Definition

Secure Digital card (SD card)

Contributor(s): Sonia Lelii

A Secure Digital (SD) card is a tiny flash memory card designed for high-capacity memory and various portable devices, such as car navigation systems, cellular phones, e-books, PDAs, smartphones, digital cameras, music players, digital video camcorders and personal computers.

In 1999, SanDisk Corp., Panasonic Corp. (formerly known as Matsushita Electric Industrial Co. Ltd.) and Toshiba Corp. agreed to develop and market the Secure Digital card standard, their improvement to the MultiMediaCard (MMC). The companies formed SD-3C LLC to license and enforce the intellectual property rights for SD memory cards and other products. In January 2000, the three formed the nonprofit SD Association to develop and promote SD technology.

A Secure Digital card is about the size of a postage stamp and weighs approximately two grams. It is similar in size to an MMC, but smaller than older memory card types, such as a SmartMedia card or CompactFlash card. An SD card features a high data transfer rate and low battery consumption, which are both primary considerations for portable devices. An SD card uses flash memory to provide nonvolatile storage, which means a power source is not required to retain stored data.

Both MMC and SD cards provide encryption capabilities for protected content to ensure secure distribution of copyrighted material, such as digital music, video and e-books. SD cards are available with storage capacities as high as 4 GB.

SD cards are more rugged than traditional storage media. They have an operating shock rating -- basically, the height you can drop them from and still have them work -- of 2,000 Gs, compared to a 100 G to 200 G rating for the mechanical drive of the typical portable computing device.


This video from ExplainingComputers
discusses SD card types, speed
classes and sizes.

This translates to a drop to the floor from 10 feet, as compared to a single foot for a mechanical disk drive. MMC and SD cards both use metal connector contacts, instead of the traditional pins and plugs, so they aren't as prone to damage during handling.

SD card formats

Secure Digital card technology includes the following:

  • SD Standard Capacity. Capacities for SDSC cards range from 128 megabytes to 2 GB. The default format for these cards is FAT16 (File Allocation Table 16).
  • SD High Capacity. Based on the SDA 2.0 specification, capacities for SDHC cards range from 4 GB to 32 GB. The default format for these cards is FAT32.
  • SD eXtended Capacity. Based on the SDA 3.0 specification, capacities for SDXC range from 64 GB to 2 terabytes. The default format for these cards is exFAT (Extended FAT).
  • SD Input Output. SDIO cards combine I/O functions with data storage. As of 2016, the prevailing formats are full or micro-size SDHC and SDXC cards.
    SD card images
    Images of an SD card, mini SD card and micro SD card.

SanDisk introduced the first 128 GB microSDXC card in February 2014, a 128 GB SDXC card in September 2014 and a 200 GB microSDXC card in March 2015. Samsung came out with the world's first 256 GB microSDXC card in May 2016.

This was last updated in January 2017

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