CompactFlash (CF) – What is it?
The format was first specified and produced by SanDisk in 1994. The physical format is now used for a variety of devices.
CompactFlash became 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 after the introduction of CompactFlash, such as SD/MMC, various Memory Stick formats, xD-Picture Card, offered stiff competition. Most of these cards are significantly smaller than CompactFlash while offering comparable capacity and read/write speed. Proprietary memory card formats intended for use in the field of professional audio and video, such as P2 and SxS, are physically larger, faster, and significantly more expensive.
CompactFlash remains popular, and has even experienced something of a comeback. For example, in 2008 Sony chose CompactFlash as the recording media for use in the HVR-MRC1K tapeless video recorder instead of using smaller MemoryStick cards or expensive SxS cards. In 2010 Canon chose CompactFlash as the recording media for use in its new lineup of professional high definition video cameras.
In November 2010, Sandisk, Sony and Nikon proposed a next generation card format targeted at high-definiton camcorders and high-resolution digital photo cameras; not backward compatible, the proposed format would come in a similar form factor as CF/CFast but be based on PCI Express instead of ATA and would offer read speed of 500 MB/s and storage capabilities beyond 2 TiB. It remains unclear, if the CFA will adopt this proposal, as CF 5.0 already supports media up to 128 PiB and CF 6.0 adds speeds up to 167 MB/s, while the SATA-based CFast already defines speeds up to 300 MB/s.
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, such as the Hasselblad CFV Digital Back for the Hasselblad series of medium format cameras. 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 PC Card standard.
CompactFlash was originally built around Intel's NOR-based flash memory, but has switched to NAND technology. CF is among the oldest and most successful formats, and has held a niche in the professional camera market especially well. It has benefited from both a better cost to memory-size ratio than other formats, (for much of its life) and generally from greater available capacity 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 to 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 that 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, is limited to a finite number of erase/write cycles for any "block," before that block can no longer be written to successfully. Typically, the controller in a CompactFlash device attempts to prevent premature wear of a sector by choosing the write location for a piece of data so as to spread the writing over the device. This process is called wear leveling.
When using CompactFlash in ATA mode as a hard drive replacement, wear leveling becomes critical. The advanced CompactFlash controllers spread the wear-leveling across the entire drive allowing all blocks to participate. The more advanced CompactFlash controllers will move data that rarely changes to ensure all blocks wear evenly.
