Secure Digital or (SD) is a non-volatile memory card format for use in portable devices, such as mobile phones, digital cameras, GPS navigation, and tablet computers.
The Secure Digital standard is maintained by the SD Card Association (SDA). SD technologies have been implemented in more than 400 brands across dozens of product categories and more than 8,000 models.
The Secure Digital format includes four card families available in three different form factors. The four families are the original Standard-Capacity (SDSC), the High-Capacity (SDHC), the extended-Capacity (SDXC), and the , which SDIO combines input/output functions with data storage. The three form factors are the original size, the "mini" size, and the "micro" size (see illustration). There are many combinations of form factors and device families.
Electrically passive adapters allow the use of a smaller card in a host device built to hold a larger card.
Host devices that comply with newer versions of the specification provide backward compatibility and accept older SD cards, but older host devices do not recognize newer cards. The SDA uses several trademark logos to enforce compliance with its specifications and assure users of compatibility. This article explains several factors that can prevent the use of a newer SD card:
- A newer card may offer greater capacity than the host device can handle.
- A newer card may use a file system the host device cannot navigate.
- Use of an SDIO card requires the host device be designed for the input/output functions the card provides.
- The organization of the card was changed starting with the SDHC family.
- Some vendors produced SDSC cards above 1 GB before the SDA had standardized a method of doing so.