Шағын көлемді дерек сақтауға арналған компьютер жады (Shag'yn k'olemdi derek saqtau'gha arnall'ghan kompyuter jady)

Computer memory used for small quantities of data

EEPROM or E2PROM (electrically erasable programmable read only memory) is a type of non volatile memory. It is used in computers, usually integrated in microcontrollers such as smart cards and remote keyless systems, or as a separate chip device, to store relatively small amounts of data by allowing individual bytes to be erased and reprogrammed. EEPROMs are organized as arrays of floating gate transistors. EEPROMs can be programmed and erased in circuit, by applying special programming signals. Originally, EEPROMs were limited to single byte operations, which made them slower, but modern EEPROMs allow multi byte page operations. An EEPROM has a limited life for erasing and reprogramming, reaching a million operations in modern EEPROMs. In an EEPROM that is frequently reprogrammed, the life of the EEPROM is an important design consideration. Flash memory is a type of EEPROM designed for high speed and high density, at the expense of large erase blocks (typically 512 bytes or larger) and limited number of write cycles (often 10,000). There is no clear boundary dividing the two, but the term "EEPROM" is generally used to describe non volatile memory with small erase blocks (as small as one byte) and a long lifetime (typically 1,000,000 cycles). Many past microcontrollers included both (flash memory for the firmware and a small EEPROM for parameters), though the trend with modern microcontrollers is to emulate EEPROM using flash. As of 2020, flash memory costs much less than byte programmable EEPROM and is the dominant memory type wherever a system requires a significant amount of non volatile solid state storage. EEPROMs, however, are still used on applications that only require small amounts of storage, like in serial presence detect.

As is described in former section, old EEPROMs are based on avalanche breakdown based hot carrier injection with high reverse breakdown voltage. But FLOTOX theoretical basis is Fowler–Nordheim tunneling hot carrier injection through a thin silicon dioxide layer between the floating gate and the wafer. In other words, it uses a tunnel junction. Theoretical basis of the physical phenomenon itself is the same as today's flash memory. But each FLOTOX structure is in conjunction with another read control transistor because the floating gate itself is just programming and erasing one data bit. Intel's FLOTOX device structure improved EEPROM reliability, in other words, the endurance of the write and erase cycles, and the data retention period. A material of study for single event effect about FLOTOX is available. Today, a detailed academical explanation of FLOTOX device structure can be found in various materials.

Nowadays, EEPROM is used for embedded microcontrollers as well as standard EEPROM products. EEPROM still requires a 2 transistor structure per bit to erase a dedicated byte in the memory, while flash memory has 1 transistor per bit to erase a region of the memory.

Because EEPROM technology is used for some security gadgets, such as credit cards, SIM cards, key less entry, etc., some devices have security protection mechanisms, such as copy protection.

EEPROM devices use a serial or parallel interface for data input/output.

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.

EEPROM memory is used to enable features in other types of products that are not strictly memory products. Products such as real time clocks, digital potentiometers, digital temperature sensors, among others, may have small amounts of EEPROM to store calibration information or other data that needs to be available in the event of power loss. It was also used on video game cartridges to save game progress and configurations, before the usage of external and internal flash memories.

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 the maximum number of rewrites being 1 million 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.

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 occupies more die area than flash memory for the same capacity, because each cell usually needs a read, a write, and an erase transistor, while flash memory 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.

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 by hot carrier injection onto the floating gate. Erase is by an ultraviolet light source, although in practice many EPROMs are encapsulated in plastic that is opaque to UV light, making them "one time programmable". Most NOR flash memory is a hybrid style—programming is through hot carrier injection and erase is through Fowler–Nordheim tunneling. Type Inject electrons onto gate(mostly interpreted as bit=0) Duration Remove electrons from gate(mostly interpreted as bit=1) Duration/mode EEPROM field electron emission 0.1—5 ms, bytewise field electron emission 0.1—5 ms, blockwise NOR flash memory hot carrier injection 0.01—1 ms field electron emission 0.01—1 ms, blockwise EPROM hot carrier injection 3—50 ms, bytewise UV light 5—30 minutes, whole chip