How Secure Digital Memory Cards Work

Secure Digital (SD) cards are flash memory cards that comply with standards set by the SD Association.
Hemera/Thinkstock

Many laptops and mobile devices you can buy today have built-in slots for Secure Digital memory cards, usually just called SD cards. An SD card is a storage device that has many useful features depending on how and where it's used. Here are some common ways you might already use SD cards:

  • You can add an SD card to a small device, like a mobile phone, to extend the storage space available for ringtones, texts, apps, music and other data.
  • When one SD card in your camera fills up with photos and videos, you can swap it out for an empty SD card instead of deleting any files.
  • You can save files to an SD card to transfer them between devices or give them to someone else.

SD cards conform to standards developed and maintained by the Secure Digital Association. Panasonic, SanDisk and Toshiba formed the SD Association in January 2000 with the goal of developing and promoting standards for flash-memory storage. SanDisk's technology, already a global leader in flash memory products, became a template for the new standards.

Headquartered in San Ramon, Calif., the SD Association doesn't make or market any products on its own. However, more than 1,000 association members, primarily major electronics manufacturers worldwide, currently contribute to the ongoing improvements in SD standards. These standards cover the SD cards themselves, as well as the hardware and device drivers used to read and write to them.

You may have used other words, such as memory cards or flash memory, to describe SD cards in conversation. The word memory, however, might be misleading to some people since it's also used to talk about Random Access Memory (RAM), which serves a completely different purpose in computing. The word flash describes the means in which the data is saved onto the solid-state material inside the SD card, which you can read about in How Flash Memory Works. Here, we'll use the term SD card since we're speaking of a flash-memory device that conforms to the SD standard and sports the SD Association's trademark.

In this article, we'll zoom in on SD's various form factors, capacities and speeds and take a look at the features, advantages and challenges of Secure Digital cards. Let's start with the types of SD cards available today.

Types of SD Cards

SD cards come in a standard set of sizes and capacity formats, and from a variety of manufacturers. Shown here are a standard SD card, two SDHC cards and a microSDHC card.
SD cards come in a standard set of sizes and capacity formats, and from a variety of manufacturers. Shown here are a standard SD card, two SDHC cards and a microSDHC card.
Photo by Stephanie Crawford

At first glance, an SD card is a small, flat, rectangular object with a notch along one edge and copper leads, called pins, embedded on one side along another edge. But there are a few different form factors. Here are the physical sizes you'll see for SD cards:

  • 32 x 24 millimeters, 2.1 millimeters thick - This is the most common size you'll see. This size has nine pins and features a write-protection switch on one side to toggle between read-only and read-write states.
  • 20 x 21.5 millimeters, 1.4 millimeters thick - This is the "mini" version with 11 pins, and it's less common than the other two sizes.
  • 15 x 11 millimeters, 1 millimeter thick - This is the "micro" version with eight pins. Because it's so small, this size has become popular in mobile devices like smartphones.

It's not just the physical makeup of the card that's standardized. SD cards also come in different capacities. These are the capacity formats for SD cards listed in order from oldest to newest:

  • SD - Standard format with up to 2 GB per card, available in all three sizes, under $10
  • SDHC - High-capacity format with 4 to 32 GB per card, available in all three sizes, $10 to $100
  • SDXC - eXtended capacity format with 32 GB to 2 TB per card, available in the larger and micro sizes only, starting around $80

That's three capacity formats, each in two or three sizes. To read and write data to the card, you'll need card-reader hardware that can handle both the size factor and the capacity class indicated. Also, if you're using a mini- or micro-size card but have the large-size reader, you can insert the smaller card into a special adapter in the shape of the larger card. Some SD card manufacturers even sell microSDHC cards together with SDHC-sized adapters to make it easier to move the cards between devices.

This is where you have to be cautious. For example, if you have an older digital camera that takes SD format cards, an SDHC card might not work, even if it fits into the camera. Check your hardware specification to find out what capacity formats your device supports. Then look for cards featuring the SD logos that correspond to the capacity-size combination you need. You can see all those logos at the SD Association Web site here.

In addition to these SD card types, there is another classification you might want to know when purchasing an SD card: its speed. Next, let's look at what activities require higher SD card speeds and how the SD Association classifies cards by speed.

Speed Class and UHC Speed Class

Another measurement you'll see on an SD card is its speed. Speed isn't very important if you're just storing files or taking still photographs. However, speed is very important when it comes to shooting and playing back video, particularly in high-definition (HD). When you have a higher speed SD card, you can capture better quality video with smoother playback.

The SD Association has two standards associated with speed: Speed Class and Ultra High Speed (UHS) Speed Class. These two standards are not compatible, so be sure to use the standard appropriate for your recording device. Let's take a closer look at each class and how to determine what speeds an SD card is capable of.

Speed Class applies to all SD cards. You'll see the Speed Class for an SD card next to its SD logo as a number inside a circular "C" shape. The following are the Speed Classes defined by the SD Association and the cards, as well as the video recording and playback qualities usually associated with those classes:

  • 2 - Standard-definition (SD), maximum speed of 12.5 MB per second
  • 6 or 4 - HD (720p, 1080i, 1080p), maximum speed 25 MB per second
  • 10 - Full HD (1080p), maximum speed of 25 MB per second

The Ultra High Speed (UHS) Speed Class is available in SD cards that feature the UHS-I bus-interface. That means the SD card has circuitry designed to read and write to memory at speeds up to 312 MB per second. A UHS SD card packaging might feature the Speed Class plus an additional class, written as a number inside a "U" shape. On the card itself, though, the UHS Speed Class is denoted by the Roman numeral "I" to the right of the SD logo.

Now that you're up to speed on SD card speeds and capacities, let's see what makes Secure Digital cards "secure."

The Security in Secure Digital

What makes a Secure Digital memory card secure? All SD cards have copyright protection for SD standard data types. This enables commercial distributors to put music or other media on the device so that it's protected from illegal copying. The technology behind this SD card feature is called Content Protection for Recordable Media (CPRM).

CPRM was developed by the 4C Entity, LCC, the organization handling copyright protection licensing for IBM, Intel, and SD co-founders Panasonic and Toshiba. 4C Entity also developed the related technology Content Protection for Pre-recorded Media (CPPM), which relates to the DVD-Audio format for music. CPRM and CPPM are generally associated with flash and DVD media formats.

CPRM doesn't apply automatically just because you're using an SD card. In order for data to be protected with CPRM, the device that's writing the data must first create a Media Identifier and Media Key Block (MKB) on the SD card. The Media Identifier and MKB are written to a physical portion of the SD card known as the Protected Area. Data stored in the Protected Area is inaccessible through the SD card's file system, but any CPRM-enabled device reading the SD card can use it for content protection operations.

To protect data with CPRM, a device uses the Media Identifier and MKB to encode the data as it's written to the SD card. To read the data, a CPRM-enabled device accesses the Media Identify and MKB and uses them to decrypt the data as it's read from the SD card. Therefore, even if you copy the CPRM-protected files from the SD card's file system, you won't be able to read them outside of that SD card.

In addition to the CPRM, there is another security feature you'll notice on the 32 x 24-millimeter SD cards (SD, SDHC and SHXC): a write-protection lock. The lock is a tiny slider along one side of the SD card. In unlocked position, you can read and write to the SD card as you would with a USB drive or any other read-write storage device. In the locked position, the SD card will not allow any changes to its file system, making it a read-only device. You might use this feature if you're concerned about accidentally overwriting data from a camera, for example, while reading the SD card on your laptop or desktop computer.

Until now we've focused on the capacity, speed and security associated with SD cards. On the next page, we're popping off the plastic cover and taking a look at the technology inside.

Inside the SD Card

What's on the inside? This is a look inside a 2 GB standard SD card by SanDisk.
What's on the inside? This is a look inside a 2 GB standard SD card by SanDisk.
Photo by Stephanie Crawford

An SD card is a solid-state device. This means it has no moving parts in order to function. This is a big improvement over older portable storage devices. Floppy disks, for example, had flimsy discs inside which spun at high speeds and were subject to data loss when placed near magnetic fields. CDs and DVDs are larger and less durable, and their storage capability degrades quickly with lots of rewrites. An SD card's components are part of its circuitry, and its packaging is small and durable compared to these predecessors.

Speaking of durable, we cracked open an SD card for the picture on this page. It wasn't easy, which is a testament to the aforementioned durability. The mini and micro sizes are more delicate, though, and you still don't want to store an SD card where it could possibly bend, chip or break.

Inside the SD card is circuitry that includes flash memory. In our article How Flash Memory Works, we cover how that circuitry controls the flow of electricity when writing or erasing data on the flash memory chip. Even though each SD card complies with standards for the form factor, capacity, speed and common input/output interface, different SD card manufacturers take different approaches in designing the flash memory and supporting components inside.

In compliance with standards, each SD card comes formatted with a File Allocation Table (FAT) file system, which is compatible with just about every operating system that might want to access the files in that system. Occasionally, you might need to reformat an SD card, either to "zero out" the data for a thorough wipe of all data, or to recover the file system after a corruption, which could happen when removing the card during critical read-write operations. When you need to reformat, the SD Association recommends using its SD Formatter software available here. One reason for this is that third-party disk formatters may not account for the Protected Area used in the SD card's CPRM functions.

So far, we've examined the SD card hardware and the technical side of how it works. Next, let's discover some of the challenges involved when using SD cards and card readers.

Challenges in Using SD Hardware

A microSD card can be tricky to handle. Here's one inside a smartphone, carefully tucked under a small metal bracket next to the microSD logo.
A microSD card can be tricky to handle. Here's one inside a smartphone, carefully tucked under a small metal bracket next to the microSD logo.
Stephanie Crawford

SD cards have a lot going for them. They're small, durable and can be used in any of thousands of devices. They do have some challenges and drawbacks, though.

One drawback involves the known limitations of flash-memory technology. According to Toshiba, the company that invented the technology, the memory cells in the solid-state SD card can only go through about 10,000 write/erase cycles before the hardware is incapable of retaining or updating its data. Though this is a proven limitation, it may rarely be a major problem for you since it's equivalent to completely writing and erasing the entire card's contents once a day, every day for 27 years. [source: Toshiba]

Perhaps more important than data lifespan are the challenges of handling the physical cards. Though each SD card is sturdy, it can fall out of a pocket, slip into tight crevices, get lost in a stack of papers, or be irreparably damaged when stepped on or submerged in a liquid. Sliding the SD card in and out of a card reader can be tricky, too, especially for the mini- and micro-sized cards. If the SD card protrudes from the device you're using it in, be careful when you're moving the device, too. To meet these challenges, always handle the card carefully, stay aware of where it is while you're using it and protect it between uses.

Another challenge with SD cards is making sure you have the right type for your device. Selecting the size you need is easy; usually you can just look at an SD card slot and know which of the three sizes is required. However, you also need to select the capacity and speed that's compatible with your device. This can be tricky when you're looking at a wall of SD cards in a store and trying to select the one that gives you the most for your money. This challenge is best met with knowledge about what SD cards your device can handle, both minimum and recommended, and what the SD logos look like for the standard you need: SD, miniSD, microSD, SDHC, miniSDHC, microSDHC, SDXC and microSDXC.

One last challenge with SD cards is something you can't get away from no matter what technology you use: the tendency for old standards to become obsolete and newer ones to replace them. For an SD card itself, this is less of an issue as newer card readers can read older cards. It's devices that use SD cards that present the bigger challenge. To continue using an older device, you'll need to find cards that conform to older standards. SD hasn't retired any standards yet, though, so look for those older standard cards to be available for a while yet.

With so much going for it, what does the future look like for SD card technology? Let's examine that on the next page.

The Future for Secure Digital Cards

Some laptops feature SD card readers, making it quick and easy to read SD cards from cameras and other devices.
Some laptops feature SD card readers, making it quick and easy to read SD cards from cameras and other devices.
Photo by Stephanie Crawford

With the boom in SD card technology in thousands of devices, it's natural to conclude that SD cards will be around for years to come. The SD Association continues to expand on SD cards, incorporating new standards as manufacturers contribute improvements to flash-memory technology. As of this writing, the UHS-II was in development to standardize even faster read-write speeds for SD cards.

With so many devices adding SD card readers, manufacturers have asked, "Can you use that slot for other things besides flash memory?" The hardware interface provides both power and data exchange for SD cards, so it seems reasonable to assume that it could do more than just read and write data. Recognizing this demand, the SD Association developed the SDIO card interface. By adding an SDIO card interface to a device, manufacturers can adapt the device for use in an existing SD card reader.

Here are just some of the functionalities covered by the SDIO card interface standard:

  • Wireless connectivity using 802.11b Wi-Fi or Bluetooth
  • Media capture through cameras and voice recorders
  • Scanners and barcode readers
  • TV and radio tuners

One amazing little device taking advantage of SDIO is the Eye-Fi card. Founded in 2005, Eye-Fi, Inc. has patent-pending technology that can turn the SD card slot in your camera into a portal for uploading photos directly to the Web. The Eye-Fi card includes a Wi-Fi radio and software designed to connect to a Wi-Fi network, and then use that connection to upload photos to your computer or a photo-sharing Web site like Flickr or Facebook. Eye-Fi cards range from $49.99 to $99.99.

In short, the future for SD cards looks bright thanks to the wide adoption of SD standards and the continued improvements and expansions to those standards. In addition, the cost of SD cards seems to be more reasonable over time, with brand competition keeping prices in check. For more information on Secure Digital memory cards, plug in to the next page.

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Sources

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  • 4C Entity, LLC. "FAQs." (Sept. 27, 2011) http://www.4centity.com/faqs.aspx
  • Eye-Fi, Inc. "About Us." (Sept. 28, 2011) http://www.eye.fi/about/us
  • Intel Corporation. "Intel and CPRM/CPPM." (Sept. 27, 2011) http://www.intel.com/standards/case/case_cp.htm
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  • SD Association. "Developers Overview." (Sept. 27, 2011) http://www.sdcard.org/developers/
  • SD Association. "SDIO." (Sept. 27, 2011) http://www.sdcard.org/developers/overview/sdio
  • Toshiba. "Toshiba America Electronic Components, Inc. Releases Performance Research on MLC NAND Flash Memory For Consumer Applications." May 20, 2004. (Sept. 27, 2011) http://www.toshiba.com/taec/news/press_releases/2004/file_04_211.jsp