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Flash Memory, Part I: MMC and SD

MMC and SD

Flash memory is available in so many formats that it can be difficult to know what will work with a particular device. Devices like MP3 players, PDAs, mobile phones, digital cameras, and personal computers can take advantage of flash memory to increase their storage capacity, but selecting the right format can be easier said than done.

Trying to tackle all the common formats in one tech tip can be a lot of reading, which is why we’re doing a two-part series on flash memory. Part I of the Flash memory series will focus on two similar, very popular and generally interchangeable formats: MMC and SD.

The basics

Before going into details, some background on each card may be appropriate. The letters ‘MMC’ stand for MultiMedia Card, which is a format jointly developed by SanDisk and Siemens in 1997. The letters ‘SD’ stand for Secure Digital, and this format is an improvement on the original MMC design and was jointly developed by SanDisk, Matsushita Electronics (better known as Panasonic) and Toshiba.

Both formats are quite durable, and the solid-state components (no moving parts) are protected by a rigid plastic case. Devices are generally unaffected by extreme temperatures and should withstand a 10-foot drop without shock damage.

physical characteristics

Both MMC and SD flash drives measure approximately 24mm x 32mm x 2.1mm, about the size of a typical postage stamp, and weigh just 2 grams. This miniature footprint may make them the easiest way to misplace your data, but it also allows for smaller devices that accept them. Personal electronic devices are shrinking as they become more powerful, and necessary accessories must keep pace. Although they share the same basic form factor, MMC and SD cards can be distinguished by two physical features, a sliding tab and the number of connections.

When looking at an MMC or SD card with the label facing you and the electrical connections facing away, there will be a notch in the upper right corner of the card. From this benchmark, there will be a small sliding tab on the left edge of an SD card, which is not found on an MMC card. Compare this 512MB MMC card to this 512MB SD card and you can see the difference if you look closely at the enlarged images. This tab slides into two positions, locked and unlocked. Allows the user to manually write-protect the data on the card, which means that with the tab in the locked position, data can be read from the card, but nothing can be written to or erased from the card.

The other physical difference is on the back of the card. An MMC card has seven electrical connections (small rectangular pads for data transfer and power reception), while an SD card has nine.

* While there may be rare exceptions, for all practical purposes SD and MMC cards can be used interchangeably in current devices, especially if they indicate “SD/MMC” compatibility.

transfer rate

SD and MMC cards are capable of similar data transfer speeds, with a slight advantage for SD cards. SD cards are now available with 60x (9MB/s) write speeds and 66x (10MB/s) read speeds, while MMC transfer speeds seem to top out at 9MB/s on any device. address. There isn’t much of a difference and both are fairly fast, but end user results will vary and may not achieve these speeds in the real world, regardless of which format is chosen.

SD and MMC cards must reference a speed as part of the technical specification, and it’s an important thing to keep in mind when comparing prices. Lower speed cards are still commercially available and may impact the performance of digital cameras or other devices where speed may be critical. Each ‘x’ in the speed rating represents 0.15MB/s, so if 45x compared to 66x doesn’t sound like much to you, maybe put it in terms of 6.75MB/s compared to 10MB/s. s will. Instead of actual speed ratings, some manufacturers will use words like “High Speed” or “Ultra” when referring to the fastest cards.

Note: Please check the actual write speed specifications of your device before purchasing “Ultra” or “High Speed” chips. You could be putting a Hemi engine in an AMC Gremlin. Don’t spend the extra money if the camera doesn’t support it.

Ability

SD cards are available in sizes up to 1GB, 2GB models are starting to appear, and the SD Card Association says models with up to 4GB and 8GB of storage capacity are also on the way. In contrast, MMC cards have a maximum capacity of 512 MB, which makes SD technology much more attractive.

Security

As mentioned in the physical features section above, SD cards offer the benefit of write protection. By ‘locking’ the card, a user can be assured that data is safe until steps are taken to unprotect it. Fears of accidentally losing or changing data can be eliminated by using an SD card instead of an MMC card, thus improving data security.

Another function supported by SD, but not by MMC, involves copyright protection. SanDisk’s website refers to this feature as “cryptographic security for the protection of copyrighted data,” and other locations refer to it as DRM or Digital Rights Management. Basically, licensed content can be written to an SD card and cannot be run except from that specific card.

Applications

In general, SD and MMC cards are interchangeable and either can be used in a compatible device. An SD card can generally cost more than an MMC card with the same capacity, but as seen in this Tech Tip, it offers more bang for the buck.

Many card readers are available for personal computers that promote the ability to read and write to a variety of common flash media formats. A 15-in-1 reader/writer such as this one can be made quite compact thanks in part to the fact that two of the 15, MMC and SD, can be read from the same device slot.

MP3 players generally come with a base amount of memory for storing music files, but having an expansion slot allows users to increase capacity and play time by adding flash memory of their choice. The Pogo RipFlash MP3 Player is such a device, providing a built-in 256MB as well as an SD/MMC slot for easy expansion.

Mobile phones and PDAs can also take advantage of increased storage space thanks to flash memory slots. The Handspring Treo 600 is a phone/PDA combination that offers an SD/MMC slot for added convenience.

And of course, digital cameras use flash memory as their “film,” where bigger, faster cards are always a welcome upgrade. odak DX7630 6.1 MP camera with 3x optical zoom and 4x digital zoom The 6.1 Megapixel Kodak DX7630 could fill the same SD/MMC card much faster than the 3.2 Megapixel Umax AstraPix 640, but one of the advantages of these cards is that the user can choose the size, as well as the amount to have on hand, to suit their particular needs and budget.

A word of warning: be sure to check on your device what card capacity it can handle. If your camera can only handle up to a 512MB card, then using a 1GB card in the camera won’t make sense (depending on the device, some won’t even be able to read the card, while others will only use up to the capacity for which either way, you’ll want to make sure the card matches the device correctly.) So, as always, check your product manual to make sure you buy compatible memory.

Last words

MMC and SD are two of the most widely used flash memory formats, but as mentioned, there are several others. Watch for next week’s tech tip: Part II of the Flash Memory Series will cover Compact Flash, Smart Media, Memory Stick and xD formats.