Pen Drive

USB flash drive

A USB flash drive is a NAND-type flash memory data storage device integrated with a USB (universal serial bus) interface. USB flash drives are typically removable and rewritable, much shorter than a floppy disk (1 to 4 inches or 2.5 to 10 cm), and weigh less than 2 ounces (60 g). Storage capacities typically range from 64 MB to 64 GB^[1] with steady improvements in size and price per gigabyte. Some allow 1 million write or erase cycles^[2][3] and have 10-year data retention,^[4] connected by USB 1.1 or USB 2.0. USB Memory card readers are also available, whereby rather than being built-in, the memory is a removable flash memory card housed in what is otherwise a regular USB flash drive, as described below.

USB flash drives offer potential advantages over other portable storage devices, particularly the floppy disk. They are more compact, faster, hold much more data, have a more durable design, and are more reliable for lack of moving parts. Additionally, it has become increasingly common for computers to ship without floppy disk drives. USB ports, on the other hand, appear on almost every current mainstream PC and laptop. These types of 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 also be faster than an optical disc drive, while storing a larger amount of data in a much smaller space.

Nothing actually moves in a flash drive: it is called a drive because it is designed to read and write data using the same system commands as a mechanical disk drive, appearing to the computer operating system and user interface as just another drive.^[3]

A flash drive consists of a small printed circuit board protected inside a plastic, metal, or rubberised case, robust enough to be carried with no additional protection, in a pocket or on a key chain for example. The USB connector is protected by a removable cap or by retracting into the body of the drive, although it is not liable to be damaged if exposed. Most flash drives use a standard type-A USB connection allowing them to be plugged into a port on a personal computer.

To access the drive it must be connected to a USB port, which powers the drive and allows it to send and receive data. Some flash drives, especially high-speed drives, may require more power than the limited amount provided by a bus-powered USB hub, such as those built into some computer keyboards or monitors. These drives will not work properly unless plugged directly into a host controller (i.e., the ports found on the computer itself) or a self-powered hub.

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 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.

Drives typically use the USB mass storage device class to communicate with the host.

Internals of a typical USB flash drive
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	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 — indicate whether the device should be in "write-protection" mode.
• Unpopulated space — provides space to include a second memory chip. Having this second space allows the manufacturer to develop only one printed circuit board that can be used for more than one storage size device, to meet the needs of the market.
• USB connector cover or cap — reduces the risk of damage and prevents the ingress of fluff or other contaminants, and 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 — the cap or the main body often contains a hole suitable for connection to a key chain or lanyard.