EEPROM

EEPROM (also written E²PROM and pronounced e-e-prom or simply e-squared), which stands for Electrically Erasable Programmable Read-Only Memory, is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration tables or device configuration.

When larger amounts of static data are to be stored (such as in USB flash drives) a specific type of EEPROM such as flash memory is more economical than traditional EEPROM devices.

EEPROMs are realized as arrays of floating-gate transistors.

History

In 1983, George Perlegos at Intel developed the Intel 2816, which was built on earlier EPROM technology, but used a thin gate oxide layer so that the chip could erase its own bits without requiring a UV source. Perlegos and others later left Intel to form Seeq Technology, which used on-device charge pumps to supply the high voltages necessary for programming EEPROMs. [cite journal
last = Rostky
first = George
title = Remembering the PROM knights of Intel
journal = EE Times
date = July 2, 2002
url = http://www.eetimes.com/issue/fp/showArticle.jhtml;?articleID=18307418
accessdate = 2007-02-08]

Functions of EEPROM

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

How the device is operated depends on the electrical interface.

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 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:
* Write Enable (WREN)
* Write Disable (WRDI)
* Read Status Register (RDSR)
* Write Status Register (WRSR)
* Read Data (READ)
* Write Data (WRITE)

Other operations supported by some EEPROM devices are:
* Program
* Sector Erase
* Chip Erase commands

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.

Failure modes

There are two limitations of stored information; endurance, and data retention.

During rewrites, the gate oxide in the floating-gate transistors gradually accumulates trapped electrons. The electric field of the trapped electrons adds to the electrons in the floating gate, lowering the window between threshold voltages for zeros vs ones. After sufficient number of rewrite cycles, the difference becomes too small to be recognizable, the cell is stuck in programmed state, and endurance failure occurs. The manufacturers usually specify minimal number of rewrites being 10⁶ or more.

During storage, the electrons injected into the floating gate may drift through the insulator, especially at increased temperature, and cause charge loss, reverting the cell into erased state. The manufacturers usually guarantee data retention of 10 years or more. [System Integration - From Transistor Design to Large Scale Integrated Circuits]

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 writeable memories compared to block-wise writable 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 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.

EEPROM manufacturers

*Aplus Flash Technology
*Mitsubishi
*Atmel
*Hitachi
*Infineon
*Maxwell Technologies
*Microchip Technology
*NXP Semiconductors
*Renesas Technology
*ROHM Electronics
*Samsung Electronics
*SmarfTech
*STMicroelectronics
*Seiko Instruments
*Winbond
*Catalyst Semiconductor Inc

See also

* NVRAM
* Flash memory
* DataFlash

References