Secure Digital card

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Secure Digital

A 512 MB SD Card
Media type: memory card
Capacity: maximum 128 GB in LBA mode (28-bit sector address)
Developed by: Secure Digital Card Association
Usage: portable devices, including digital cameras and handheld computers
Extended from: MultiMediaCard (MMC)
A USB device for reading various kinds of flash memories, with a SD card plugged in
A USB device for reading various kinds of flash memories, with a SD card plugged in

Secure Digital (SD) is a flash (non-volatile) memory card format developed by Matsushita, SanDisk and Toshiba for use in portable devices, including digital cameras, handheld computers, PDAs and GPS units. As of 2006, SD card capacities include 8, 16, 32, 64, 128, 256, and 512 MB, 1, 2, 4, 6, 8 GB (4-32 GB: SDHC)[1].

The format has proven to be very popular. However, compatibility issues between older devices and the newer 2GB and larger cards and the SDHC format[2] have caused considerable confusion for consumers.

Contents

[edit] History

In August of 1999, Matsushita (best known by its Panasonic brand name), SanDisk, and Toshiba first announced an agreement on a comprehensive collaboration to jointly develop, specify and widely promote a next generation secure memory card called the SD Memory Card. With a physical profile of 24 mm × 32 mm × 2.1 mm, the new card provided both an SDMI-compliant (Secure Digital Music Initiative), high-level of copyright protection and high-density memory capacity for the time. The new memory card format was designed to compete with Sony's Memory Stick format that was released in 1998.

The “Secure” in Secure Digital comes from the card’s origin. To create the SD card, Toshiba added encryption hardware to the already-existent MMC card, to calm music industry concerns that MMC cards would allow for easy piracy of music. (A similar scheme is the MagicGate standard used in Memory Sticks.) In theory, the encryption would allow some enforcement of Digital rights management[1] schemes on digital music, but the capability is rarely used.

The signature “SD” logo was actually developed for another use entirely: it was originally used for “Super Density Disc”, a prototype format by Toshiba created during the development of DVD. This is why the “D” looks similar to half of an optical disc, possibly reinforced by the SD card's music industry features and consumer's familiarity with music on optical discs.

At the 2000 CES trade show Matsushita, SanDisk and Toshiba Corporation announced that a new industry-wide association will be created to set industry standards for their proprietary SD (Secure Digital) Memory Card and promote its wide acceptance in digital applications. The new organization, named the SDAssociation (SDA), is headquartered in California and its executive membership includes some 30 world-leading high-tech companies and major content companies. Sampling of the SD Memory Card began in the first quarter of 2000, and production shipments commenced in the second quarter of 2000. The card was initially available in 32 and 64 megabyte capacities.

In April 2006, the SDA released a detailed specification for the non-security related portions of the SD Memory Card standard. In addition, they released specifications for the SDIO cards and the standard SD host controller. During the same year, specifications were finalised for the small form-factor microSD (formerly known as TransFlash) and SDHC, with capacities in excess of 2 GB and a minimum sustained read/write speed of 2.2 MB/s.

[edit] Form factor

SD cards are based on the older MultiMediaCard (MMC) format, but have a number of differences:

  • The SD card is asymmetrically shaped so that it cannot be inserted upside down, whereas an MMC card will go in most of the way but not make contact if inverted.
  • Most SD cards are physically thicker than MMC cards. SD cards generally measure 32 mm × 24 mm × 2.1 mm, but can be as thin as 1.4 mm, just like MMC cards (see below).
  • The contacts are recessed beneath the surface of the card (like Memory Stick cards), protecting the contacts from contact with the fingers
  • SD cards typically have higher data transfer rates, but this is always changing, particularly in light of recent improvements to the MMC standard.
  • Digital rights management features are available but are seldom used.

Devices with SD slots can use the thinner MMC cards, but the standard SD cards will not fit into the thinner MMC slots. SD cards can be used in CompactFlash or PC Card slots with an adapter. miniSD and microSD cards can be used directly in SD slots with a physical interface adapter. There are some SD cards with a USB connector for dual-purpose use, and there are card readers which allow SD cards to be accessed via many connectivity ports such as USB, FireWire, and the common parallel port. SD cards can also be accessed via a floppy disk drive with a FlashPath adapter.

[edit] Optional write-protect tab

When looking at the card from the top (see pictures) there is one required notch on the right side (the side with the diagonal notched corner). On the left side there is usually a tab. This is the write-protect tab. The MMC card has neither notch. The tab / notch works the same way as the notches on compact audio cassettes and videotape cassette tapes or floppy disks.

When this write-protect tab is in the down position (away from the end that is inserted) then it is write protected and read-only. When the tab is in the up position it is write enabled. Since the tab is optional, the card can have no switch and no notch, which makes the card always writable, or it can have an empty notch and be a ROM card, or always write-protected and read-only. If the tab becomes broken or falls off then the card will become a write-protected ROM card and no longer be writable. A possible troubleshooting solution would be to apply tape over the notched area (avoiding the connectors and the other notch) to configure the card in a permanent writable state.

The write protect tab feature is optional within the Secure Digital Association guidelines. Some manufacturers claim that the write switch is easily broken, and do not include it on all their card models[3].

[edit] Speeds

There are different speed grades available which are measured with the same system as CD-ROMs, in multiples of 150 kB/s. Basic cards transfer data up to six times the data rate of the standard CD-ROM speed (900 kB/s, vs. 150 kB/s). High-speed cards are made with higher data transfer rates like 66x (10 MB/s), and high-end cards have speeds of 150x or higher. Some digital cameras require high-speed cards to record video smoothly or capture multiple still photographs in rapid succession. As of December 2005, most devices comply to the SD card specification 1.01, with maximum speed of 66x. Higher speeds of up to 133x are defined by specification 1.1.

[edit] Openness of standards

MMC cards also work in SD slots
MMC cards also work in SD slots

Like most memory card formats, SD is covered by numerous patents and trademarks.

There are three versions of the SD specification: 1.0, 1.1 and 2.0. Access to the 1.1 and 2.0 versions are only available after agreeing to a non-disclosure agreement which prohibits the development of an open source driver, a fact that generates a fair amount of consternation in the open-source and free software communities.

However, the SDCA have made access to a simplified version of the 1.0 specification under a less-restrictive licence[4]. This has aided the development of at least one Linux driver[5].

The usual workaround is to develop an open-source wrapper for a closed-source SD driver available on the particular platform, but this is far from ideal. Another common workaround is to use the older MMC mode, which all SD cards are required to support by the SD standard.

This means that SD is less open than CompactFlash or USB flash memory drives, which can be implemented for free but require licensing fees for the associated logos and trademarks, but far more open than xD or Memory Stick, where no public documentation nor any documented legacy implementation is available.

[edit] Technical explanation

All SD memory and SDIO cards are required to support the older SPI/MMC mode which supports the slightly slower four-wire serial interface (clock, serial in, serial out, chip select) that is compatible with SPI ports on many microcontrollers. Many digital cameras, digital audio players, and other portable devices probably use MMC mode exclusively. Documentation for this mode can be purchased from the MMCA for $500; however, partial documentation for SDIO is free and there is free documentation available for memory cards as part of some manufacturers' datasheets.

MMC mode does not provide access to the proprietary encryption features of SD cards, and the free SD documentation does not describe these features. As the SD encryption exists primarily for media producers, it is not of much use to consumers who typically use SD cards to hold unprotected data.

There are three transfer modes supported by SD: SPI mode (separate serial in and serial out), one-bit SD mode (separate command and data channels and a proprietary transfer format), and four-bit SD mode (uses extra pins plus some reassigned pins) to support four-bit-wide parallel transfers.

Low speed cards support 0 to 400 kbit/s data rate and SPI and one-bit SD transfer modes. High speed cards support 0 to 100 Mbit/s data rate in four-bit mode and 0–25 Mbit/s in SPI and one-bit SD modes.

Royalties for SD/SDIO licenses are imposed for manufacture and sale of memory cards and host adapters ($1000 per year plus membership at $1500/year) but SDIO cards can be made without royalties and MMC host adapters do not require a royalty. MMC cards had a seven-pin interface; SD and SDIO expanded this to nine pins.

See Legitimacy of standards for background info.

[edit] DRM features

The digital rights management scheme embedded in the SD cards is defined as the Content Protection for Recordable Media (CPRM) by the 4C Entity and is centered around use of the Cryptomeria cipher (also known as C2). The specification is kept secret and is only accessible to licensees. DVD-Audio use a very similar scheme known as Content Protection for Prerecorded Media (CPPM).

[edit] Different types of MMC/SD cards

The SD card is not the only flash memory card standard ratified by the Secure Digital Card Association. Other SD Card Association formats include miniSD, microSD (formerly known as TransFlash before ratification by the SD Card Association), and SDHC (Secure Digital High Capacity, for capacities above 2GB). SDHC is not fully compatible with the format that it extends, in that SD devices that do not specifically support SDHC will not work with the newer cards.

These smaller cards are usable in full size MMC/SD/SDIO slots with an adapter (which must route the electrical connections as well as making physical contact). It should be noted, however, that it is already difficult to create I/O devices in the SD form factor and this will be even more difficult in the smaller sizes. However, a WiFi card for mini-SDIO is already available from Spectec.

As SD slots still support MMC cards, the separately-evolved smaller MMC variants are also compatible with SD-supporting devices. Unlike miniSD and microSD (which are sufficiently different from SD to make mechanical adapters impractical), RS-MMC slots maintain backward compatibility with full-sized MMC cards, because the RS-MMC cards are simply shorter MMC cards. More information on these variants can be found in the article about the MultiMediaCard standard.

It is also important to note, that unlike for the data storage (which typically works everywhere where SD slot is present), the SDIO device must be supported and equipped with drivers and applications for the host system and usually doesn't work outside of manufacturer's scope (which means, for example, that an HP SDIO camera usually does not work in PDAs for which it is not listed as an accessory). This behavior is often not expected by end users (who expect that only the SD slot is required) and is similar to compatibility problems among Bluetooth devices.

Technical comparison
Type MMC RS-MMC MMC Plus SecureMMC SD SDIO miniSD microSD
SD Socket Yes Mechanical adapter Yes Yes Yes Yes Electro-mechanical adapter Electro-mechanical adapter
Pins 7 7 13 7 9 9 11 8
Form factor Thin Thin/short Thin Thin Thick (exceptions possible) Thick Narrow/short/thin Narrow/short/extrathin
Width 24 mm 24 mm 24 mm 24 mm 24 mm 24 mm 20 mm 11 mm
Length 32 mm 18 mm 32 mm 32 mm 32 mm 32 mm+ 21.5 mm 15 mm
Thickness 1.4 mm 1.4 mm 1.4 mm 1.4 mm 2.1 mm (exceptions possible) 2.1 mm 1.4 mm 1 mm
SPI mode Optional Optional Optional Required Required Required Required Optional
1 bit mode Yes Yes Yes Yes Yes Yes Yes Yes
4 bit mode No No Yes  ? Optional Optional Optional Optional
8 bit mode No No Yes  ? No No No No
Xfer clock 0–20 MHz 0–20 MHz 0–52 MHz 0–20 MHz? 0–25 MHz - 0–50 MHz 0–25 MHz 0–25 MHz? 0–25 MHz?
Max Xfer 20 Mbit/s 20 Mbit/s 416 Mbit/s 20 Mbit/s? 100 Mbit/s - 200 Mbit/s 100 Mbit/s 100 Mbit/s 100 Mbit/s
Max SPI XFR 20 Mbit/s 20 Mbit/s 52 Mbit/s 20 Mbit/s 25 Mbit/s 25 Mbit/s 25 Mbit/s 25 Mbit/s
DRM No No No Yes Yes N/A Yes Yes
User encrypt No No No Yes No No No No
Simplified Spec Yes Yes No Not yet? Yes Yes No No
Memb cost $2500/yr (not required) $2000/yr (General), $4500/yr (Executive)
Spec cost $500  ?  ? Member Member Member Member
Host license No No No No $1000/yr + memb
Mem card royalties Yes Yes Yes Yes Yes Yes Yes Yes
I/O card royalties N/A N/A N/A N/A N/A $1000/yr+memb N/A N/A
Open source compatible Yes Yes Yes? Yes? Yes Yes Yes Yes
Type MMC RS-MMC MMC Plus SecureMMC SD SDIO miniSD microSD

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

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

Most, possibly all, current MMC flash memory cards support SPI mode even if not officially required as failure to do so would severely affect compatibility. All cards currently made by SanDisk, Ritek/Ridata, and Kingmax digital appear to support SPI. Also, MMC cards may be electrically identical to SD cards but in a thinner package and with a fuse blown to disable SD functionality (so no SD royalties need to be paid). Some MicroSD cards do not support SPI mode.

MMC defined the SPI and one-bit MMC/SD protocols. The underlying SPI protocol has existed for years as a standard feature on many microcontrollers. From a societal perspective, the justification for a new incompatible SD/MMC protocol is questionable; the development of a new incompatible and unnecessary protocol may help trade associations collect licensing and membership fees but it raises the cost of hardware and software in many ways. The new protocol used open collector signalling to allow multiple cards on the same bus but this actually causes problems at higher clock rate. While SPI used three shared lines plus a separate chip select to each card, the new protocol allows up to 30 cards to be connected to the same three wires (with no chip select) at the expense of a much more complicated card initialization and the requirement that each card have a unique serial number for plug and play operation; this feature is rarely used and its use is actively discouraged in new standards (which recommend a completely separate channel to each card) because of speed and power consumption issues. The quasi-proprietary one-bit protocol was extended to support four bit wide (SD and MMC) and eight bit (MMC only) transfers for more speed while much of the rest of the computer industry is moving to higher speed narrower channels; standard SPI could simply have been clocked at higher data rates (such as 133 MHz) for higher performance than offered by four-bit SD — embedded CPUs that did not already have higher clock rates available would not have been fast enough to handle the higher data rates anyway. The SD card association dropped support for some of the old one-bit MMC protocol commands and added support for additional commands related to copy protection.

[edit] Compatibility issues with 2GB and larger cards

Compatibility with 2 gigabyte and larger SD cards has been poor, due to the SD/MMC protocol using a 32-bit address field denominated in bytes. The SDHC standard addresses this limitation by using 32-bit block addresses instead. Both SD and SDHC are traditionally accessed as 512-byte blocks on 512-byte boundaries, so the change to host software or firmware is minor but required. Before SDHC was standardized, various manufacturers "extended" the SD control block fields for their 2GB and 4GB cards in different ways. Those cards are incompatible with many SD and some SDHC devices, as they conform to neither standard. All SDHC readers work with standard SD cards.[6]

Many older devices will not accept the 2GB size even though it is in the revised standard. The following statement is from the SD association specification:

"To make 2GByte card, the Maximum Block Length (READ_BL_LEN=WRITE_BL_LEN) shall be set to 1024 bytes. However, the Block Length, set by CMD16, shall be up to 512 bytes to keep consistency with 512 bytes Maximum Block Length cards (Less than and equal 2GByte cards)."[7]

Since all cards up to and including the 1GB card uses a fixed 512 block size, some device drivers do not handle the larger block size and will not even recognize the 2GB card.[citation needed] Citation for this is more difficult as the SD organization hides this problem. SanDisk web site shows examples of devices such as the iPAQ 1910 that will only support 1GB cards and the Epson Photo RX300 Technical support says they only support 1GB cards in the SD slot.[citation needed] Many early card readers only support 1GB, based on user experiences.[citation needed]

[edit] SDHC

A new SD format, SDHC (Secure Digital High Capacity), allows capacities in excess of 2GB (4GB to 32GB)[8]. SDHC uses the same form factor as SD, but the SD 2.0 standard in SDHC uses a different memory addressing method (sector addressing vs byte addressing[9]). SDHC cards only work in SDHC compatible devices, but standard SD cards work in both SD and SDHC devices. The SDHC trademark is licensed to ensure compatibility.[10]

SDHC cards have SD Speed Class Ratings defined by the SD Association. The SD Speed Class Ratings specify the following minimum sustained write speed onto empty SDHC cards:

  • Class 2: 2 MB/s
  • Class 4: 4 MB/s
  • Class 6: 6 MB/s

As of April 2007, examples of devices which support the SDHC format include:

[edit] SD and SDHC compatibility issues

As of early 2007, the simultaneous availability of non-standard 4GB SD and 4GB SDHC cards, and incompatibilities between SD and SDHC have caused confusion among various for consumers looking to buy memory devices.

The following compatibility issues between SD and SDHC cards and devices can arise:

  • Devices that do not specifically support SDHC do not recognize SDHC memory cards.
  • Some manufacturers have produced 4GB SD cards that conform to neither the SD2.0/SDHC spec nor existing SD devices[11].

[edit] SDIO

A camera that uses the SDIO interface
A camera that uses the SDIO interface

SDIO stands for Secure Digital Input Output.

SD slots can actually be used for more than flash memory cards. Devices that support SDIO (typically PDAs like the Palm Treo, but occasionally laptops or cell phones) can use small devices designed for the SD form factor, like GPS receivers, Wi-Fi or Bluetooth adapters, modems, Ethernet adapters, barcode readers, IrDA adapters, FM radio tuners, TV tuners, RFID readers, digital cameras, or other mass storage media such as hard drives.

A number of other devices have been proposed but not yet implemented, including RS-232 serial adapters, fingerprint scanners, SDIO to USB host/slave adapters (which would allow an SDIO-equipped handheld device to use USB peripherals and/or interface to PCs), magnetic strip readers, combination Bluetooth/Wi-Fi/GPS transceivers, cellular modems (PCS, CDPD, GSM, etc.), and APRS/TNC adapters.

SDIO cards are fully compatible with SD Memory Card host controller (including mechanical, electrical, power, signaling and software). When an SDIO card is inserted into a non SDIO-aware host, it will cause no physical damage or disruption to device or host controller. SPI bus topology is mandatory for SDIO, unlike SD Memory. Most of the SD Memory commands are not supported in SDIO. SDIO cards can contain 8 separate logical cards, though at the moment this is at most a memory and IO function.

[edit] Market penetration

Secure Digital cards are used as storage media in these devices:

SD/MMC cards have all but replaced Toshiba's SmartMedia as the dominant memory card format used in compact digital cameras. In 2001 SmartMedia had achieved nearly 50% use, but by 2005 SD/MMC had achieved over 40% of the digital camera market and SmartMedia’s share had plummeted. A notable majority of the world’s leading digital camera manufacturers use SD in their product lines, including Casio, Canon, Nikon, Pentax, Kodak, Panasonic and Konica Minolta. Three major brands, however, have stuck to their own proprietary formats in their cameras: Olympus and Fuji using xD cards, and Sony using Memory Stick. Additionally, SD has not conquered the Digital SLR market, where CompactFlash remains the most popular format, except in some consumer-level models, like Pentax's *istDS/DL/DS2 and K100D/K110D/K10D lines, D40, D50 and D80 by Nikon (some high-end professional DSLRs — namely Canon's EOS-1Ds Mark II and EOS-1D Mark II N — do support SD cards as well, in addition to CompactFlash).

With a media card reader — typically a small USB device — users can access the SD card's contents using their computers. Some newer computers have such card readers built in.

A recent development is SD cards with built-in USB plugs, to eliminate the need for an SD/USB adapter or SD slot on a PC, though at higher initial cost. One design marketed by SanDisk features a folding flap to expose the plug. Although SanDisk was not the first to market a USB enabled SD card, the move did encourage other manufacturers to follow suit.

[edit] See also

[edit] External links

[edit] References

  1. ^ http://www.dramexchange.com/WeeklyResearch/Post/1/492.aspx
  2. ^ Pocket PC Users Steer Clear of SDHC... For Now, Pocket PC Central Press, July 18, 2006
  3. ^ Kingmax FAQ, www.kingmaxdigi.com, 2006
  4. ^ Sharp Linux PDA promotes use of proprietary SD card, but more open MMC works just fine
  5. ^ Linux/Drivers/sdhci
  6. ^ SD Compatibility, CARDSPEED - Card Readers and Memory Cards, December 1, 2006
  7. ^ SD Group Technical Committee (September 25, 2006). "Section 4: SD Memory Card Functional Description; 4.3.2: 2 GByte Card", SD Specifications, Part 1: Physical Layer Simplified Specification (PDF, HTML), Version 2.00, SD Card Association, p. 19. Retrieved on February 23, 2007. 
  8. ^ SDHC (SD High Capacity) Memory Card, SD Card Association, 2004
  9. ^ SDHC vs. SD memory addressing
  10. ^ What is SDHC?
  11. ^ Techgage review, including an OCZ 4GBan OCZ 4GB SD (non-SDHC) card
 v • d • e Memory cards
CompactFlash (CF) | JEIDA | Memory Stick | MultiMediaCard (MMC) | PC card | SmartMedia | Secure Digital (SD) | xD-Picture
Comparison of memory cards
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