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.

EEPROM:-

         EEPROM (also written E2PROM and pronounced "e-e-prom," "double-e prom" or simply "e-squared") stands for Electrically Erasable Programmable Read-Only Memory and 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.

EEPROM is user-modifiable read-only memory (ROM) that can be erased and reprogrammed (written to) repeatedly through the application of higher than normal electrical voltage generated externally or internally in the case of modern EEPROMs. EPROM usually must be removed from the device for erasing and programming, whereas EEPROMs can be programmed and erased in circuit. Originally, EEPROMs were limited to single byte operations which made them slower, but modern EEPROMs allow multi-byte page operations. It also has a limited life - that is, the number of times it could be reprogrammed was limited to tens or hundreds of thousands of times. That limitation has been extended to a million write operations in modern EEPROMs. In an EEPROM that is frequently reprogrammed while the computer is in use, the life of the EEPROM can be an important design consideration. It is for this reason that EEPROMs were used for configuration information, rather than random access memory.

History:-In 1978, 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

APPLICATION OF EPROM

           For large volumes of parts (thousands of pieces or more), mask-programmed ROMs are the lowest cost devices to produce. However, these require many weeks lead time to make, since the artwork for an IC mask layer must be altered to store data on the ROMs. Initially, it was thought that the EPROM would be too expensive for mass production use and that it would be confined to development only. It was soon found that small-volume production was economical with EPROM parts, particularly when the advantage of rapid upgrades of firmware was considered.

Some microcontrollers, from before the era of EEPROMs and flash memory, use an on-chip EPROM to store their program. Such microcontrollers include some versions of the Intel 8048, the Freescale 68HC11, and the "C" versions of the PIC microcontroller. Like EPROM chips, such microcontrollers came in windowed (expensive) versions that were useful for debugging and program development. The same chip came in (somewhat cheaper) opaque OTP packages for production. Leaving the die of such a chip exposed to light can also change behavior in unexpected ways when moving from a windowed part used for development to a non-windowed part for production.

Erasure can also be accomplished with X-rays:

           "Erasure, however, has to be accomplished by non-electrical methods, since the gate electrode is not accessible electrically. Shining ultraviolet light on any part of an unpackaged device causes a photocurrent to flow from the floating gate back to the silicon substrate, thereby discharging the gate to its initial, uncharged condition. This method of erasure allows complete testing and correction of a complex memory array before the package is finally sealed. Once the package is sealed, information can still be erased by exposing it to X radiation in excess of 5*104 rads, a dose which is easily attained with commercial X-ray generators." (5*104 rad = 500 J/kg)[5]

"In other words, to erase your EPROM, you would first have to X-ray it and then put it in an oven at about 600 degrees Celsius (to anneal semiconductor alterations caused by the x-rays). The effects of this process on the reliability of the part would have required extensive testing so they decided on the window instead. (any temperature between 450 - 1410 °C should work).

EPROMs had a limited but large number of erase cycles; the silicon dioxide around the gates would accumulate damage from each cycle, making the chip unreliable after several thousand cycles. EPROM programming is slow compared to other forms of memory. Because higher-density parts have little exposed oxide between the layers of interconnects and gate, ultraviolet erasing becomes less practical for very large memories. Even dust inside the package can prevent some cells from being erased.

DETAIL OF EPROM

As the quartz window is expensive to make, OTP (one-time programmable) chips were introduced; here, the die is mounted in an opaque package so it cannot be erased after programming - this also eliminates the need to test the erase function, further reducing cost. OTP versions of both EPROMs and EPROM-based microcontrollers are manufactured. However, OTP EPROM (whether separate or part of a larger chip) is being increasingly replaced by EEPROM for small amounts where the cell cost isn't too important and flash for larger amounts.

              A programmed EPROM retains its data for about ten to twenty years and can be read an unlimited number of times. The erasing window must be kept covered with an opaque label to prevent accidental erasure by sunlight. Old PC BIOS chips were often EPROMs, and the erasing window was often covered with a label containing the BIOS publisher's name, the BIOS revision, and a copyright notice. The practice of covering the BIOS chip with a label is still commonly seen as of today, even though current BIOS chips are actually EEPROMs or NOR flashes, with no erase windows.

Erasure of the EPROM begins to occur with wavelengths shorter than 400 nm. Exposure time for sunlight of 1 week or 3 years for room fluorescent lighting may cause erasure. The recommended erasure procedure is exposure to UV light at 253.7 nm of at least 15 W-sec/cm2 for 20 to 30 minutes, with the lamp at a distance of about 1 inch.