EEPROMs.Functions of EEPROM:-

      There are different types of electrical interfaces to EEPROM devices. Main categories of these interface types are:

1)Serial bus                                         2)Parallel bus

1)Serial bus devices:-Most common serial interface types are SPI, I²C, Microwire, UNI/O, and 1-Wire. These interfaces require between 1 and 4 control signals for operation, resulting in a memory device in an 8 pin (or less) package.The serial EEPROM (or SEEPROM) typically operates in three phases: OP-Code Phase, Address Phase and Data Phase. The OP-Code is usually the first 8-bits input to the serial input pin of the EEPROM device (or with most I²C devices, is implicit); followed by 8 to 24 bits of addressing depending on the depth of the device, then data to be read or written.Each EEPROM device typically has its own set of OP-Code instructions to map to different functions. Some of the common operations on SPI EEPROM devices are:

a)Write Enable (WREN)    b)Write Disable (WRDI)   c)Read Status Register (RDSR)   d)Write Status Register (WRSR)   e)Read Data (READ)   f)Write Data (WRITE)

Other operations supported by some EEPROM devices are:-

a)Program   b)Sector Erase    c)Chip Erase commands

2)Parallel bus devices:-Parallel EEPROM devices typically have an 8-bit data bus and an address bus wide enough to cover the complete memory. Most devices have chip select and write protect pins. Some microcontrollers also have integrated parallel EEPROM.Operation of a parallel EEPROM is simple and fast when compared to serial EEPROM, but these devices are larger due to the higher pin count (28 pins or more) and have been decreasing in popularity in favor of serial EEPROM or Flash.

Related types:-Flash memory is a later form of EEPROM. In the industry, there is a convention to reserve the term EEPROM to byte-wise erasable memories compared to block-wise erasable flash memories. EEPROM takes more die area than flash memory for the same capacity because each cell usually needs both a read, write and erase transistor, while in flash memory the erase circuits are shared by large blocks of cells (often 512×8).Newer non-volatile memory technologies such as FeRAM and MRAM are slowly replacing EEPROMs in some applications, but are expected to remain a small fraction of the EEPROM market for the foreseeable future

Comparison with EPROM and EEPROM/Flash:-The difference between EPROM and EEPROM lies in the way that the memory programs and erases. EEPROM can be programmed and erased electrically using field electron emission (more commonly known in the industry as "Fowler–Nordheim tunneling").EPROMs can't be erased electrically, and are programmed via hot carrier injection onto the floating gate. Erase is via an ultraviolet light source, although in practice many EPROMs are encapsulated in plastic that is opaque to UV light, and are "one-time programmable".Most NOR Flash memory is a hybrid style—programming is through hot carrier injection and erase is through Fowler–Nordheim tunneling.