A USB flash drive consists of flash memory data storage device integrated with a USB (Universal Serial Bus) 1.1 or 2.0 interface. USB flash drives are typically removable and rewritable, much smaller than a floppy disk, and most weigh less than 30 g (1 oz).[1] Storage capacities in 2010 can be as large as 256 GB[2] with steady improvements in size and price per capacity. Some allow 1 million write or erase cycles[3][4] and have a 10-year data retention cycle.[citation needed]
USB flash drives are often used for the same purposes as floppy disks were. They are smaller, faster, have thousands of times more capacity, and are more durable and reliable because of their lack of moving parts. Until approximately 2005, most desktop and laptop computers were supplied with floppy disc drives, but most recent equipment has abandoned floppy disk drives in favor of USB ports.
Flash drives use the USB mass storage standard, supported natively by modern operating systems such as Windows, Mac OS X, Linux, and other Unix-like systems. USB drives with USB 2.0 support can store more data and transfer faster than a much larger optical disc drive and can be read by most other systems such as the PlayStation 3.
Nothing moves mechanically in a flash drive; the term drive persists because computers read and write flash-drive data using the same system commands as for a mechanical disk drive, with the storage appearing to the computer operating system and user interface as just another drive.[4] Flash drives are very robust mechanically, and can withstand anything that does not actually break the circuit board or connector.
A flash drive consists of a small printed circuit board carrying the circuit elements and a USB connector, insulated electrically and protected inside a plastic, metal, or rubberized case which can be carried in a pocket or on a key chain, for example. The USB connector may be protected by a removable cap or by retracting into the body of the drive, although it is not likely to be damaged if unprotected. Most flash drives use a standard type-A USB connection allowing plugging into a port on a personal computer, but drives for other interfaces also exist.
Most USB flash drives draw their power from the USB connection, and do not require a battery. Some devices that combine the functionality of a digital audio player with flash-drive-type storage require a battery for the player function.
Flash memory combines a number of older technologies, with lower cost, lower power consumption and small size made possible by recent[update] advances in microprocessor technology. The memory storage was based on earlier EPROM and EEPROM technologies. These had very limited capacity, were very slow for both reading and writing, required complex high-voltage drive circuitry, and could only be re-written after erasing the entire contents of the chip.
Hardware designers later developed EEPROMs with the erasure region broken up into smaller "fields" that could be erased individually without affecting the others. Altering the contents of a particular memory location involved copying the entire field into an off-chip buffer memory, erasing the field, modifying the data as required in the buffer, and re-writing it into the same field. This required considerable computer support, and PC-based EEPROM flash memory systems often carried their own dedicated microprocessor system. Flash drives are more or less a miniaturized version of this.
The development of high-speed serial data interfaces such as USB made semiconductor memory systems with serially accessed storage viable, and the simultaneous development of small, high-speed, low-power microprocessor systems allowed this to be incorporated into extremely compact systems. Serial access requires far fewer electrical connections for the memory chips than does parallel access, which has simplified the manufacture of multi-gigabyte drives.
Computers access modern[update] flash memory systems very much like hard disk drives, where the controller system has full control over where information is actually stored. The actual EEPROM writing and erasure processes are, however, still very similar to the earlier systems described above.
Many low-cost MP3 players simply add extra software and a battery to a standard flash memory control microprocessor so it can also serve as a music playback decoder. Most of these players can also be used as a conventional flash drive, for storing files of any type.
[edit] History
[edit] First commercial product
Trek Technology and IBM began selling the first USB flash drives commercially in 2000. The Singaporean Trek Technology sold a model under the brand name "ThumbDrive", and IBM marketed the first such drives in North America with its product named the "DiskOnKey" -which was developed and manufactured by the Israeli company M-Systems. IBM's USB flash drive became available on December 15, 2000,[5] and had a storage capacity of 8 MB, more than five times the capacity of the then-common floppy disks.
In 2000 Lexar introduced a Compact Flash (CF) card with a USB connection, and a companion card read/writer and USB cable that eliminated the need for a USB hub.
In 2002 Netac Technology, a Shenzen consumer electronics company which claims to have invented the USB flash drive in the late 1990s,[6] was granted a Chinese patent for the device.[7]
Both Trek Technology and Netac Technology have tried to protect their patent claims. Trek won a Singaporean suit,[8] but a court in the United Kingdom revoked one of Trek's UK patents.[9] While Netac Technology has brought lawsuits against PNY Technologies,[7] Lenovo,[10] aigo,[11] Sony,[12][13][14] and Taiwan's Acer and Tai Guen Enterprise Co,[14] most companies that manufacture USB flash drives do so without regard for Trek and Netac's patents.
[edit] Second generation
Modern flash drives have USB 2.0 connectivity. However, they do not currently use the full 480 Mbit/s (60MB/s) the USB 2.0 Hi-Speed specification supports because of technical limitations inherent in NAND flash. The fastest drives currently available use a dual channel controller, although they still fall considerably short of the transfer rate possible from a current generation hard disk, or the maximum high speed USB throughput.
Typical overall file transfer speeds vary considerably, and should be checked before purchase. Speeds may be given in Mbyte per second, Mbit per second, or optical drive multipliers such as "180X" (180 times 150 KiB per second). Typical fast drives claim to read at up to 30 megabytes/s (MB/s) and write at about half that, about 20 times faster than older "USB full speed" devices, which are limited to a maximum speed of 12 Mbit/s (1.5 MB/s).
[edit] Design and implementation
One end of the device is fitted with a single male type-A USB connector. Inside the plastic casing is a small printed circuit board. Mounted on this board is some power circuitry and a small number of surface-mounted integrated circuits (ICs). Typically, one of these ICs provides an interface to the USB port, another drives the onboard memory, and the other is the flash memory.
Drives typically use the USB mass storage device class to communicate with the host.
|
| |
| 1 | USB connector |
|---|---|
| 2 | USB mass storage controller device |
| 3 | Test points |
| 4 | Flash memory chip |
| 5 | Crystal oscillator |
| 6 | LED |
| 7 | Write-protect switch (Optional) |
| 8 | Space for second flash memory chip |
[edit] Essential components
There are typically four parts to a flash drive:
- Male type-A USB connector – provides an interface to the host computer.
- USB mass storage controller – implements the USB host controller. The controller contains a small microcontroller with a small amount of on-chip ROM and RAM.
- NAND flash memory chip – stores data. NAND flash is typically also used in digital cameras.
- Crystal oscillator – produces the device's main 12 MHz clock signal and controls the device's data output through a phase-locked loop.
[edit] Additional components
The typical device may also include:
- Jumpers and test pins – for testing during the flash drive's manufacturing or loading code into the microprocessor.
- LEDs – indicate data transfers or data reads and writes.
- Write-protect switches – Enable or disable writing of data into memory.
- Unpopulated space – provides space to include a second memory chip. Having this second space allows the manufacturer to use a single printed circuit board for more than one storage size device.
- USB connector cover or cap – reduces the risk of damage, prevents the ingress of fluff or other contaminants, and improves overall device appearance. Some flash drives use retractable USB connectors instead. Others have a swivel arrangement so that the connector can be protected without removing anything.
- Transport aid – the cap or the body often contains a hole suitable for connection to a key chain or lanyard. Connecting the cap, rather than the body, can allow the drive itself to be lost.
- Some drives offer expandable storage via an internal memory card slot, much like a memory card reader.[15][16]
0 comments:
Post a Comment