Secure Digital card Facts for Kids

An SDHC card, with 8 GB capacity; this one is class 6. This means it has a minimum data rate of 6 MB/s

A Secure Digital card (SD card) is a kind of memory card. Such cards are often used to store images or data in digital cameras. As of 2008, different capacities between 4 megabytes and 32 gigabytes have been made. The card has a rectangular design, but one edge is chipped off. This means that the cards cannot be inserted into the cameras (or other devices) the wrong way.

There are different kinds of cards:

Those labeled SD, with capacities up to 2 GB
Those labeled SDHC, with capacities between 4GB and 32GB
Those labeled SDXC, with capacities of up to 2 TB (largest made is currently 512GB for SDXC or microSDXC).

SD and SDHC are not compatible, but devices that accept SDHC also accept SD cards. The interface of SDHC and SDXC cards is the same, but SDXC uses a different file system. Some devices (for example the Wii) that originally shipped only with support for SD can be made to support SDHC with a firmware update.

There are also different classes. These refer to the read and write speeds. Currently in use are Class 2 - which means 2 MBytes/second, Class 4 - 4 MB/sec and Class 6 at 6 MB/sec. The problem with this is that different manufacturers measure different things with it, some measure write speeds, others read speeds. There are also differences because some give guarantees, and others simply say that the speeds they indicate are the maximum achievable under good conditions.

Still another measurement is called rating. A rating of 1 corresponds to 150 KBytes per second, the read speed of a CD. That way, a class 2 is equal to a speed of 13x.

microSD cards do the same thing but are much smaller.

Images for kids

microSD card in a smartphone
This microSDHC card holds 8 billion bytes. Beneath it is a section of a magnetic-core memory (used until the 1970s) that holds eight bytes using 64 cores. The card covers approximately 20 bits (2 1/2 bytes)
Back side of a Lexar UHS-II microSDHC card, showing the additional row of UHS-II connections
Front and back of an SD Express card
64GB SanDisk Ultra microSDXC card (with UHS-I and UHS Speed Class 1 markings)
32GB Lexar 1000x microSDHC card (with UHS-II and UHS Speed Class 3 markings)
The front and back of the Sony 64GB SF-M Tough Series UHS-II SDXC Memory Card.
Sony 64GB SF-M Tough Series UHS-II SDXC Memory Card is one of the few cards in the market without a sliding tab on the write protect notch.
MicroSD to SD adapter (left), microSD to miniSD adapter (middle), microSD card (right)
SD cards with dual interfaces: SD and USB
Camera using the SDIO interface to connect to some HP iPAQ devices
Samsung Pro 64 GB microSDXC original (left) and counterfeit (right): The counterfeit claims to have 64 GB in capacity, but only 8 GB (Class 4 speed) are usable: When trying to write more than 8 GB, data loss occurs. Also used for SanDisk 64 GB fakes.
Images of genuine, questionable, and fake/counterfeit microSD (Secure Digital) cards before and after decapsulation. Details at source, photo by Andrew Huang.
SD card in a DSLR camera
A shield (daughterboard) that gives Arduino prototyping microprocessors access to SD cards
Size comparison of families: SD (blue), miniSD (green), microSD (red)
Official pin numbers for each card type (top to bottom): MMC, SD, miniSD, microSD. This shows the evolution from the older MMC, on which SD is based. NOTE: This drawing doesn't show 8 new UHS-II contacts that were added in spec 4.0.
Inside a 512 MB SD card: NAND flash chip that holds the data (bottom) and SD controller (top)
Inside a 2 GB SD card: two NAND flash chips (top and middle), SD controller chip (bottom)
Inside a 16 GB SDHC card
4 GB SDSC card
Dismantled microSD to SD adapter showing the passive connection from the microSD card slot on the bottom to the SD pins on the top
Size comparison of various flash cards: SD, CompactFlash, MMC, xD

Secure Digital card facts for kids

Images for kids

See also