USB flash drive
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A USB Flash drive or keydrive is a small computer storage device that connects directly to a USB port with a built-in connector. Many can be put on keyrings. They come in many different designs and colours. Keydrives store files on flash memory. Many different capacities exist, the most commonly used ones are 256 or 512 megabytes, but capacities from 8 megabytes up to 4 gigabytes have been produced. Some people use keydrives instead of floppy disks because they hold more files and are much more durable.
Some keydrives have a light that blinks to show if it is being used by the computer. Other features include a switch to protect files from being erased, password protection, and a fingerprint reader.
Other common names for a keydrive include: USB memory key, USB flash drive, pendrive and jumpdrive. Sandisk and Lexar make a very large portion of the keydrives sold, but many small companies make them as well.
The memory chips used on such a drive are usually NAND gates, to provide for flash memory. This memory has been put together with an USB 1.1 or 2.0 type adapter. The different USB versions are mainly different as to transfer speed.
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[edit] Components
This photograph shows both sides of the printed circuit board (PCB) inside a typical flash drive (circa 2004). The flash drive in this photograph is a 64 MB USB 2.0 device with its plastic case removed.
One end of the device is fitted with a single male type-A USB connector. Inside the plastic casing is a small, highly cost-engineered, printed circuit board. Mounted on this board is some simple 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.
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 |
8 | Unpopulated space for second flash memory chip |
[edit] Essential components
The parts of a typical flash drive are as follows:
- Male type-A USB connector - provides an interface to the host computer. (item 1 in the diagram)
- USB mass storage controller - implements the USB host controller and provides a seamless linear interface to block-oriented serial flash devices while hiding the complexities of block-orientation, block erasure, and wear balancing. The controller contains a small RISC microprocessor and a small amount of on-chip ROM and RAM. (item 2 in the diagram)
- A NAND flash memory chip - stores data. NAND flash is typically also used in digital cameras. (item 4 in the diagram)
- Crystal oscillator - produces the device's main 12 MHz clock signal and controls the device's data output through a phase-locked loop (the crystal itself is item 5 in the diagram)
[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. (item 3 in the diagram)
- LEDs - indicates data transfers or data reads and writes. (item 6 in the diagram)
- Write-protect switch - indicates whether the device should be in "write-protection" mode. (item 7 in the diagram)
- Unpopulated space - provides space to include a second memory chip. Having this second space allows the manufacturer to develop only one PCB that can be used for more than one storage size device, to meet the needs of the market. (item 8 in the diagram)
- USB connector cover or cap - reduces the risk of damage due to static electricity, improves overall device appearance. Some flash drives do not feature a cap, but instead have retractable USB connectors. Other flash drives have a "swivel" cap that is permanently connected to the drive itself and eliminates the chance of losing the cap.
- Transport aid - In some cases, the cap contains the hole suitable for connection to a key chain or lanyard or to otherwise aid transport and storage of the USB flash device. However, this increases the risk of the device being lost during transport. For this reason, most devices now have the hole on the main device body, though this requires the entire keychain to be attached to the computer. Some, therefore, have holes in both places.
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USB flash drives