Flash memory
Flash memory is an electronic (i.e. no moving parts) non-volatile computer storage device that can be electrically erased and reprogrammed.
Flash memory was developed from EEPROM (electrically erasable programmable read-only memory). There are two main types of flash memory, which are named after the NAND and NOR logic gates. The internal characteristics of the individual flash memory cells exhibit characteristics similar to those of the corresponding gates.
Whereas EEPROMs had to be completely erased before being rewritten, NAND type flash memory may be written and read in blocks (or pages) which are generally much smaller than the entire device. The NOR type allows a single machine word (byte) to be written or read independently.
The NAND type is primarily used in main memory memory cards, USB flash drives, solid-state drives, and similar products, for general storage and transfer of data. The NOR type, which allows true random access and therefore direct code execution, is used as a replacement for the older EPROM and as an alternative to certain kinds of ROM applications. However, NOR flash memory may emulate ROM primarily at the machine code level; many digital designs need ROM (or PLA) structures for other uses, often at significantly higher speeds than (economical) flash memory may achieve. NAND or NOR flash memory is also often used to store configuration data in numerous digital products, a task previously made possible by EEPROMs or battery-powered static RAM.
Example applications of both types of flash memory include personal computers, PDAs, digital audio players, digital cameras, mobile phones, synthesizers, video games, scientific instrumentation, industrial robotics, medical electronics, and so on. In addition to being non-volatile, flash memory offers fast read access times, as fast as dynamic RAM, although not as fast as static RAM or ROM. Its mechanical shock resistance helps explain its popularity over hard disks in portable devices; as does its high durability, being able to withstand high pressure, temperature, immersion in water, etc.
Although flash memory is technically a type of EEPROM, the term "EEPROM" is generally used to refer specifically to non-flash EEPROM which is erasable in small blocks, typically bytes. Because erase cycles are slow, the large block sizes used in flash memory erasing give it a significant speed advantage over old-style EEPROM when writing large amounts of data. Flash memory now costs far less than byte-programmable EEPROM and has become the dominant memory type wherever a significant amount of non-volatile, solid state storage is needed.
Serial flash
Serial flash is a small, low-power flash memory that uses a serial interface, typically Serial Peripheral Interface Bus (SPI), for sequential data access. When incorporated into an embedded system, serial flash requires fewer wires on the PCB than parallel flash memories, since it transmits and receives data one bit at a time. This may permit a reduction in board space, power consumption, and total system cost.
There are several reasons why a serial device, with fewer external pins than a parallel device, can significantly reduce overall cost:
Many ASICs are pad-limited, meaning that the size of the die is constrained by the number of wire bond pads, rather than the complexity and number of gates used for the device logic. Eliminating bond pads thus permits a more compact integrated circuit, on a smaller die; this increases the number of dies that may be fabricated on a wafer, and thus reduces the cost per die.
Reducing the number of external pins also reduces assembly and packaging costs. A serial device may be packaged in a smaller and simpler package than a parallel device.
Smaller and lower pin-count packages occupy less PCB area.
Lower pin-count devices simplify PCB routing.
There are two major SPI flash types. The first type is characterized by small pages and one or more internal SRAM page buffers allowing a complete page to be read to the buffer, partially modified, and then written back (for example, the Atmel AT45 DataFlash or the Micron Technology Page Erase NOR Flash). The second type has larger sectors. The smallest sectors typically found in an SPI flash are 4 kB, but they can be as large as 64 kB. Since the SPI flash lacks an internal SRAM buffer, the complete page must be read out and modified before being written back, making it slow to manage. SPI flash is cheaper than DataFlash and is therefore a good choice when the application is code shadowing.
The two types are not easily exchangeable, since they do not have the same pinout, and the command sets are incompatible.