dimanche 6 juin 2010
Flash Memory
We store and transfer all kinds of files on our computers --
digital photographs, music files, word processing documents,
PDFs and countless other forms of media. But sometimes your
computer's hard drive isn't exactly where you want your
information. Whether you want to make backup copies of files
that live off of your systems or if you worry about your
security, portable storage devices that use a type of
electronic memory called flash memory may be the right
solution.
Electronic memory comes in a variety of forms to serve
a variety of purposes. Flash memory is used for easy and fast
information storage in computers, digital cameras and home
video game consoles. It is used more like a hard drive than
as RAM. In fact, flash memory is known as a solid state
storage device, meaning there are no moving parts --
everything is electronic instead of mechanical.
Here are a few examples of flash memory:
* Your computer's BIOS chip.
* CompactFlash (most often found in digital cameras).
* SmartMedia (most often found in digital cameras).
* Memory Stick (most often found in digital cameras).
* PCMCIA Type I and Type II memory cards (used as
solid-state disks in laptops).
* Memory cards for video game consoles.
Flash memory is a type of EEPROM chip, which stands for
Electronically Erasable Programmable Read Only Memory. It has
a grid of columns and rows with a cell that has two
transistors at each intersection.
The two transistors are separated from each other by a thin
oxide layer. One of the transistors is known as a floating
gate, and the other one is the control gate. The floating
gate's only link to the row, or wordline, is through the
control gate. As long as this link is in place, the cell has
a value of 1. To change the value to a 0 requires a curious
process called Fowler-Nordheim tunneling.
Flash Memory: Tunneling and Erasing
Tunneling is used to alter the placement of electrons in the
floating gate. An electrical charge, usually 10 to 13 volts,
is applied to the floating gate. The charge comes from the
column, or bitline, enters the floating gate and drains to
a ground.
This charge causes the floating-gate transistor to act like
an electron gun. The excited electrons are pushed through and
trapped on other side of the thin oxide layer, giving it
a negative charge. These negatively charged electrons act as
a barrier between the control gate and the floating gate.
A special device called a cell sensor monitors the level of
the charge passing through the floating gate. If the flow
through the gate is above the 50 percent threshold, it has
a value of 1. When the charge passing through drops below the
50-percent threshold, the value changes to 0. A blank EEPROM
has all of the gates fully open, giving each cell a value of
1.
The electrons in the cells of a flash-memory chip can be
returned to normal ("1") by the application of an electric
field, a higher-voltage charge. Flash memory uses in-circuit
wiring to apply the electric field either to the entire chip
or to predetermined sections known as blocks. This erases the
targeted area of the chip, which can then be rewritten. Flash
memory works much faster than traditional EEPROMs because
instead of erasing one byte at a time, it erases a block or
the entire chip, and then rewrites it.
You may think that your car radio has flash memory, since
you're able to program the presets and the radio remembers
them. But it's actually using flash RAM. The difference is
that flash RAM has to have some power to maintain its
contents, while flash memory will maintain its data without
any external source of power. Even though you've turned the
power off, the car radio is pulling a tiny amount of current
to preserve the data in the flash RAM. That is why the radio
will lose its presets if your car battery dies or the wires
are disconnected.
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