Secure Digital (SD) is a non-volatile memory card format for use in portable devices. It was derived from an earlier effort called the Multimediacard (MMC) which is a flash memory memory card standard. While these devices are called memory cards technically they are really Solid State Disk drives (SSD) since they are generally accessed like disk drives and are formatted to emulate a disk drive.
They are both about the size of a postage stamp: 24 mm x 32 mm. The MMC is 1.4mm thick while the SD card is 2.1mm thick.
MMC was unveiled in 1997 by Siemens AG and SanDisk, it is based on Toshiba's NAND-based flash memory, and is therefore much smaller than earlier systems based on Intel NOR-based memory such as CompactFlash (CF). In August of 1999, Panasonic, SanDisk, and Toshiba first agreed to develop and market the SD (Secure Digital) Memory Card which added support for DRM and a higher memory density. The SD card also introduced the Serial Peripheral Interface Bus. This is a master/slave bus with the computer initiating the connection.
The original MMC card has a 7 pin interface and supports serial data transfer. The SD card has a 9 pin interface and supports the MMC data mode plus a faster 4 bit parallel mode. An SD card slot can typically support either card type however a card slot specifically targeted for MMC will be too thin to accept the thicker SD card or if it accepts the card it may only use the slower serial interface. The data is clock synchronized using a variable clock rate up to 50 MHz. Using 4 bit parallel could theoretically support a 25 MB/s bus transfer rate.
The data on these cards is organized similar to a disk drive with discrete blocks of information being transferred. Originally all of the data was arranged in 512 byte blocks using the FAT file system pioneered on PC's. MMC and SD file formats are compatible up to a 1 GB card size. At two Gigabytes the SD file system doubled the block size to 1 KB. Many older devices do not support the 2 GB card due to this change in the formatting. A newer MMC standard has overcome this limitation.
 SD specifics
An SD card uses an asymmetric physical design with a corner cut off to avoid its being plugged in backward or upside down. It has a cutout notch on one side that allows it to be tested for Read only or R/W ability. A small switch can be used to cover the slot to enable or disable writing to the card.
A portion of the memory capacity is not available for standard storage. This portion is used to support a DRM scheme that allows protected content to be sold on the device although this capability is seldom used. The rest of the card is formatted to mimic a standard disk drive and is pre-formatted when it is sold. Many devices permit reformatting the card and this is sometimes required when the card is used on a device with special formatting requirements.
A standard SD card permits capacities from 16 MB up to 2 GB. Some non-standard implementations have extended this capability to 4 GB but may not work on all devices. A card is typically formatted as a FAT16 volume but some devices may prefer a FAT32 formatted file system.
Many SD cards are advertised to be high speed capable. This generally refers to the write speed and is particularly important for cameras which wish to record a picture at a very high speed. This speed usually does not equate the read speed that is the dominant need of an eBook reader. Most of the time you will need no difference or very little difference in performance when used on an eBook reader.
The SD interface standard is a closed standard and only available to a select few although a simplified interface description has now been made available to the public. Many SD interface drivers and cards were developed by reverse engineering the standard and in some cases they may not have gotten everything exactly right. This is often the reason a particular card may not work in a particular device. It is a variant of SPI.
When memory technology permitted capacities beyond 2 GB the SD card association decided to come out with a new standard called SDHC (Secure Digital High Capacity). This standard is based on a FAT32 file system and several other changes. Many device controllers cannot support this standard and cannot work with SD cards that are built to this standard. A controller that accepts SDHC will also work with older cards. SDHC cards have capacities of 4 GB up to 32 GB and are rated as to speed.
Some devices may not be able to format an SDHC card but can otherwise use one. For other devices it is not clear whether they can do SDHC or not. Generally the eBook reader manufacturers have been less than forthcoming about SDHC so basically we depend on users to tell us if they work.
SDHC also includes guaranteed speed classes for write speed. The three SDHC speed classes — Class 2, Class 4 and Class 6 — transfer data at least 2, 4 and 6 MB per second. This is typically under worst case conditions, generally a device will be much faster than the guaranteed numbers sometimes achieving 20 MBps or a bit more.
Some users have reformatted SDXC cards to FAT32 to exceed the 32GB limit. This has worked successfully allowing increased storage capacity on some devices that only support SDHC. It is not known if there are performance issues.
The Secure Digital Extended Capacity (SDXC) format was unveiled at CES 2009. The theoretical maximum capacity has been increased to 2TB (2048 GB), with a maximum transfer rate of 300 MBps. SDXC Cards will utilize Microsoft's exFAT file system. Initially released devices are expected to reach 110 MBps. Typical sizes include: 64GB, 128GB, 256GB, 512GB, 1TB, 2TB.
exFAT (extended file allocation table), also known as FAT64, was released with WinCE 6. It is useful when there is a desire for high capacity with less system overhead than is required for the Windows NTFS disk format. It has been released on later Windows OS versions including Vista and Windows 7, Windows 8 in both 32 bit and 64 bit versions and Windows 10 in both versions. Typically you can download the driver and update an SD card slot. Note that you must only format one of these cards using the exFAT format. Plugging one of these cards into a device that does not support this format may give an error message and suggest that you reformat the card. Do not do that just because of the message. NTFS might be used on such devices.
A card that is intended for Windows use or any hardware that supports regular disk drives can also support NTFS files system which may provide a slight speed advantage. The capacities are the same as a disk drive and certainly an alternative to SDXC for 64 GB and above. No card is delivered in this format but you can reformat the card on any Windows computer.
Similar to CF card slots the SD card standard also defines an interface for I/O (Input/Output) devices. A particular implementation may or may not support I/O devices. I/O devices such as wifi cards, GPS cards, Bluetooth cards and many other devices can be plugged into an SDIO capable card slot. There is also SDIO capability in some mini-SD card slots. Whether the card will actually work or not depends on the availability of a specific driver for the device on the operating system that supports I/O on the card slot.
An SDIO card could be the same size as an SD card but will typically be longer. The extra length will be for an antenna, a connector, or possibly the entire device with the card slot only serving as a connection to the parent device. This extra length and possibly width of the extended portion needs to be considered to determine if the card will actually work in the device.
 smaller SD cards
There are SD cards, mini-SD cards, and micro-SD cards. These are physical changes in the size of the card. Most eBook devices use standard sized SD cards physically but there are adapters for micro SD cards and mini SD cards that allow them to be used in an SD card slot. Micro SD cards were originally called trans flash or TF cards. Some of these cards predate SDHC capability.
While MultiMediaCard was the earlier product, see history, the SD card seems to have captured the market. Even so the MMCA organization has continued to develop improvements to the interface that is available as an open standard. Most recently, however, the MMCA has turned over their designs to the Jedec organization which is a standards organization for the solidstate industry.
Recent MMC implementations include the MiCard, a card that can be plugged directly in a USB slot, as well as advanced MMC capabilities. Newer MMC products include MMCPlus, MMC mobile HC, and higher capacity and faster MMC cards. These include 4 bit bus transfer and even 8 bit bus transfer devices. MMC cards are now available as Reduced Size MMC cards and higher capacity cards up to 4 GB cards with 8 GB on the way.
 Device Support
Most dedicated eBook Readers have support for SD cards and by extension MMC cards 1 GB and below. However only the newest devices support the SDHC standard. In some cases a controller/driver will support this new standard but the operating system was not modified to understand the increased capacity or to provide the ability to format the card. A 32 bit operating system can typically only count to 2G using signed integer numbers. A card with more capacity might be shown with a negative number or be wildly inaccurate.
Devices that will allow the use of an SDHC card include:
Devices that do not work with SDHC include:
- Bookeen Cybook Gen3 - uses up to 2 GB FAT32 cards
- iRex iLiad - only supports up to 1 GB cards. see Iliad Memory Cards
- Sony PRS500 - up to 2 GB see Sony Reader Memorycards