Hardware Reference
In-Depth Information
1 to a 0. Once a flash cell is programmed (that is, changed to a 0), the only way it can be changed
back to a 1 is by erasing it. The problem with this is that, although you can program individual cells
or pages, you can only erase cells or pages on a block basis, and a block usually consists of
thousands of cells (512KB in most cases). The actual programming and erasing process coaxes
electrons into and out of the transistor's floating gate by a process known as Fowler-Nordheim
tunneling.
The two major types of flash memory technology are called NOR (Not OR) and NAND (Not AND).
Both use the same basic transistor (cell) design, but they differ in how the cells are interconnected.
NOR flash works more like dynamic RAM (DRAM), providing high-speed random-access
capabilities with the ability to read or write data in single byte quantities. NOR flash is the type of
memory used for flash ROMs, such as those found in motherboards, cell phones, and most devices
that have updatable firmware.
On the other hand, NAND flash works more like a storage device, reading and writing data in pages
or blocks instead of individual bytes. NAND flash is used in devices that store file-oriented data,
such as SSDs, USB key or thumb drives, digital cameras and digital film media, music players, and
more. NAND flash is denser than NOR flash, storing more data in a given amount of die space and
costing less overall for a given amount of storage.
The speed, low power requirements, and compact size of recent flash memory and SSD devices have
made flash memory a perfect counterpart for portable devices such as laptop computers and digital
cameras, which often refer to flash memory devices as so-called “digital film.” Unlike real film,
digital film can be erased and reshot. Ultra-compact, USB flash memory drives have all but replaced
traditional floppy drives, Zip/SuperDisk drives, and even optical discs for transporting data between
systems.
Several types of flash memory devices have been popular, including the following:
• CompactFlash (CF)
• SmartMedia (SM)
• MultiMediaCard (MMC)
• SecureDigital (SD)
• Memory Stick
• ATA Flash
• xD-Picture Card
• Solid-state drive (SSD)
• USB flash devices
Some of these are available in different sizes (Type I/Type II).
Table 10.1
shows the various types of
solid-state storage used in digital cameras and other devices, listed in order of introduction.
Table 10.1. Different Flash Memory Devices and Physical Sizes
