An EEPROM (English acronym for electrically erasable programmable read-only memory, literally, electrically erasable programmable read-only memory, even E2PROM ) is a non -volatile, electronic memory chip, the stored information can be erased electrically. It is related to other erasable memory such as the EPROM erasable with UV light, and which is also electrically erasable flash memory. It is used for storing smaller amounts of data in electrical equipment, in which the information must be maintained even without supply voltage or in which individual storage elements or data words can be changed simply must. A typical example is to store telephone numbers of a telephone. For storing large amounts of data such as the BIOS in flash memory PC systems are usually more economical.
The term " EEPROM" describes only the properties of the memory that this is non-volatile and can be removed only with electric power ( in contrast to the UV light only erasable EPROM). It includes, therefore, strictly speaking, the now commonly referred to as EEPROM word or byte- erasable memory, as well as the newer block -erasable flash memory. Since the latter can be omitted otherwise necessary per memory cell write, read and erase transistors, is with them a significantly higher storage density achievable.
EEPROM is composed of a matrix of field-effect transistors with insulated gate terminal ( the floating gate ) in which each transistor representing a bit. During the programming process is applied to the floating gate, a charge that can only be removed by the deletion again. In normal operation, the charge on the insulated gate is maintained completely.
For UV erasable EPROMs charge by hot-carrier injection (English hot carrier injection, HCl) is placed on the gate and can be removed only by irradiation with UV light again, while both the EEPROM when writing and erasing the charge is applied or removed by Fowler- Nordheim tunneling on the insulated gate. With a flash, however, the charge is applied when writing by HCI on the gate and removed when deleting by Fowler- Nordheim tunneling.
For programming of the EEPROM, a high voltage pulse is applied to the control gate, wherein a tunnel current therefrom by the insulative dielectric in the floating gate flow. This high voltage needed to be applied in the EPROM on the outside of the memory device, as it is generated in the EEPROM and the flash memory in block internally.
After the writing of the memory, i.e., the selective application of charge to the floating gate, the written data is represented by a bit pattern of charged / uncharged gates. These data can now be read out via the drain- source terminals of the transistors as often as the normal operating voltage reading is far below the programming voltage. The number of rewrites of the individual memory cells is limited, however, the manufacturers usually guarantee some 10,000 to 1,000,000 write cycles. This is achieved in part by redundant memory cells.
Areas of application
EEPROMs can be written and erased byte by byte as opposed to flash EEPROMs. In comparison to the flash EEPROM, which may take anywhere from 1 microseconds and 1 millisecond for a write cycle, conventional EEPROMs 1 ms to 10 ms are significantly slower. EEPROMs are therefore preferably used if individual data bytes changed at longer intervals and protected against power failure must be stored, such as in configuration data or runtime meters.
As a replacement for the earlier serving as a program or table memory ROM or EPROM, the EEPROM is not suitable due to the significantly higher cost of production, the role was later taken over by the flash memory. The higher cost of EEPROM technology meant that first stand-alone EEPROM devices were usually connected via a serial interface to the microcontroller. Later on-chip EEPROMs were then offered at a number of microcontrollers. Since microcontrollers are now mostly anyway manufactured in robust Flash technologies that allow more frequent erasing and programming, usually also an area of the flash memory for variable data can be used. For this purpose, a portion of the flash memory is allocated and written to and read some with special algorithms. It has a page before the deletion, as well as the total reserved area, only be fully utilized before it is rewritten. This method makes in many cases the EEPROM in microcontrollers superfluous.
However, an EEPROM can not be replaced in all applications through Flash. Currently, it is not yet possible to describe flash over such a wide temperature range as EEPROM reliable, but here makes the process technology advances and temperature compensation when writing improves the performance. In addition, it can be problematic to have a non-deterministic write time, which may result in EEPROM emulation by flash when a page needs to be deleted in certain applications.
Next blocks with parallel lead-out address and data busses there are also cases in the EEPROM, for example, with only 8 connections, in which addresses and data over a serial bus such as I ² C to be replaced. Such EEPROMs can be used for example to SDRAM from the manufacturer to storage of the product parameters which can then be read by the CPU. With this information in the SPD EEPROM, the memory modules can be automatically configured on the PC.
Failure symptoms and life
In EEPROM stored data can be affected by two types of deficits, the limited life or writability and the limited conservation capacity of the memory state of the individual memory locations in the EEPROM.
In the oxide layer of the gates of the floating gate transistors used in EEPROMs trapped electrons accumulate. The electric field of the trapped electrons adds up to the field of the floating gate and thus reduces the window between the threshold voltages that represent the memory states or low value. After a certain number of write operations, the difference is too small to be distinguishable, and the memory location is permanently available to the programmed value. Manufacturers usually specify the maximum number of write operations with 106 or more.
The introduced during the storage in the floating gate electrons may leak through the insulation layer, this is particularly at elevated temperatures, thereby causing a loss of the charge state and back to enable the memory location to the cleared state. Manufacturers usually guarantee the stability of data stored for a period of 10 years.