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| Flash Memory | |
| Flash memory is solid state, nonvolatile memory that
relies on stored electrical charge, rather than moving magnetic media,
to store data. Like bubble memory, which was touted during the 1980s as
a solution for high-density, non-volatile, rewritable storage, it has no
moving parts and is relatively rugged and durable. But unlike bubble
memory, flash is light, compact, energy-efficient, and ever less
expensive-making it an ideal storage medium for digital cameras, smart
cards, automotive control systems, and many other embedded applications.
Today's ultra-high-density disk drives are the undisputed kings of the hill in terms of megabytes per dollar, and are likely to remain so for the foreseeable future. But flash is the best choice in applications where mechanical shocks, environmental extremes, strict power constraints, or the need for small, rugged, swappable modules rules out the use of disk drives.
Flash vs. EEPROMOne of the most frequently asked questions about flash is how it differs from EEPROM, or electronically eraseable programmable read-only memory. While the two technologies are converging, each has a different heritage and intended use-and hence, different traditional strengths and weaknesses. Flash memory was originally created as a replacement for mass storage media such as floppy and hard disks. It's therefore designed for maximum capacity and density, minimum power consumption, and a high number of write cycles. (All non-volatile solid-state memory-whether it's EEPROM or flash-can endure a limited number of write cycles; it will, eventually, wear out.) Information stored in flash memory is usually written in blocks rather than a byte or word at a time (though, as I'll mention later, hardware and software can make them appear to be accessible one byte at a time). EEPROMs, on the other hand, are descendents of the venerable EPROM. They were originally designed to hold configuration information, or to lend field reprogrammability to devices that would otherwise require a change of chips. They usually have far smaller capacities-kilobits, as opposed to megabits-and are programmable in smaller chunks, often byte-by-byte. (Some are accessed serially, bit-by-bit.) Read access times tend to be shorter than for flash (though flash is catching up). And because they were meant for occasional field upgrades rather than constant updating, EEPROMs tend to be built to allow fewer write cycles. Finally, small EEPROMs are much cheaper than flash (not per bit, but per device), because they're mature products based on much older semiconductor process technology. As time goes by, the newer flash process technologies are sure to be
incorporated into devices that more resemble EEPROMs, blurring the
distinctions between the two categories of nonvolatile memory. Two kinds of flashWhile the innards of flash memory chips all work by pretty much the same principles, designers can buy and incorporate flash into their products in one of two forms. Linear flash, as the name implies, is laid out and addressed linearly, in blocks. The same address always maps to the same physical block of memory, and the chips and modules contain only memory plus address decoding and buffer circuits. This makes them simple, cheap, and energy-efficient. Linear flash is the obvious choice for nonvolatile memory that's built permanently into an embedded system. And if you write removable linear flash memory cards in a standard "flash file system" format, you can usually interchange linear flash memory modules between systems (though the odds of compatibility are a bit better with ATA flash, as described below). The second form of flash is called ATA flash. As the name implies, an ATA flash memory module interfaces with the rest of the system using the de facto "AT Attachment" standard. The memory appears as if it were sectors on a hard disk and is accessed via the same register interface used by the original IBM PC/AT's hard disk controller (and, more recently, IDE disk drives). The main advantages of ATA flash, from the embedded system developer's perspective, are flexibility and interchangeability with hard disks. While linear flash modules aren't 100% interchangeable between devices that accept removable memory modules, ATA flash virtually always is. Why? Because the memory module looks just like a disk drive to the system to which it's connected. It can thus be accessed using an operating system's standard disk access code and familiar file system APIs-not to mention the standard library calls used in C and other high-level languages. Another advantage is compatibility. So long as the memory is written according to the conventions of a standard file system (such as the MS-DOS "FAT" file system), it can be exchanged between any number of devices that recognize removable disk drives. (You can even substitute a disk drive for the flash module during development, or in versions of the product tailored for applications where ruggedness or power savings aren't as important.) For example, a memory card written by a digital camera that uses flash memory in a PC Card format can be read directly by any computer with a PC Card slot. This aids in debugging, and saves the developer the time and effort of developing special software and hardware interfaces to connect the camera to a computer. The disadvantages of ATA flash, as you might expect, are expense and power. Because the cards have built-in intelligence, they cost more and suck up more power than simple memory modules. They may also require you to include, and pay royalties on, the disk access component of your embedded operating system. If that code, and the disk runtime library code, would not otherwise be needed, this might increase your system's memory requirements. Flash Design ConsiderationsFlash memory offerings vary widely in capacity, price, speed, and features. Some flash memories can run at lower voltages (as low as 3V or less), which saves power, but works more slowly. Others run at higher speeds but require five or even 12V. Some vendors-including Intel-have created single products that can work at several speeds and power levels, depending on the voltage applied. Many flash memories incorporate built-in RAM buffers that essentially act as caches, holding the contents of a block of memory from the flash array for fast access. Others provide just the raw memory-thus leaving the option of creating a cache to the system designer. Another consideration when specifying flash memory is block size. ATA flash devices, because they emulate disk drives, naturally read and write data in "sectors"-usually 512-byte blocks. But linear flash memories have a block structure as well, though the circuitry may be designed to hide this from the rest of the system. Flash and Smart CardsIn the future, your credit card won't be just a piece of plastic with a magnetic strip on the back; it'll be a flash memory-perhaps with added intelligence. Several vendors, such as Xicor, have addressed the need for these specialized memories by creating chips that include built-in password protection, serial communications buses, and 100-year data retention. While these chips will find their most widespread uses in debit cards, credit cards, and similar applications, they're also useful for any embedded system where access control is a consideration-including cash registers, door locks, car alarms, automatic gates, parental supervision devices, or even sobriety testing systems. TrendsMemory vendors are moving to increase the density of flash memory by storing more than one bit per memory cell. In EPROMs, EEPROMs, and older flash memories, only one bit is stored in each cell, which (as in dynamic RAM chips) is a tiny capacitor. Either there's a sufficient amount of charge in the cell or there's not; the amount of charge determines whether the cell contains a 1 or a 0. Newer flash memory, however, allows four or more possible amounts of charge per cell and can thus store two or more bits in each. This should drive the cost per bit down even further, making flash a more attractive solution. Contact Information:
AMD |
The PC Card comes in
three different types - Type I, Type II and Type III. The Type I
& II cards are solid state type memory cards (i.e. they store
the information on memory chips) while the Type III cards are
usually tiny hard drives. All three card types measure the same
length and width and use the same 68-pin connector. The only
difference between card types is thickness. The thicknesses are 3.3,
5.0, and 10.5 millimeters for Type I, Type II, and Type III cards
respectively. Because they differ only in thickness, a thinner card
can be used in a thicker slot, but a thicker card can not be used in
a thinner slot. The Type III cards have been slowly disappearing
from the market over the past few years or so and it is currently
difficult to find these cards in retail outlets. 
The Compact Flash Type II, or more commonly known as the Microdrive Card, is similar in
size to the Compact Flash card discussed above, but is a bit
thicker, This is not a solid state memory card, but actually
contains a tiny hard drive. These cards are available in very large
capacities (currently up to 4 GB) and are fairly inexpensive at a
cost of less than 30 cents per megabyte. A significant problem with
this type of memory card is their lack of durability. Though the
card is well shielded from magnetic or electromagnetic fields, they
are fairly sensitive to impact and vibration and can be damaged if
dropped or jarred. They also do not tolerate drastic changes in
temperature well. 
The SmartMedia memory card (sometimes referred to as an SSFDC
- Solid State Floppy Disc Card) is a more recent product which
became commonly available in the mid 1990's. This memory card is
smaller and much thinner than the Compact Flash card. Due to it's
smaller size, camera makers were able to begin developing even
smaller digital cameras that were similar in size to some of the
smallest point and shoot film cameras. The SmartMedia Cards are also
more efficient with regards to power drain, and the batteries in the
digital cameras that use this type of card tend to last a bit
longer. Another advantage of this card type is that they are a bit
less expensive to produce than CompactFlash cards with a typical
cost of under 60 cents per megabyte. There are some drawbacks to the
SmartMedia card system. One problem is that the cards have no
shielding, so they are susceptible to data loss and damage from
electromagnetic fields (such as airport x-rays). Also, these cards
are not as widely compatible as the other types of memory cards -
Particularly CompactFlash cards. CompactFlash cards have a control
circuit built right into the memory card. This circuit allows the
card to "tell" the camera or computer how much capacity it contains
so the camera or computer can take advantage of all the available
memory. The SmartMedia Cards do not have this controller circuit, so
the memory controller needs to be built into the camera. What this
means to the user, is that the camera you purchase will not be able
to read any cards of larger capacity than was available when the
camera was released. For example, say you purchased an Olympus
D-600L camera. This camera was released in early 1998 when the
largest SmartMedia card available was 16MB. Because the Controller
chip in the camera was designed for 16MB cards, you cannot use cards
larger than 16MB capacity in this camera - larger capacity cards
will not be recognized by the Olympus D-600L camera. 
The Memory Stick Memory card was developed by Sony and was
first used in Sony digital cameras in 1998. The Memory Stick memory
card is well shielded and durable like the Compact Flash cards but
is much smaller to allow it to be used in smaller products such as
ultra compact digital cameras, and MPEG players. At this time, the
Memory Stick is used mostly with Sony products, but there are now
several digital photo products from other manufacturers that accept
the Memory Stick cards. There is also a version of this card with
copyright protection technology called the MagicGate Memory Stick.
This card is a bit more expensive than the standard Memory Stick
card, but that additional cost pays for the potential royalty fees
when recording copyrighted MPEG music. 
The MultiMedia Card (MMC) was developed to
try and incorporate the best features of SmartMedia and Compact
Flash cards in to one system. This memory card system is very
similar to the Memory Stick card with regard to its capabilities but
is a non-proprietary form. The MultiMedia Card has become fairly
common in products such as ultra compact digital cameras, PDA
(Personal Digital Assistant) units, cell phones and MPEG players.
These cards cost about the same as the Compact Flash type cards at a
typical cost of slightly more than 60 cents per Megabyte.
where the modem or
network interface is installed. To use this slot as a card reader,
you will need to purchase an appropriate card adapter and insert
your memory card with the adapter into the card slot. When the
computer recognizes the memory card, it should show up as a hard
drive. Then, all you need to do is simply drag and drop your image
files to a folder on your computer hard drive, and you are done. The
PC Card slot is usually much faster than the camera interface or a
card reader for transferring the files. Card adapters are available
for just about every memory card type on the market. 
floppy disk drive on your computer. The FlashPath adapter
looks like a floppy disk. You insert the memory card into the
adapter and then put the adapter into your floppy disk drive. You
can now view the files on the floppy drive of your computer and
drag-and drop the files to your hard drive. This device does not
usually work on laptop computers or external floppy drives, and is
very slow. However, for the computer novice, it is the easiest way
to get the files in to your computer.
