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 128Gb on MMC card sizes. The original specification defined a 1-bit serial interface but that increased, initially to 4-bits and then 8-bits, to significantly increase access and data transfer speeds. The standard is open so has been embraced by many manufacturers of cards and devices, with Samsung in particular becoming a very strong supporter.
MMC begat a number of different forms as the standards migrated to suit new developments. The most significant of those formats are:
|Reduced Size (RS-MMC)||Intended for use in mobile phones, these cards came in a reduced size format; a physically cut down version of the original MMC card, at 18mm long rather than 32mm, that proved popular with Siemens and Nokia in particular. Usually supplied with a simple adapter to allow it to replicate a full-size MMC card when in use with a PC-based card reader. MMCmobile (see below) superseded these cards, using the same physical form factor.|
|Dual-Voltage (DV-MMC)||Originally MMC cards operated at 3.3v but a 1.8v standard was introduced for lower power consumption in mobile devices such as PDAs. These were rendered obsolete by the MMCplus and MMCmobile formats (below) that emerged in 2005.|
|MMCplus||Introduced in 2005 using Version 4 of the MMC standard, these cards introduced the higher rate 4-bit and 8-bit data buses and operate at a higher frequency, again increasing speed.|
|MMCmobile||Introduced alongside MMCplus in 2005, MMCmobile represents the RS-MMC form of Version 4 and introduces the same new features, with the exception of the 8-bit data bus, which can’t be implemented in the RS-MMC form factor because of the lack of extra pins.|
|Secure Digital (SD)||Not strictly an MMC variant, the SD card format warrants a mention here to put it in context, although it has an article here (link) in it’s own right. SD was also designed by SanDisk and Toshiba but this time joined by Matsushita, owners of the Panasonic brand name. It was an all-new specification, based on the MMC format but with some major changes and was designed to compete head-on with Sony’s proprietary Memory Stick format, with built-in copyright protection. It’s important to note that although MMC and SD cards look very similar (particularly with the respective micro formats), they are not compatible in any way.|
Fortunately backward compatibility was considered important by the MMC consortium when they brought out MMCplus and MMCmobile, and as they are electrically the same as the older card formats, they will work in card readers designed for the earlier MMC and RS-MMC formats. But the facilities such as the 4 and 8-bit bus, higher frequency operating speeds and dual-voltage capability will not work unless it is possible to get a firmware upgrade for the reader.
At time of publication, although MMC cards are still available and are very cheap, they are slowly being overtaken in most areas by SD cards. The most enduring format appears to be MMCmobile.
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