April 7th, 2008 SD vs. MMC

May 9th, 2008 Adapting the Sample Host Controller

May 14th, 2008 Understanding Critical Section Deadlocks in Windows CE 6

More > > > > > >

Email:

. . . . . . . . . . . . . . . . . . . . . . RETURN TO BLOG HOME

VISIT WWW.BSQUARE.COM

This is the second in a series of posts about SD architecture. If you’re not familiar with the technology, start with my SD 101 post.

While Secure Digital (SD) and MultiMediaCard (MMC) memory cards are interchangeable in many ways, there are some key differences. Choosing the right card for your embedded project depends on your needs.

Physical characteristics
SD cards are based on the design of MMCs. They have the same length and width, but their thickness can vary. Both types of cards measure 32 mm by 24 mm. All MMC cards, however, are 1.4 mm thick. The thickness of the SD cards varies. Some SD cards are as thick as 2.1 mm. Others are as thick as MMC cards.

This results in one major interchangeability issue. While all MMC cards fit in SD slots, not all SD cards work in MMC slots.

The cards also have a different number of pins. The original MMC specification calls for a seven-pin interface; the latest version of the MMC specification supports an 8-bit data bus, larger than is currently supported in the latest SD specification. SD and SDIO have nine pins.

Despite the difference in the number of pins, the cards are mechanically interchangeable. However, software drivers and electrical issues need to be addressed.

Data protection
Another major difference between the cards is the issue of data protection. The S in SD stands for secure, as in copyright security.

If copyright protection is an issue in your device, this capability may be a consideration. If not, be aware that the security will consume some space on the card, whether or not you use it.

Information on the SD security specification is available only from the SD Card Association. The MultiMediaCard Association also offers a competing standard called Secure MMC.

SD cards also offer write protection. They have a write protection tab – similar to the tab on old floppy disks. If that tab is open, the card is read-only.

Speed
At first, the main difference between SD and MMC cards was speed. The first SD specification offered maximum transfer rates four times greater than MMC. Today, the cards are more closely matched.

Each new generation offers much more power and the rival associations are fiercely competitive. If high performance cards are critical to your project, make sure you consult the latest specifications from both associations.

Next week, I’ll offer some tips for adapting the BSQUARE sample SDIO host controller.

. . . . . . . . . . . . . . . . . . . . . . .
Raymond Yip
Senior Software Engineer
Professional Engineering Services
BSQUARE Corporation | Contact Me!