CompactFlash
CompactFlash (CF) is a mass storage device format used in portable electronic devices. For storage, CompactFlash typically uses flash memory in a standardized enclosure.
The format was first specified and produced by SanDisk in 1994. The physical format is now used for a variety of devices.
CompactFlash became a popular storage medium for digital cameras. In recent years it has been widely replaced by smaller cards on the consumer end, but it is still a preferred format for D-SLR cameras, for its superior capacity and reliability.
There are two main subdivisions of CF cards, Type I (3.3 mm thick) and the thicker Type II (CF2) cards (5 mm thick). The CF Type II slot is used by Microdrives and some other devices. There are four main speeds of cards including the original CF, CF High Speed (using CF+/CF2.0), a faster CF 3.0 standard and a yet faster CF 4.0 standard that is being adopted as of 2007. The thickness of the CF card type is dictated by the preceding PCMCIA card type standard which was used for data storage in previous years.
CompactFlash was originally built around Intel's NOR-based flash memory, but it has switched over to NAND. CF is among the oldest and most successful formats, and has held on to a niche in the professional camera market especially well. It has benefited from having both a better cost to memory size ratio than other formats for much of its life, and generally having larger capacities available than other formats.
CF cards can be used directly in a PC Card slot with a plug adapter, used as an ATA (IDE) or PCMCIA storage device with a passive adapter or with a reader, or attached various other types of ports such as USB or FireWire. As some newer card types are smaller, they can be used directly in a CF card slot with an adapter. Formats which can be used this way include SD/MMC, Memory Stick Duo, xD-Picture Card in a Type I slot, and SmartMedia in a Type II slot, as of 2005. Some multi-card readers use CF for I/O as well.
Flash memory, regardless of format, can take only a limited number of erase/write cycles to a particular "sector" before that sector can no longer be written. Typically, the controller in a CompactFlash device attempts to prevent premature wearout of a sector by choosing the location for a piece of data at write time so as to spread out the writing over the device. This process is called wear levelling.
NOR-based flash has lower density than newer NAND-based systems, and CompactFlash is therefore the physically largest of the three memory card formats that came out in the early 1990s, the other two being Miniature Card (MiniCard) and SmartMedia (SSFDC). However, CF did switch to NAND type memory later on. The IBM Microdrive format implements the CF Type II interface, but is not solid-state memory.
CompactFlash defines a physical interface which is smaller than, but electrically identical to, the ATA interface. That is, it appears to the host device as if it were a hard disk. The CF device contains an ATA controller. CF devices operate at 3.3 volts or 5 volts, and can be swapped from system to system. CF cards with flash memory are able to cope with extremely rapid changes in temperature. Industrial versions of flash memory cards can operate at a range of −45 to +85 °C.
CF has managed to be the most successful of the early memory card formats, outliving Miniature Card, SmartMedia, and PC Card Type I in mainstream popularity. The memory card formats that came out in the late 1990s through the early 2000s (SD/MMC, various Memory Stick formats, xD-Picture Card, etc.) offered stiff competition. The new formats were significantly smaller than CF, in some cases by an even greater fraction than CF had been smaller than PC Card. These new formats would eventually dominate the memory card market for compact consumer electronic devices.
Speed
Flash memory devices are non-volatile and solid-state, and thus are more robust than disk drives. Cards consume around 5% of the power required by small disk drives and still have reasonable transfer rates of over 45 MB/s for the more expensive 'high speed' cards.
Card speed is usually specified in "x" ratings, e.g. 8x, 20x, 133x. This is the same system used for CD-ROMs and gives the data rate as a multiple of the data rate of the first CD-ROMs (i.e. the data rate of an audio CD). The base rate is 150 kB/s, so for example, 20x = 20 - 150 kB/s = 3.0 MB/s.
The following table lists some common ratings and their respective maximum transfer rates.