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Secure Digital

Secure digital (SD) is a non-volatile memory card format developed by the SD Card Association (SDA) for portable devices.

The standard was jointly launched by SanDisk, Matsushita Electric and Toshiba in August 1999. It is an improvement of MMC and has become an industry standard. The three companies form sd-3c, LLC, a company that licenses and enforces intellectual property rights related to SD memory cards and SD host and ancillary products.

The two companies also set up the non-profit organization SD Association (SDA) in January 2000 to promote and create SD card standards. Today, SDA has about 1000 member companies. SDA uses multiple trademark logos owned and licensed by sd-3c to enforce compliance with its specifications and ensure user compatibility.

Secure digital includes five card families in three different sizes. These five series are original standard capacity (SDSC), large capacity (SDHC), extended capacity (SDXC), super capacity (sduc) and SDIO, which combine input / output functions with data storage. The three sizes are original, minimum and micro. Electrical passive adapters allow smaller cards to be installed and run in devices built for larger cards. SD card takes up little space and is an ideal storage medium for smaller, thinner and more portable electronic devices.

SD card speed is usually evaluated by its sequential read or write speed. Sequential performance is most relevant to storing and retrieving large files (relative to the internal block size of flash memory), such as images and multimedia. Small data (such as file names, sizes, and timestamps) fall within much lower speed limits of random access, which may be a limiting factor in some use cases.

For earlier SD cards, some card manufacturers specified speeds as "multiples" that compared the average speed of data read to the average speed of the original CD-ROM drive. The speed level rating replaces this level, which guarantees the minimum rate at which data can be written to the card.

The newer SD card family increases the speed of the card by increasing the bus speed (the frequency of strobe information to the card and clock signals selected from the card). Regardless of the bus speed, the card can signal the host that it is "busy" until the read or write operation is completed. Compliance with a higher speed level is a guarantee of the card's limited use of the "busy" indication.