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 such devices as digital
cameras and home video game
consoles. It is used more as a hard
drive than as RAM. In fact,
Flash memory is considered a solid state storage
device. Solid state means that 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
In this edition of HowStuffWorks,
we'll find out how Flash memory works and look at some of the
forms it takes and types of devices that use it.
The Basics We discussed the underlying
technology of Flash memory in How ROM Works,
but here's a quick review:
Flash memory is a type of EEPROM chip. It has a grid
of columns and rows with a cell that has two transistors at
each intersection (see image below). 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
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
This charge causes the floating-gate transistor to act like
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 greater than 50 percent of the charge, 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
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
You may think that your car radio has
Flash memory, since you are able to program the presets and
the radio remembers them. But it is 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
have 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
In the following sections, we will concentrate on removable
Flash memory products.
Removable Flash Memory Cards While your
computer's BIOS chip is
the most common form of Flash memory, removable solid-state
storage devices are becoming increasingly popular.
SmartMedia and CompactFlash cards are both
well-known, especially as "electronic film" for digital
cameras. Other removable Flash memory products include Sony'sMemory Stick, PCMCIA memory cards, and memory cards for
game systems such as Nintendo's N64, Sega's Dreamcast
and Sony's PlayStation.
We will focus on SmartMedia and CompactFlash, but the
essential idea is the same for all of these products. Every
one of them is simply a form of Flash memory.
There are several reasons to use Flash memory instead of a
Flash memory is noiseless.
It allows faster access.
It is smaller in size.
It is lighter.
It has no moving parts.
So why don't we just use
Flash memory for everything? Because the cost per megabyte for
a hard disk is drastically cheaper, and the capacity is
substantially more. You can buy a 40-gigabyte (40,000-MB) hard
drive for less than $200, while a 192-MB CompactFlash card
will generally cost you more.
SmartMedia The solid-state floppy-disk
card (SSFDC), better known as SmartMedia, was originally
developed by Toshiba.
cards are available in capacities ranging from 2 MB to 128 MB.
The card itself is quite small, approximately 45 mm long, 37
mm wide and less than 1 mm thick. This is amazing when you
consider what is packed into such a tiny package!
As shown below, SmartMedia cards are elegant in their
simplicity. A plane electrode is connected to the
Flash-memory chip by bonding wires. The Flash-memory
chip, plane electrode and bonding wires are embedded in a
resin using a technique called over-molded thin
package (OMTP). This allows everything to be integrated
into a single package without the need for soldering.
The OMTP module is glued to a base card to create
the actual card. Power and data is carried by the electrode to
the Flash-memory chip when the card is inserted into a device.
A notched corner indicates the power requirements of the
SmartMedia card. Looking at the card with the electrode facing
up, if the notch is on the left side, the card needs 5 volts.
If the notch is on the right side, it requires 3.3 volts.
SmartMedia cards erase, write and read memory in small
blocks (256- or 512-byte increments). This approach means that
they are capable of fast, reliable performance while allowing
you to specify which data you wish to keep. They are small,
lightweight and easy to use. They are less rugged than other
forms of removable solid-state storage, so you should be very
careful when handling and storing them.
CompactFlash CompactFlash cards were
developed by Sandisk
in 1994, and they are different from SmartMedia cards in two
They are thicker.
They utilize a controller chip.
consists of a small circuit board with Flash-memory chips and
a dedicated controller chip, all encased in a rugged shell
that is several times thicker than a SmartMedia card.
As shown below, CompactFlash cards are 43 mm wide and 36 mm
long, and come in two thicknesses: Type I cards are 3.3
mm thick, and Type II cards are 5.5 mm thick.
CompactFlash cards support dual voltage and will operate at
either 3.3 volts or 5 volts.
The increased thickness of the card allows for greater
storage capacity than SmartMedia cards. CompactFlash sizes
range from 8 MB to 192 MB. The onboard controller can increase
performance, particularly on devices that have slow processors.
The case and controller chip add size, weight and complexity
to the CompactFlash card when compared to the SmartMedia card.
Both of these types of removable storage, as well as PCMCIA
Type I and Type II memory cards, adhere to standards developed
by the Personal
Computer Memory Card International Association
(PCMCIA). Because of these standards, it is easy to use
CompactFlash and SmartMedia products in a variety of devices.
You can also buy adapters that allow you to access these cards
through a standard floppy
port or PCMCIA card slot (like the one you find on a laptop
computer). Sony's Memory Stick is available in a
large array of products offered by Sony, and is now showing up
in products from other manufacturers as well.
Although standards are flourishing, there are many
Flash-memory products that are completely proprietary in
nature, such as the memory cards in video game systems. But it
is good to know that as electronic components become
increasingly interchangeable and learn to communicate with
each other (by way of technologies such as Bluetooth),
memory will allow you to keep your world close at hand.
For more information on Flash memory, other forms of
computer memory and related topics, check out the links on the