 | |
 From top to bottom: SD, miniSD, microSD | |
| Media type | Memory card |
|---|---|
| Capacity |
|
| Block size | Variable |
| Read mechanism |
|
| Write mechanism | Variable |
| Standard | SD Standard |
| Developed by | SD Association |
| Dimensions |
|
| Weight |
|
| Usage | Portable devices, such as digital cameras and mobile phones (including many smartphones) |
| Extended from | MultiMediaCard |
| Released | August 1999 |
Secure Digital, officially abbreviated asSD, is aproprietary,non-volatile,flash memory card format theSD Association (SDA) developed for use in portable devices.
Because of their small physical dimensions, SD cards became widely used in many consumer electronic devices, such asdigital cameras,camcorders,video game consoles,mobile phones,action cameras such as theGoPro Hero series, andcamera drones.[1][2]
The standard was introduced in August 1999 bySanDisk,Panasonic (Matsushita) andToshiba as an improvement onMultiMediaCards (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 theSD Association (SDA), a non-profit organization to create and promote SD card standards.[5] As of 2023[update], 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]
In 1999,SanDisk,Panasonic (Matsushita) andToshiba agreed to develop and market the Secure Digital (SD) memory card.[7] The card was derived from theMultiMediaCard (MMC)[8] and provideddigital rights management (DRM) based on theSecure Digital Music Initiative (SDMI) standard and a highmemory 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 theMemory Stick, a flash storage format with DRMSony 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 theSuper Density Disc, which was the unsuccessful Toshiba entry in theDVD format war. For this reason, the letter "D" is styled to resemble an optical disc.[10]
At the 2000Consumer Electronics Show (CES), the three companies announced the creation of theSD Association (SDA) to promote SD cards. The SD Association, which was headquartered inSan 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 64megabyte (MB)[a] cards became available three months later.[citation needed] The first 64 MB cards were offered for sale for US$200.[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]
At March 2003CeBIT, SanDisk Corporation introduced, announced and demonstrated theminiSD 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 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]
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]
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 theSamsung Galaxy S III andSamsung Galaxy Note II mobile phones, to expand their available storage to several hundreds ofgigabytes.
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 theexFAT file system by default.[24] SDXC was announced at theConsumer Electronics Show (CES) 2009 (January 7–10). At the same show,SanDisk andSony also announced a comparableMemory Stick XC variant with the same 2 TB[b] maximum as SDXC,[25] andPanasonic 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, includingSony'sHandycam HDR-CX55Vcamcorder,Canon'sEOS 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 fromToshiba (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) andLexar (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 introducedMicroP2 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 currentP2 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 nmNAND 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 2016Western Digital (SanDisk) announced that a prototype of the first 1 TB[c] SDXC card would be demonstrated atPhotokina.[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 2018Kingston 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 andSanDisk unveiled their microSDXC cards of 1 TB capacity.[52]
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 atNAB 2024, which will make use of the SDUC format. It is set to release in 2025.[53]
Secure Digital includes five card families available inthree form factors. The five families are the original standard capacity (SDSC), high capacity (SDHC), extended capacity (SDXC), ultra capacity (SDUC) andSDIO, which combinesinput/output functions with data storage.[54][55][56]
| SD | SDHC | SDXC | SDUC | ||
|---|---|---|---|---|---|
| Logo | |  |  |  | |
| Capacity | Minimum | — | 2 GB | 32 GB | 2 TB |
| Maximum | 2 GB | 32 GB | 2 TB | 128 TB | |
| File system | FAT12,FAT16 | FAT32 | exFAT | ||
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The second-generation Secure Digital (SDSC or Secure Digital Standard Capacity) card was developed to improve on theMultiMediaCard (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]
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 (seebelow), and the fact that SDHC cards are shipped preformatted with theFAT32 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]
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'sexFAT 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 like8K.[72][73] The new ratings define a minimal write speed of 90 MB/s.[74][75]
SDXC cards are required to be formatted usingexFAT,[58] but many operating systems will support others.[citation needed]
Windows Vista (SP1) and later[76] andOS X (10.6.5 and later) have native support for exFAT.[77][78] (Windows XP and Server 2003 can support exFAT via an optional update from Microsoft.)[79]
MostBSD andLinux 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.[80] Users of older kernels or BSD can manually install third-party implementations of exFAT (as aFUSE module) in order to be able to mount exFAT-formatted volumes.[81] However, SDXC cards can be reformatted to use any file system (such asext4,UFS,VFAT orNTFS), alleviating the restrictions associated with exFAT availability.
The SD Association provides a formatting utility for Windows and Mac OS X that checks and formats SD, SDHC, SDXC and SDUC cards.[82]
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 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] regardless of form factor, either micro or full size, or interface type including UHS-I, UHS-II, UHS-III or SD Express.[86]
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 theflash memory), such as images and multimedia. Small data (such as file names, sizes and timestamps) falls under the much lowerspeed 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 originalCD-ROM drive. This was superseded by theSpeed 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 | Card standards | Specification | |||||||
|---|---|---|---|---|---|---|---|---|---|
| Interface | Logo | Speed | PCI Express | Duplex | SD | SDHC | SDXC | SDUC | |
| Default Speed | — | 12.5 MB/s | — | Half | Yes | Yes | Yes | Yes | 1.01 |
| High Speed | — | 25 MB/s | 1.10 | ||||||
| UHS-I |  | 50 MB/s | Half, Full | No | 3.01 | ||||
| 104 MB/s | Half | ||||||||
| UHS-II |  | 156 MB/s | Full | 4.00 | |||||
| 312 MB/s | Half | ||||||||
| UHS-III |  | 312 MB/s | Full | 6.00 | |||||
| 624 MB/s | |||||||||
| SD Express | .svg) | 985 MB/s | 3.1 ×1 | 7.00, 7.10 | |||||
| 1,970 MB/s | 3.1 ×2, 4.0 x1 | 8.0 | |||||||
| 3,940 MB/s | 4.0 ×2 | ||||||||
Host Card | UHS-I | UHS-II | UHS-III | Express | |||
|---|---|---|---|---|---|---|---|
| UHS50 | UHS104 | Full | Half | ||||
| UHS-I | UHS50 | 50 | 50 | 50 | 50 | 50 | 50 |
| UHS104 | 50 | 104 | 104 | 104 | 104 | 104 | |
| UHS-II | Full | 50 | 104 | 156 | 156 | 156 | 104 |
| Half | 50 | 104 | 156 | 312 | 312 | 104 | |
| UHS-III | 50 | 104 | 156 | 312 | 624 | 104 | |
| Express | 50 | 104 | 104 | 104 | 104 | 985 | |
SD cards will read and write at speeds of 12.5 MB/s.
High-Speed Mode (25 MB/s) was introduced to support digital cameras with 1.10 spec version.[93]
The Ultra High Speed (UHS) bus is available on some SDHC and SDXC cards.[94][95][96]
Cards that comply with UHS show Roman numerals 'I', 'II' or 'III' next to the SD card logo,[94][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 Vlow-voltage differential signaling (LVDS) on a second row of pins.[97] 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:
Specified in SD version 3.01.[98] 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 asUHS104 (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.[99][100] 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).
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[101] (a total of 17 pins for full-size and 16 pins for micro-size cards).[94] 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 2025, 100 DSLR and mirrorless cameras support UHS-II.[102]
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.[103]
The SD Express bus was released in June 2018 with SD specification 7.0. It uses a singlePCIe lane to provide full-duplex 985 MB/s transfer speed. Supporting cards must also implement theNVM 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.[104]
Hosts which implement version 7.0 of the spec allow SD Cards to dodirect memory access, which increases the attack surface of the host dramatically in the face of malicious SD cards.[105]
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.[106]
Version 9.0 was released in February 2022.[107]
Version 9.1 was announced in October 2023.[108]
In February 2019, the SD Association announced microSD Express.[109] 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.[110]
TheSD 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:[98][90][111][112]
The SD Association defines three types of Speed Class ratings: the original Speed Class, UHS Speed Class and Video Speed Class.
| Minimum sequential writing speed | Speed Class | Video format[e] | ||||||
|---|---|---|---|---|---|---|---|---|
| Speed Class | UHS Speed Class | Video Speed Class | SD Express Speed Class | SD | HD /Full HD | 4K | 8K | |
| 2 MB/s | Class 2 (C2) | — | — | — | Yes | No | No | No |
| 4 MB/s | Class 4 (C4) | Yes | ||||||
| 6 MB/s | Class 6 (C6) | Class 6 (V6) | Yes | |||||
| 10 MB/s | Class 10 (C10) | Class 1 (U1) | Class 10 (V10) | |||||
| 30 MB/s | — | Class 3 (U3) | Class 30 (V30) | Yes | ||||
| 60 MB/s | — | Class 60 (V60) | ||||||
| 90 MB/s | Class 90 (V90) | |||||||
| 150 MB/s | — | Class 150 (E150) | ||||||
| 300 MB/s | Class 300 (E300) | |||||||
| 450 MB/s | Class 450 (E450) | |||||||
| 600 MB/s | Class 600 (E600) | |||||||
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.[98] 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.[98] 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).
| Rating | Approx. (MB/s) | Comparable speed class |
|---|---|---|
| 16× | 2.34 | (13×) |
| 32× | 4.69 | (27×) |
| 48× | 7.03 | (40×) |
| 100× | 14.6 |  (67×) |
The "×" rating, which was used by some card manufacturers and made obsolete by speed classes, is a multiple of the standardCD-ROM drive speed of 150 KB/s[f] (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, includingTranscend andKingston, report their cards' write speed.[114] When a card lists both a speed class and an "×" rating, the latter may be assumed a read speed only.[citation needed]
UHS-I and UHS-II cards can useUHS 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 recording4K video.[115] Before November 2013, the rating was brandedUHS 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 withtapeless camcorders while performing other functions. It is also suitable for real-time broadcasts and capturing large HD videos.
Video Speed Class defines a set of requirements for UHS cards to match the modernMLC 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).
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.[108]
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 minimumIOPS 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.[116] 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.[117]
| Name | Logo | Minimum randomIOPS | Minimum sustained sequential writing | |
|---|---|---|---|---|
| Read | Write | |||
| Class 1 (A1) |  | 1,500 IOPS | 500 IOPS | 10 MB/s |
| Class 2 (A2) |  | 4,000 IOPS | 2,000 IOPS | |
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-definitioncamcorder 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 adigital camera records large photographs or videos) and writing a large number of small files (arandom-access use common insmartphones). 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 notnecessarily 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]
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]
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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]
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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 theNokia N8 to reformat the card for subsequent use in other devices.[123]
A smartSD memory card is a microSD card with an internal "secure element" that allows the transfer of ISO 7816Application 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 byDeviceFidelity, Inc.,[125][126] a pioneer innear-field communication (NFC) andmobile payments, with the introduction of In2Pay and CredenSE products, later commercialized and certified for mobile contactless transactions byVisa 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 2012Good Technology partnered with DeviceFidelity to use microSD cards with secure elements formobile identity andaccess 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, includingBank of America,US Bank andWells Fargo,[134][135][136] while others were part of innovative new direct-to-consumerneobank programs such asmoneto, first launched in 2012.[137][138][139][140]
microSD cards with Secure Elements have also been used for securevoice 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 withArizona State University to launch campus access solutions for students using microSD with Secure Element andMiFare technology provided byDeviceFidelity, Inc.[142][143] This was the first time regular mobile phones could be used to open doors without need for electronic access keys.
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Vendors have sought to differentiate their products in the market through various vendor-specific features:
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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 thePalm Treo, but occasionally laptops or mobile phones).[citation needed] These devices can use the SD slot to supportGPS receivers,modems,barcode readers,FM radio tuners, TV tuners,RFID readers,digital cameras and interfaces toWi-Fi,Bluetooth,Ethernet andIrDA. 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 standardizedUSB device classes andBluetooth profiles.)[citation needed]
TheSDIO family comprises Low-Speed and Full-Speed cards. Both types of SDIO cards supportSerial 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]
Host devices that comply with newer versions of the specification providebackward 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:
Card Slot | SDSC | SDHC | SDHC UHS | SDXC | SDXC UHS | SDIO |
|---|---|---|---|---|---|---|
| SDSC | Partial[g] | FAT16, < 4 GB[g] | FAT16, < 4 GB[g] | No | No | No |
| SDHC | Yes | Yes | In non-UHS mode | FAT32 | FAT32 in non-UHS mode | No |
| SDHC UHS | In non-UHS mode | In non-UHS mode | In UHS mode | FAT32 in non-UHS mode | FAT32 in UHS mode | No |
| SDXC | Yes | Yes | In non-UHS mode | Yes | In non-UHS mode | No |
| SDXC UHS | In non-UHS mode | In non-UHS mode | In UHS mode | In non-UHS mode | In UHS mode | No |
| SDIO | Varies | Varies | Varies | Varies | Varies | Yes |
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 asdigital cameras,camcorders andvideo game consoles, tend to use full-sized cards. Devices in which small size is paramount, such asmobile phones,action cameras such as theGoPro Hero series, andcamera drones, tend to use microSD cards.[1][2]
The microSD card has helped propel the smartphone market by giving both manufacturers and consumers greater flexibility and freedom.
Whilecloud storage depends on stable internet connection and sufficiently voluminousdata plans, memory cards in mobile devices provide location-independent andprivate storage expansion with much higher transfer rates and nonetwork delay, enabling applications such asphotography andvideo recording. While data stored internally onbricked devices isinaccessible, data stored on the memory card can be salvaged and accessed externally by the user asmass storage device. A benefit overUSB on the go storage expansion is uncompromisedergonomy. 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 portableFTP server hosting overWiFi Direct. Due to thetechnical 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 asWindows Mobile andAndroid 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 asCompactFlash. 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 thePC card, ExpressBus,USB,FireWire and theparallel printer port. Active adapters also let SD cards be used in devices designed for other formats, such asCompactFlash. TheFlashPath adapter lets SD cards be used in afloppy disk drive.
Some devices such as theSamsung Galaxy Fit (2011) andSamsung 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 andSIM card.
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 detectcounterfeit 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 withrandom 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 tocompress 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, repeating0xFFs).
Secure Digital memory cards can be used in SonyXDCAM EXcamcorders with an adapter.[162]
Although manypersonal 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 primaryhard disk through the onboard ATA controller, because none of the SD card variants support ATA signalling. Primaryhard disk use requires a separate SD host controller[163] or an SD-to-CompactFlash converter. However, on computers that supportbootstrapping 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.
Inlaptop andtablet computers, memory cards in an integratedmemory card reader offer anergonomical benefit over USBflash 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, newerApple computers with installed SD card readers have been able to boot inmacOS from SD storage devices, when properly formatted toMac OS Extended file format and the default partition table set toGUID Partition Table.[164]
SD cards are increasing in usage and popularity among owners ofvintage computers likeAtari 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]
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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 theMMC standard became the de facto standard for embedded systems. SanDisk provides such embedded memory components under the iNAND brand.[167]
While some modernmicrocontrollers integrate SDIO hardware which uses the faster proprietary four-bit SD bus mode, almost all modern microcontrollers at least haveSPI units that can interface to an SD card operating in the slower one-bit SPI bus mode. If not, SPI can also be emulated bybit banging (e.g. a SD card slotsoldered to aLinksysWRT54G-TM router and wired toGPIO pins usingDD-WRT'sLinux kernel achieved only1.6Mbit/s throughput).[168]
Prerecorded microSDs have been used to commercialize music under the brandsslotMusic andslotRadio bySanDisk andMQS byAstell & Kern.
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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.[169]
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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]
| MMC pin | SD pin | miniSD pin | microSD pin | Name | I/O | Logic | Description |
|---|---|---|---|---|---|---|---|
| 1 | 1 | 1 | 2 | nCS | I | PP | SPI Card Select [CS] (Negative logic) |
| 2 | 2 | 2 | 3 | DI | I | PP | SPI Serial Data In [MOSI] |
| 3 | 3 | 3 | VSS | S | S | Ground | |
| 4 | 4 | 4 | 4 | VDD | S | S | Power |
| 5 | 5 | 5 | 5 | CLK | I | PP | SPI Serial Clock [SCLK] |
| 6 | 6 | 6 | 6 | VSS | S | S | Ground |
| 7 | 7 | 7 | 7 | DO | O | PP | SPI Serial Data Out [MISO] |
| 8 | 8 | 8 | NC nIRQ | . O | . OD | Unused (memory cards) Interrupt (SDIO cards) (negative logic) | |
| 9 | 9 | 1 | NC | . | . | Unused | |
| 10 | NC | . | . | Reserved | |||
| 11 | NC | . | . | Reserved |
| 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 asynchronous 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.[98]
Through issuing various commands, the host device can:[98]
The command interface is an extension of theMultiMediaCard (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 3.3 volt electrical interface. On command, SDHC and SDXC cards can switch to 1.8 V operation.[98]
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.[98]
After determining that the SD card supports it, the host device can also command the SD card to switch to ahigher 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.[98] Between commands, the host device can stop the clock entirely.
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Most SD cards ship preformatted with one or moreMBR partitions, where the first or only partition contains afile system. This lets them operate like thehard disk of apersonal 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.[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 onflash 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[h] (65,536 logical sectors or more), but not more than 2 GB,[d]FAT16B with partition type06h andEBPB 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 KBclusters, which are not widely supported.) FAT16B does not support cards above 4 GB[d] at all.
The SDXC specification mandates the use ofMicrosoft'sproprietaryexFAT file system,[173] which sometimes requires appropriate drivers (e.g.exfat-utils/exfat-fuse on Linux).
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 usewear 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]
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 fromTwinMOS 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]
| 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,[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]
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.[98] 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]
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 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.[98]
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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.[98] Because of these redefinitions, older host devices do not correctly identify SDHC or SDXC cards nor their correct capacity.
Capacity is calculated thus:
Capacity = (C_SIZE + 1) × 524288where for SDHC 4112 ≤C_SIZE ≤ 65375 ≈2 GB ≤ Capacity ≤ ≈32 GBwhere 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]
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Like most memory card formats, SD is covered by numerouspatents andtrademarks. ExcludingSDIO 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 anon-disclosure agreement (NDA) prohibiting development ofopen-source drivers. However, the system was eventuallyreverse-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 SDIOBluetooth Type-A.[183]
The Simplified Specification[184] is available.
Again, most of the information had already been discovered andLinux had a fully free driver for it. Still, building a chip conforming to this specification caused theOne 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 affectsembedded systems, laptop computers and some desktop computers; many desktop computers do not have card slots, instead usingUSB-basedcard readers if necessary.[citation needed] These card readers present a standardUSB 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]
TheSPI-bus interface mode is the only type that does not require a host license for accessing SD cards.[citation needed]
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]
Speed class considered irrelevant: our benchmarking reveals that the "speed class" marking on SD cards is not necessarily indicative of application performance; although the class rating is meant for sequential performance, we find several cases in which higher-grade SD cards performed worse than lower-grade ones overall.
Variations in 4k small block performance saw a difference of approximately 300-fold between the fastest and slowest cards. Distressingly, many of the tested cards were mediocre to poor on that metric, which may explain why running updates on Linux running off SD cards can take a very long time.
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