Secure Digital (SD) Cards – popular digital camera memory card format

Secure Digital (SD) is a memory card format used to transfer data between PCs and smaller devices, such as digital cameras, PDAs, MP3 players and mobile phones. Because the memory used is solid state, i.e. it remembers what’s been written to it when the power is removed, the information stays on the card when it’s unplugged. In order to get the data onto a PC, a card reader, either integrated into the PC or connected via a USB port, is required. Laptop users can also use an adapter that takes SD cards and fits into a PC Card slot. A consortium of Matsushita (aka Panasonic), Toshiba and SanDisk developed SD because they had been outflanked by Sony’s introduction of Memory Stick, their own proprietary card format, in 1998. This had two features that neither MultiMediaCard (MMC) nor CompactFlash, (CF) the other major player in memory cards at the time, possessed: a write-protect switch and integrated copyright protection. SD came onto the market in 1999 and was based, at least physically, on the MMC format. SD cards are slightly thicker but this allows the contacts to be recessed to protect them from damage. The other major physical change from the MMC design was an asymmetrical profile, which prevents SD cards from being inserted the wrong way round. The closeness of the two designs means that MMC cards can be used in SD slots, but not the other way round. Multi-function card readers are available with a slot than can accept SD cards as well as MMC, SmartMedia, Memory Stick and xD-Picture Card. One interesting and innovative adaptation is SD Plus,...

MultiMediaCard (MMC) – SanDisk and Siemens NAND memory card for phones and smaller devices

Ten years after releasing the CompactFlash card and watching it become very successful, SanDisk had a go at repeating the trick, this time in collaboration with German company Siemens, releasing the MultiMediaCard (MMC). This was designed to take over from CompactFlash in most markets but also reach newer, smaller devices that the CF format is simply too big to fit, such as mobile phones, smartphones and PDAs. An MMC card is about the size of a postage stamp whereas a CF card is closer to a book of matches. One disadvantage of the form factor is that it is possible to insert an MMC card into a card reader slot the wrong way round. As the pins won’t connect there won’t be any damage, but it can be frustrating. The size reduction was made possible by the adoption of NAND flash memory developed by Toshiba and introduced in their SmartMedia card format in 1995. NAND flash memory allows greater storage densities, is cheaper to make and the data states are retained for longer than NOR flash memory, the method used by CF. NAND flash memory also has to be written to and erased in block mode, rather than bit-by-bit. This property is a disadvantage for directly accessing memory, as you do with a PC’s RAM, but makes it a good match for emulating secondary storage such as hard disks or optical disks. MMC came on the market in 1997 and the available memory sizes have constantly increased since then, although not to the same extent as cards that are available in CF form. There is a theoretical limit of...

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