Tuesday, October 7, 2008

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

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.

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.

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