

SDHC host devices are required to accept older SD cards. [61] However, older host devices do not recognize SDHC or SDXC memory cards, although some devices can do so through a firmware upgrade. [62] [ better source needed ] Older Windows operating systems released before Windows 7 require patches or service packs to support access to SDHC cards. [63] [64] [65]

SDXC

Secure Digital eXtended Capacity logo. The specification defines cards with a capacity of more than 32 GB up to 2 TB. SDXC.svg
Secure Digital eXtended Capacity logo. The specification defines cards with a capacity of more than 32 GB up to 2 TB.

The Secure Digital eXtended Capacity (SDXC) format, announced in January 2009 and defined in version 3.01 of the SD specification, supports cards up to 2 TB, [b] compared to a limit of 32 GB [d] for SDHC cards in the SD 2.0 specification. SDXC adopts Microsoft's exFAT file system as a mandatory feature. [66]

Version 3.01 also introduced the Ultra High Speed (UHS) bus for both SDHC and SDXC cards, with interface speeds from 50 MB/s to 104 MB/s for four-bit UHS-I bus. [67] (this number has since been exceeded with SanDisk proprietary technology for 170 MB/s read, which is not proprietary anymore, as Lexar has the 1066x running at 160 MB/s read and 120 MB/s write via UHS 1, and Kingston also has their Canvas Go! Plus, also running at 170 MB/s). [68] [69] [70] [71]

Version 4.0, introduced in June 2011, allows speeds of 156 MB/s to 312 MB/s over the four-lane (two differential lanes) UHS-II bus, which requires an additional row of physical pins. [67]

Version 5.0 was announced in February 2016 at CP+ 2016, and added "Video Speed Class" ratings for UHS cards to handle higher resolution video formats like 8K. [72] [73] The new ratings define a minimal write speed of 90 MB/s. [74] [75]

SDUC

Secure Digital Ultra Capacity (SDUC) logo. The specification defines cards with a capacity of more than 2 TB up to 128 TB. SDUC.svg
Secure Digital Ultra Capacity (SDUC) logo. The specification defines cards with a capacity of more than 2 TB up to 128 TB.

The Secure Digital Ultra Capacity (SDUC) format, described in the SD 7.0 specification, and announced in June 2018, supports cards up to 128 TB [b] and offers speeds up to 985 MB/s, regardless of form factor, either micro or full size, or interface type including UHS-I, UHS-II, UHS-III or SD Express. [76] The SD Express interface can also be used with SDHC and SDXC cards.

exFAT filesystem

SDXC and SDUC cards are required to be formatted using exFAT, [58] but many operating systems will support others.[ citation needed ]

Windows Vista (SP1) and later [77] and OS X (10.6.5 and later) have native support for exFAT. [78] [79] (Windows XP and Server 2003 can support exFAT via an optional update from Microsoft.) [80]

Most BSD and Linux distributions did not have exFAT support for legal reasons, though in Linux kernel 5.4 Microsoft open-sourced the spec and allowed the inclusion of an exFAT driver. [81] Users of older kernels or BSD can manually install third-party implementations of exFAT (as a FUSE module) in order to be able to mount exFAT-formatted volumes. [82] However, SDXC cards can be reformatted to use any file system (such as ext4, UFS, VFAT or NTFS), alleviating the restrictions associated with exFAT availability.

Except for the change of file system, SDXC cards are mostly backward compatible with SDHC readers, and many SDHC host devices can use SDXC cards if they are first reformatted to the FAT32 file system. [83] [84] [85]

The SD Association provides a formatting utility for Windows and Mac OS X that checks and formats SD, SDHC, SDXC and SDUC cards. [86]

Speed

SD card speed is customarily rated by its sequential read or write speed. The sequential performance aspect is the most relevant for storing and retrieving large files (relative to block sizes internal to the flash memory), such as images and multimedia. Small data (such as file names, sizes and timestamps) falls under the much lower speed limit of random access, which can be the limiting factor in some use cases. [87] [88] [89]

With early SD cards, a few card manufacturers specified the speed as a "times" ("×") rating, which compared the average speed of reading data to that of the original CD-ROM drive. This was superseded by the Speed Class Rating, which guarantees a minimum rate at which data can be written to the card. [90]

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

Bus

Default Speed

SD cards will read and write at speeds of 12.5 MB/s.

High Speed

High-Speed Mode (25 MB/s) was introduced to support digital cameras with 1.10 spec version. [91]

Ultra High Speed (UHS)

The Ultra High Speed (UHS) bus is available on some SDHC and SDXC cards. [92] [93] [94]

Cards that comply with UHS show Roman numerals 'I', 'II' or 'III' next to the SD card logo, [92] [90] and report this capability to the host device. Use of UHS-I requires that the host device command the card to drop from 3.3-volt to 1.8-volt operation over the I/O interface pins and select the four-bit transfer mode, while UHS-II requires 0.4-volt operation.

The higher speed rates of UHS-II and III are achieved by using two-lane 0.4 V low-voltage differential signaling (LVDS) on a second row of pins. [95] Each lane is capable of transferring up to 156 MB/s. In full-duplex mode, one lane is used for Transmit while the other is used for Receive. In half-duplex mode both lanes are used for the same direction of data transfer allowing a double data rate at the same clock speed. In addition to enabling higher data rates, the UHS-II interface allows for lower interface power consumption, lower I/O voltage and lower electromagnetic interference (EMI).

The following ultra-high speeds are specified:

UHS-I

Specified in SD version 3.01. [96] Supports a clock frequency of 100 MHz (a quadrupling of the original "Default Speed"), which in four-bit transfer mode could transfer 50 MB/s (SDR50). UHS-I cards declared as UHS104 (SDR104) also support a clock frequency of 208 MHz, which could transfer 104 MB/s. Double data rate operation at 50 MHz (DDR50) is also specified in Version 3.01, and is mandatory for microSDHC and microSDXC cards labeled as UHS-I. In this mode, four bits are transferred when the clock signal rises and another four bits when it falls, transferring an entire byte on each full clock cycle, hence a 50 MB/s operation could be transferred using a 50 MHz clock.

There is a proprietary UHS-I extension, called DDR200, originally created by SanDisk that increases transfer speed further to 170 MB/s. Unlike UHS-II, it does not use additional pins. It achieves this by using the 208 MHz frequency of the standard SDR104 mode, but using DDR transfers. [97] [98] This extension has since then been used by Lexar for their 1066x series (160 MB/s), Kingston Canvas Go Plus (170 MB/s) and the MyMemory PRO SD card (180 MB/s).

UHS-II
Back side of a Lexar UHS-II microSDHC card, showing the additional row of UHS-II connections Lexar 1000x MicroSDHC UHS-II U3 Class 10 - Back.jpg
Back side of a Lexar UHS-II microSDHC card, showing the additional row of UHS-II connections

Specified in version 4.0, further raises the data transfer rate to a theoretical maximum of 156 MB/s (full-duplex) or 312 MB/s (half-duplex) using an additional row of pins for LVDS signalling [99] (a total of 17 pins for full-size and 16 pins for micro-size cards). [92] While first implementations in compact system cameras were seen three years after specification (2014), it took many more years until UHS-II was implemented on a regular basis. At the beginning of 2024, almost 90 DSLR and mirrorless cameras support UHS-II. [100]

UHS-III

Version 6.0, released in February 2017, added two new data rates to the standard. FD312 provides 312 MB/s while FD624 doubles that. Both are full-duplex. The physical interface and pin-layout are the same as with UHS-II, retaining backward compatibility. [101]

SD Express

Front and back of an SD Express card SD Express Highest Speed Card Standard.jpg
Front and back of an SD Express card

The SD Express bus was released in June 2018 with SD specification 7.0. It uses a single PCIe lane to provide full-duplex 985 MB/s transfer speed. Supporting cards must also implement the NVM Express storage access protocol. The Express bus can be implemented by SDHC, SDXC and SDUC cards. For legacy application use, SD Express cards must also support High-Speed bus and UHS-I bus. The Express bus re-uses the pin layout of UHS-II cards and reserves the space for additional two pins that may be introduced in the future. [102]

Hosts which implement version 7.0 of the spec allow SD Cards to do direct memory access, which increases the attack surface of the host dramatically in the face of malicious SD cards. [103]

Version 8.0 was announced on 19 May 2020, with support for two PCIe lanes with an additional row of contacts and PCIe 4.0 transfer rates, for a maximum bandwidth of 3,938 MB/s. [104]

Version 9.0 was released in February 2022. [105]

Version 9.1 was announced in October 2023. [106]

microSD Express

In February 2019, the SD Association announced microSD Express. [107] The microSD Express cards offer PCI Express and NVMe interfaces, as the June 2018 SD Express release did, alongside the legacy microSD interface for continued backwards compatibility. The SDA also released visual marks to denote microSD Express memory cards to make matching the card and device easier for optimal device performance. [108]

Bus speed comparison

Comparison of SD card bus speeds [109]
Bus interfaceBus logoBus speed PCI Express lanes Duplex Card typesSpec version
SDSDHCSDXCSDUC
Default Speed—00,012.5 MB/s— ?YesYesYesYes1.01
High Speed0025 MB/s1.10
UHS-I UHS-I.svg 0050 MB/sHalf / FullNo3.01
0104 MB/sHalf
—0180 MB/s [e] —
UHS-II UHS-II.svg 0156 MB/sFull4.00
0312 MB/sHalf
UHS-III UHS-III.svg 0312 MB/sFull6.0
0624 MB/s
SD Express SDex(short).svg
SDexpress.svg 		0985 MB/s3.1 ×17.0
1,969 MB/s3.1 ×28.0
4.0 ×1
3,938 MB/s4.0 ×2

Compatibility

Bus speed of host and card combinations (in MB/s) [110]
Host
Card
UHS-IUHS-IIUHS-IIIExpress
UHS50UHS104FullHalf
UHS-IUHS50505050505050
UHS10450104104104104104
UHS-IIFull50104156156156104
Half50104156312312104
UHS-III50104156312624104
Express50104104104104985

NOTE: If the card reader uses the DDR208 controller on the UHS 1 pins, the card reader will perform at 180 MB/s on applicable UHS 1 cards

Class

32 GB SanDisk Ultra microSDHC card (with Speed Class 10, UHS-I, UHS Speed Class 1, and Application Performance Class 1 markings) SanDisk Ultra microSDHC card, 32GB.png
32 GB SanDisk Ultra microSDHC card (with Speed Class 10, UHS-I, UHS Speed Class 1, and Application Performance Class 1 markings)
32 GB Lexar 1000x microSDHC card (with UHS-II and UHS Speed Class 3 markings) Lexar 1000x MicroSDHC UHS-II U3 Class 10 - Front.jpg
32 GB Lexar 1000x microSDHC card (with UHS-II and UHS Speed Class 3 markings)
The front and back of the Sony 64 GB SF-M Tough Series UHS-II SDXC Memory Card Sony Tough SF-M64T SDUHSIIBEIDE.png
The front and back of the Sony 64 GB SF-M Tough Series UHS-II SDXC Memory Card

The SD Association defines standard speed classes for SDHC/SDXC cards indicating minimum performance (minimum serial data writing speed). Both read and write speeds must exceed the specified value. The specification defines these classes in terms of performance curves that translate into the following minimum read-write performance levels on an empty card and suitability for different applications: [96] [90] [111] [112]

The SD Association defines three types of Speed Class ratings: the original Speed Class, UHS Speed Class and Video Speed Class.

(Original) Speed Class

Speed Class ratings 2, 4 and 6 assert that the card supports the respective number of megabytes per second as a minimum sustained write speed for a card in a fragmented state.

Class 10 asserts that the card supports 10 MB/s as a minimum non-fragmented sequential write speed and uses a High Speed bus mode. [96] The host device can read a card's speed class and warn the user if the card reports a speed class that falls below an application's minimum need. [96] By comparison, the older "×" rating measured maximum speed under ideal conditions, and was vague as to whether this was read speed or write speed.

The graphical symbol for the speed class has a number encircled with 'C' (C2, C4, C6 and C10).

UHS Speed Class

UHS-I and UHS-II cards can use UHS Speed Class rating with two possible grades: class 1 for minimum write performance of at least 10 MB/s ('U1' symbol featuring number 1 inside 'U') and class 3 for minimum write performance of 30 MB/s ('U3' symbol featuring 3 inside 'U'), targeted at recording 4K video. [113] Before November 2013, the rating was branded UHS Speed Grade and contained grades 0 (no symbol) and 1 ('U1' symbol). Manufacturers can also display standard speed class symbols (C2, C4, C6 and C10) alongside, or in place of UHS speed class.

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

Video Speed Class

Video Speed Class defines a set of requirements for UHS cards to match the modern MLC NAND flash memory [74] and supports progressive 4K and 8K video with minimum sequential writing speeds of 6 – 90 MB/s. [72] [90] [111] The graphical symbols use a stylized 'V' followed by a number designating write speed (i.e. V6, V10, V30, V60 and V90).

SD Express Speed Class

Version 9.1 of the SD specification, introduced in October 2023, defines new SD Express speed classes. The graphical symbols use a stylized 'E' followed by a number designating the minimum read/write speed. The specified classes are E150, E300, E450 and E600. [106]

Comparison

Panasonic CFEXPRESS CARD 128 GB & SAMSUNG SD CARD PRO PLUS 256 GB (2021 EDITION) FRONT.jpg
Panasonic CFEXPRESS CARD 128 GB & SAMSUNG SD CARD PRO PLUS 256 GB (2021 EDITION) REAR.jpg
Top: A 128 GB CFexpress Type B card by Panasonic (1700 MBit/s reading) next to 256 GB SD card V30 (160 MBit/s reading) by Samsung
Bottom: Bottom view showing SD card contacts
Comparison of SD card Speed Class ratings [114]
Minimum sequential
writing speed		Suggested
max. bitrate		Speed Class Video format [f]
Speed ClassUHS Speed ClassVideo Speed Class SD HD / Full HD 4K 8K
02 MB/s 15 Mbit/s SDHC Speed Class 2.svg Class 2 (C2)——YesNoNoNo
04 MB/s30 Mbit/s SDHC Speed Class 4.svg Class 4 (C4)Yes
06 MB/s45 Mbit/s SDHC Speed Class 6.svg Class 6 (C6) SD Video Speed Class 6.svg Class 6 (V6)Yes
10 MB/s75 Mbit/s SD Class 10.svg Class 10 (C10) SD UHS Speed Class 1.svg Class 1 (U1) SD Video Speed Class 10.svg Class 10 (V10)
30 MB/s220 Mbit/s SD UHS Speed Class 3.svg Class 3 (U3) SD Video Speed Class 30.svg Class 30 (V30)Yes
60 MB/s460 Mbit/s SD Video Speed Class 60.svg Class 60 (V60)
90 MB/s700 Mbit/s SD Video Speed Class 90.svg Class 90 (V90)

Application Performance Class

A 64 GB microSDXC card from Raspberry Pi (with Speed Class 30, UHS-I, UHS Speed Class 3, Video Speed Class V30 and Application Performance Class 2 markings) 26389-Raspberry-Pi-A2-Class-SD-Card-64GB-Front (cropped).jpg
A 64 GB microSDXC card from Raspberry Pi (with Speed Class 30, UHS-I, UHS Speed Class 3, Video Speed Class V30 and Application Performance Class 2 markings)

Application Performance Class is a newly defined standard from the SD Specification 5.1 and 6.0 which not only define sequential Writing Speeds but also mandates a minimum IOPS for reading and writing. Class A1 requires a minimum of 1,500 reading and 500 writing operations per second using 4 kbytes blocks, while class A2 requires 4,000 and 2,000 IOPS. [115] A2 class cards require host driver support as they use command queuing and write caching to achieve their higher speeds. Without they are guaranteed to at least reach A1 speeds. As of Linux kernel 5.15, it fully supports A2. [116]

Comparison of SD card Application Performance Class ratings [117]
NameMinimum random IOPS Minimum sustained
sequential writing
ReadWrite
SD App Performance 1.svg Application Performance Class 1 (A1)1,500 IOPS0500 IOPS10 MB/s
SD App Performance 2.svg Application Performance Class 2 (A2)4,000 IOPS2,000 IOPS

"×" rating

RatingApprox.
(MB/s)		Comparable
speed class
16×2.34 SDHC Speed Class 2.svg (13×)
32×4.69 SDHC Speed Class 4.svg (27×)
48×7.03 SDHC Speed Class 6.svg (40×)
100×14.6 SD Class 10.svg (67×)

The "×" rating, which was used by some card manufacturers and made obsolete by speed classes, is a multiple of the standard CD-ROM drive speed of 150  KB/s [g] (approximately 1.23  Mbit/s). Basic cards transfer data at up to six times (6×) the CD-ROM speed; that is, 900 kbit/s or 7.37 Mbit/s. The 2.0 specification[ clarification needed ] defines speeds up to 200×, but is not as specific as Speed Classes are on how to measure speed. Manufacturers may report best-case speeds and may report the card's fastest read speed, which is typically faster than the write speed. Some vendors, including Transcend and Kingston, report their cards' write speed. [118] When a card lists both a speed class and an "×" rating, the latter may be assumed a read speed only.[ citation needed ]

Real-world performance

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

The speed class rating does not totally characterize card performance. Different cards of the same class may vary considerably while meeting class specifications. A card's speed depends on many factors, including:

In addition, speed may vary markedly between writing a large amount of data to a single file (sequential access, as when a digital camera records large photographs or videos) and writing a large number of small files (a random-access use common in smartphones). A study in 2012 found that, in this random-access use, some Class 2 cards achieved a write speed of 1.38  MB/s, while all cards tested of Class 6 or greater (and some of lower Classes; lower Class does not necessarily mean better small-file performance), including those from major manufacturers, were over 100 times slower. [87] In 2014, a blogger measured a 300-fold performance difference on small writes; this time, the best card in this category was a class 4 card. [88]

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. [119] [120]

Write-protect notch

Unlocked and locked SD cards SD card unlocked and locked.svg
Unlocked and locked SD cards
Sony 64 GB SF-M Tough Series UHS-II SDXC Memory Card is one of the few cards in the market without a sliding tab on the write protect notch. Vorderseite Sony Tough SF-M64T SDUHSII011CORRklein.png
Sony 64 GB SF-M Tough Series UHS-II SDXC Memory Card is one of the few cards in the market without a sliding tab on the write protect notch.

Most full-size SD cards have a "mechanical write protect switch" allowing the user to advise the host computer that the user wants the device to be treated as read-only. This does not protect the data on the card if the host is compromised: "It is the responsibility of the host to protect the card. The position [i.e., setting] of the write protect switch is unknown to the internal circuitry of the card." [121] Some host devices do not support write protection, which is an optional feature of the SD specification, and drivers and devices that do obey a read-only indication may give the user a way to override it.[ citation needed ]

The switch is a sliding tab that covers a notch in the card. The miniSD and microSD formats do not directly support a write protection notch, but they can be inserted into full-size adapters which do.[ citation needed ]

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

On the left side, there may be a write-protection notch. If the notch is omitted, the card can be read and written. If the card is notched, it is read-only. If the card has a notch and a sliding tab which covers the notch, the user can slide the tab upward (toward the contacts) to declare the card read/write, or downward to declare it read-only.[ citation needed ] The diagram to the right shows an orange sliding write-protect tab in both the unlocked and locked positions.[ citation needed ]

Cards sold with content that must not be altered are permanently marked read-only by having a notch and no sliding tab.[ citation needed ]

Card password

MicroSD-to-SD adapter (left), microSD-to-miniSD adapter (middle), microSD card (right) Sdadaptersandcards.jpg
MicroSD-to-SD adapter (left), microSD-to-miniSD adapter (middle), microSD card (right)

A host device can lock an SD card using a password of up to 16 bytes, typically supplied by the user.[ citation needed ] A locked card interacts normally with the host device except that it rejects commands to read and write data.[ citation needed ] 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.[ citation needed ]

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". [122] 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. [123]

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. [124]

Some of the earliest versions of microSD memory cards with secure elements were developed in 2009 by DeviceFidelity, Inc., [125] [126] 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. [127] 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. [128] [129] [130]

Various implementations of smartSD cards have been done for payment applications and secured authentication. [131] [132] In 2012 Good Technology partnered with DeviceFidelity to use microSD cards with secure elements for mobile identity and access control. [133]

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, [134] [135] [136] while others were part of innovative new direct-to-consumer neobank programs such as moneto, first launched in 2012. [137] [138] [139] [140]

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. [141] 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. [142] [143] This was the first time regular mobile phones could be used to open doors without need for electronic access keys.

Vendor enhancements

Eye-Fi Wifi Card.jpg
Eye-Fi Mobi 16 GB
FlashAir 014 (15247275839) (cropped).jpg
Toshiba FlashAir 16 GB
Pqi-air-card Kai Xiang (8536928274) (cropped).jpg
PQI air card 4 GB
Transcend WiFi SD-Card.jpg
Transcend Wi-Fi 16 GB
SD cards with dual interfaces: SD and USB USB-SD-Cards.jpg
SD cards with dual interfaces: SD and USB

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

SDIO cards

Secure Digital Input Output (SDIO) logo. SDIO Vector.svg
Secure Digital Input Output (SDIO) logo.
Camera using the SDIO interface to connect to some HP iPAQ devices HP PhotoSmart SDIO Kamera (cropped).jpg
Camera using the SDIO interface to connect to some HP iPAQ devices

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

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

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

The SDIO family comprises Low-Speed and Full-Speed cards. Both types of SDIO cards support Serial Peripheral Interface (SPI) and one-bit SD bus types. Low-Speed SDIO cards are allowed to also support the four-bit SD bus; Full-Speed SDIO cards are required to support the four-bit SD bus. To use an SDIO card as a "combo card" (for both memory and I/O), the host device must first select four-bit SD bus operation. Two other unique features of Low-Speed SDIO are a maximum clock rate of 400 kHz for all communications, and the use of Pin 8 as "interrupt" to try to initiate dialogue with the host device. [149]

Compatibility

Wi-Fi/DAB+/FM/CD/MP3/WMA/USB/SDHC/podcast stereo recorder with iPod dock Stream64i.jpg
Wi-Fi/DAB+/FM/CD/MP3/WMA/USB/SDHC/podcast stereo recorder with iPod dock

Host devices that comply with newer versions of the specification provide backward compatibility and accept older SD cards. [61] 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, [55] there are several factors that arise:

SD compatibility table
Card
Slot
SDSCSDHCSDHC
UHS		SDXCSDXC
UHS		SDIO
SDSCPartial [h] FAT16, < 4 GB [h] FAT16, < 4 GB [h] NoNoNo
SDHCYesYesIn non-UHS modeFAT32FAT32 in non-UHS modeNo
SDHC UHSIn non-UHS modeIn non-UHS modeIn UHS modeFAT32 in non-UHS modeFAT32 in UHS modeNo
SDXCYesYesIn non-UHS modeYesIn non-UHS modeNo
SDXC UHSIn non-UHS modeIn non-UHS modeIn UHS modeIn non-UHS modeIn UHS modeNo
SDIOVariesVariesVariesVariesVariesYes

Markets

An internal microSD card reader, taken from a Nintendo Switch Micro-SD-Kartenleser der Nintendo Switch 20230405 HOF09182 RAW-Export.png
An internal microSD card reader, taken from a Nintendo Switch

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 such as the GoPro Hero series, and camera drones, tend to use microSD cards. [1] [2]

Mobile phones

The microSD card has helped propel the smartphone market by giving both manufacturers and consumers greater flexibility and freedom.

While cloud storage depends on stable internet connection and sufficiently voluminous data plans, memory cards in mobile devices provide location-independent and private storage expansion with much higher transfer rates and no network delay, enabling applications such as photography and video recording. While data stored internally on bricked devices is inaccessible, data stored on the memory card can be salvaged and accessed externally by the user as mass storage device. A benefit over USB on the go storage expansion is uncompromised ergonomy. The usage of a memory card also protects the mobile phone's non-replaceable internal storage from weardown from heavy applications such as excessive camera usage and portable FTP server hosting over WiFi Direct. Due to the technical development of memory cards, users of existing mobile devices are able to expand their storage further and priceworthier with time. [150] [151] [152]

Recent versions of major operating systems such as Windows Mobile and Android allow applications to run from microSD cards, creating possibilities for new usage models for SD cards in mobile computing markets, as well as clearing available internal storage space. [153]

SD cards are not the most economical solution in devices that need only a small amount of non-volatile memory, such as station presets in small radios. They may also not present the best choice for applications that require higher storage capacities or speeds as provided by other flash card standards such as CompactFlash. These limitations may be addressed by evolving memory technologies, such as the new SD 7.0 specifications which allow storage capabilities of up to 128 TB. [b] [154]

Many personal computers of all types, including tablets and mobile phones, use SD cards, either through built-in slots or through an active electronic adapter. Adapters exist for the PC card, ExpressBus, USB, FireWire and the parallel printer port. Active adapters also let SD cards be used in devices designed for other formats, such as CompactFlash. The FlashPath adapter lets SD cards be used in a floppy disk drive.

Some devices such as the Samsung Galaxy Fit (2011) and Samsung Galaxy Note 8.0 (2013) have an SD card compartment located externally and accessible by hand, while it is located under the battery cover on other devices. More recent mobile phones use a pin-hole ejection system for the tray which houses both the memory card and SIM card.

Counterfeits

Samsung Pro 64 GB microSDXC original (left) and counterfeit (right): The counterfeit claims to have 64 GB in capacity, but only 8 GB (Class 4 speed) are usable: When trying to write more than 8 GB, data loss occurs. Also used for SanDisk 64 GB fakes. Samsung Pro 64gb micro-SDXC original and falsification front and back.jpg
Samsung Pro 64 GB microSDXC original (left) and counterfeit (right): The counterfeit claims to have 64 GB in capacity, but only 8 GB (Class 4 speed) are usable: When trying to write more than 8 GB, data loss occurs. Also used for SanDisk 64 GB fakes.
Images of genuine, questionable and counterfeit microSD (Secure Digital) cards before and after decapsulation. Details at source, photo by Andrew Huang Decapsulated microSD memory card lineup-genuine, questionable, and fake-counterfeit.jpg
Images of genuine, questionable and counterfeit microSD (Secure Digital) cards before and after decapsulation. Details at source, photo by Andrew Huang

Commonly found on the market are mislabeled or counterfeit Secure Digital cards that report a fake capacity or run slower than labeled. [155] [156] [157] Software tools exist to check and detect counterfeit products, [158] [159] [160] and in some cases it is possible to repair these devices to remove the false capacity information and use its real storage limit. [161]

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).

Digital cameras

SD card in a DSLR camera Pentax K10D with SanDiskSD card.jpg
SD card in a DSLR camera

Secure Digital memory cards can be used in Sony XDCAM EX camcorders with an adapter. [162]

Personal computers

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 [163] 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 ergonomical 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. [164]

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). [165]

Embedded systems

A shield (daughterboard) that gives Arduino prototyping microprocessors access to SD cards Arduino Ethernet Shield (pre-production sample).jpg
A shield (daughterboard) that gives Arduino prototyping microprocessors access to SD cards

In 2008, the SDA specified Embedded SD, "leverag[ing] well-known SD standards" to enable non-removable SD-style devices on printed circuit boards. [166] 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. [167]

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 1.6 Mbit/s throughput). [168]

Music distribution

Prerecorded microSDs have been used to commercialize music under the brands slotMusic and slotRadio by SanDisk and MQS by Astell & Kern.

Technical details

microSD card and standard SD card adapter
SD card adapter.jpg
SD card adapters.jpg

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

Size comparison of families: SD (blue), miniSD (green), microSD (red) MicroSD MemoryCard 002.jpg
Size comparison of families: SD (blue), miniSD (green), microSD (red)
  • SD (SDSC), SDHC, SDXC, SDIO, SDUC
  • 32 mm × 24 mm × 2.1 mm (1+17⁄64 in × 15⁄16 in × 5⁄64 in)
  • 32 mm × 24 mm × 1.4 mm (1+17⁄64 in × 15⁄16 in × 1⁄16 in) (as thin as MMC) for Thin SD (rare)

MiniSD

  • miniSD, miniSDHC, miniSDIO
  • 21.5 mm × 20 mm × 1.4 mm (27⁄32 in × 25⁄32 in × 1⁄16 in)

microSD

The micro form factor is the smallest SD card format. [169]

  • microSD, microSDHC, microSDXC, microSDUC
  • 15 mm × 11 mm × 1 mm (19⁄32 in × 7⁄16 in × 3⁄64 in)

Transfer modes

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

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. [170]

Official pin numbers for each card type (top to bottom): MMC, SD, miniSD, microSD. This shows the evolution from the older MMC, on which SD is based. NOTE: This drawing does not show 8 new UHS-II contacts that were added in spec 4.0. MMC-SD-miniSD-microSD-Color-Numbers-Names.gif
Official pin numbers for each card type (top to bottom): MMC, SD, miniSD, microSD. This shows the evolution from the older MMC, on which SD is based. NOTE: This drawing does not show 8 new UHS-II contacts that were added in spec 4.0.
SPI bus mode
MMC
pin		SD
pin		miniSD
pin		microSD
pin		NameI/OLogicDescription
1112nCSIPPSPI Card Select [CS] (Negative logic)
2223DIIPPSPI Serial Data In [MOSI]
333VSSSSGround
4444VDDSSPower
5555CLKIPPSPI Serial Clock [SCLK]
6666VSSSSGround
7777DOOPPSPI Serial Data Out [MISO]
888NC
nIRQ		.
O		.
OD		Unused (memory cards)
Interrupt (SDIO cards) (negative logic)
991NC..Unused
10NC..Reserved
11NC..Reserved
One-bit SD bus mode
MMC
pin		SD
pin		miniSD
pin		microSD
pin		NameI/OLogicDescription
1112CDI/O.Card detection (by host) and
non-SPI mode detection (by card)
2223CMDI/OPP,
OD		Command,
Response
333VSSSSGround
4444VDDSSPower
5555CLKIPPSerial clock
6666VSSSSGround
7777DAT0I/OPPSD Serial Data 0
888NC
nIRQ		.
O		.
OD		Unused (memory cards)
Interrupt (SDIO cards) (negative Logic)
991NC..Unused
10NC..Reserved
11NC..Reserved
Four-bit SD bus mode
MMC
pin		SD
pin		miniSD
pin		microSD
pin		NameI/OLogicDescription
.112DAT3I/OPPSD Serial Data 3
.223CMDI/OPP,
OD		Command,
Response
.33VSSSSGround
.444VDDSSPower
.555CLKIPPSerial clock
.666VSSSSGround
.777DAT0I/OPPSD Serial Data 0
888DAT1
nIRQ		I/O
O		PP
OD		SD Serial Data 1 (memory cards)
Interrupt Period (SDIO cards share pin via protocol)
991DAT2I/OPPSD Serial Data 2
10NC..Reserved
11NC..Reserved

Notes:

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

Interface

Inside a 512 MB SD card: NAND flash chip that holds the data (bottom) and SD controller (top) Sd insides.png
Inside a 512 MB SD card: NAND flash chip that holds the data (bottom) and SD controller (top)
Inside a 2 GB SD card: two NAND flash chips (top and middle), SD controller chip (bottom) SD-extreMEmory 2GB alt innen.jpg
Inside a 2 GB SD card: two NAND flash chips (top and middle), SD controller chip (bottom)
Inside a 16 GB SDHC card Pretec 16GB SDHC without cover 20090420.jpg
Inside a 16 GB SDHC card

Command interface

SD cards and host devices initially communicate through a synchronous one-bit interface, where the host device provides a clock signal that strobes single bits in and out of the SD card. The host device thereby sends 48-bit commands and receives responses. The card can signal that a response will be delayed, but the host device can abort the dialogue. [96]

Through issuing various commands, the host device can: [96]

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

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

Electrical interface

All SD card families initially use a 3.3  volt electrical interface. On command, SDHC and SDXC cards can switch to 1.8 V operation. [96]

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. [96]

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. [96] 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:

  • For SDSC cards:
  • For SDHC cards:
    • Capacity of less than 16,450,560 logical sectors (smaller than 7.8 GB): FAT32 with partition type 0Bh and EBPB 7.1
    • Capacity of at least 16,450,560 logical sectors (larger than 7.8 GB): FAT32 with partition type 0Ch and EBPB 7.1
  • For SDXC cards: exFAT with partition type 07h

Most consumer products that take an SD card expect that it is partitioned and formatted in this way. Universal support for FAT12, FAT16, FAT16B 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.[ citation needed ]

On such SD cards, standard utility programs such as Mac OS X's " Disk Utility " or Windows' SCANDISK can be used to repair a corrupted filing system and sometimes recover deleted files. Defragmentation tools for FAT file systems may be used on such cards. The resulting consolidation of files may provide a marginal improvement in the time required to read or write the file, [172] 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.[ citation needed ] 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.[ citation needed ]

When reformatting an SD card with a capacity of at least 32 MB [i] (65,536 logical sectors or more), but not more than 2 GB, [d] FAT16B with partition type 06h and EBPB 4.1 [171] is recommended if the card is for a consumer device. (FAT16B is also an option for 4 GB cards, but it requires the use of 64 KB clusters, which are not widely supported.) FAT16B does not support cards above 4 GB [d] at all.

The SDXC specification mandates the use of Microsoft's proprietary exFAT file system, [173] which sometimes requires appropriate drivers (e.g. exfat-utils/exfat-fuse on Linux).

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. [174] 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. [175]

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. [175]

Power consumption

The power consumption of SD cards varies by its speed mode, manufacturer and model.[ citation needed ]

During transfer it may be in the range of 66–330 mW (20–100 mA at a supply voltage of 3.3 V). Specifications from TwinMOS Technologies list a maximum of 149 mW (45 mA) during transfer. Toshiba lists 264–330 mW (80–100 mA). [176] Standby current is much lower, less than 0.2 mA for one 2006 microSD card. [177] If there is data transfer for significant periods, battery life may be reduced noticeably; for reference, the capacity of smartphone batteries is typically around 6 Wh (Samsung Galaxy S2: 1650 mAh @ 3.7 V).

Modern UHS-II cards can consume up to 2.88 W, if the host device supports bus speed mode SDR104 or UHS-II. Minimum power consumption in the case of a UHS-II host is 720 mW.[ citation needed ]

Card requirements in different bus speed modes [178]
Bus speed
mode		Max. bus
speed
[MB/s]		Max. clock
frequency
[MHz]		Signal
voltage
[V]		SDSC
[W]		SDHC
[W]		SDXC
[W]
HD312312520.4–2.882.88
FD156156520.4–2.882.88
SDR1041042081.8–2.882.88
SDR50501001.8–1.441.44
DDR5050501.8–1.441.44
SDR2525501.8–0.720.72
SDR1212.5251.8–0.360.36 / 0.54
High Speed25503.30.720.720.72
Default Speed12.5253.30.330.360.36 / 0.54

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, [179] 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. [180] [181]

SDSC cards above 1 GB

4 GB SDSC card Sd4gy crop.jpg
4 GB SDSC card

SD version 1.00 assumed 512 bytes per block. This permitted SDSC cards up to 4,096 × 512 × 512 B = 1 GB. [d]

Version 1.01 let an SDSC card use a 4-bit field to indicate 1,024 or 2,048 bytes per block instead. [96] Doing so enabled cards with 2 GB and 4 GB capacity, such as the Transcend 4 GB SD card, the Memorette 4 GB SD card and the Hoco 4 GB microSD card.[ citation needed ]

Storage capacity calculations

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

Version 1

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

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

Later versions state (at Section 4.3.2) that a 2 GB SDSC card shall set its READ_BL_LEN (and WRITE_BL_LEN) to indicate 1,024 bytes, so that the above computation correctly reports the card's capacity, but that, for consistency, the host device shall not request (by CMD16) block lengths over 512 B. [96]

Versions 2 and 3

In the definition of SDHC cards in version 2.0, the C_SIZE portion of the CSD is 22 bits and it indicates the memory size in multiples of 512 KB (the C_SIZE_MULT field is removed and READ_BL_LEN is no longer used to compute capacity). Two bits that were formerly reserved now identify the card family: 0 is SDSC; 1 is SDHC or SDXC; 2 and 3 are reserved. [96] Because of these redefinitions, older host devices do not correctly identify SDHC or SDXC cards nor their correct capacity.

  • SDHC cards are restricted to reporting a capacity not over 32 GB.[ citation needed ]
  • SDXC cards are allowed to use all 22 bits of the C_SIZE field. An SDHC card that did so (reported C_SIZE > 65,375 to indicate a capacity of over 32 GB) would violate the specification. A host device that relied on C_SIZE rather than the specification to determine the card's maximum capacity might support such a card, but the card might fail in other SDHC-compatible host devices.[ citation needed ]

Capacity is calculated thus:

Capacity = (C_SIZE + 1) × 524288 where for SDHC  4112 ≤ C_SIZE ≤ 65375  ≈2 GB ≤ Capacity ≤ ≈32 GB where for SDXC  65535 ≤ C_SIZE  ≈32 GB ≤ Capacity ≤ 2 TB[ citation needed ]

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

Openness of specification

Dismantled microSD to SD adapter showing the passive connection from the microSD card slot on the bottom to the SD pins on the top MicroSD to SD adapter, disassembled.png
Dismantled microSD to SD adapter showing the passive connection from the microSD card slot on the bottom to the SD pins on the top

Like most memory card formats, SD is covered by numerous patents and trademarks. Excluding SDIO cards, royalties for SD card licenses are imposed for manufacture and sale of memory cards and host adapters (US$1,000/year plus membership at US$1,500/year)[ citation needed ]

Early versions of the SD specification were available under a non-disclosure agreement (NDA) prohibiting development of open-source drivers. However, the system was eventually reverse-engineered and free software drivers provided access to SD cards not using DRM. Subsequent to the release of most open-source drivers, the SDA provided a simplified version of the specification under a less restrictive license helping reduce some incompatibility issues. [182]

Under a disclaimers agreement, the simplified specification released by the SDA in 2006 – as opposed to that of SD cards – was later extended to the physical layer, ASSD extensions, SDIO and SDIO Bluetooth Type-A. [183]

The Simplified Specification [184] is available.

Again, most of the information had already been discovered and Linux had a fully free driver for it. Still, building a chip conforming to this specification caused the One Laptop per Child project to claim "the first truly Open Source SD implementation, with no need to obtain an SDI license or sign NDAs to create SD drivers or applications." [185]

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

The SPI-bus interface mode is the only type that does not require a host license for accessing SD cards.[ citation needed ]

Size comparison of various flash cards: SD, CompactFlash, MMC, xD Flash memory cards size.jpg
Size comparison of various flash cards: SD, CompactFlash, MMC, xD

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. [186] [187]

See also

Footnotes

  1. ↑ except where indicated otherwise, 1 MB equals one million bytes
  2. 1 2 3 4 5 6 here, 1 TB = 10244 B
  3. ↑ except where stated otherwise, in this article 1 TB = 1000 billion bytes
  4. 1 2 3 4 5 6 7 here, 1 GB = 1 GiB = 230 B
  5. ↑ This speed is achievable using DDR208 controller.
  6. ↑ The necessary recording and playback speed class requirements may vary by device.
  7. ↑ 1 KB = 1024 B
  8. 1 2 3 See discussion about storage capacity and compatibilities.
  9. 1 2 3 4 here, MB = 10242 B

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