Introduction to EPROM and EEPROM

Catalog

Ⅰ EPROM

EPROM is a kind of ROM memory with an erasable function and can be reprogrammed after erasing. Before writing, the contents inside must be irradiated with ultraviolet rays on the transparent window on its IC card to clear it. This type of chip is easier to identify. The package contains a "quartz glass window". The "quartz glass window" of a programmed EPROM chip is generally covered with black adhesive paper to prevent direct sunlight.

EPROM chips can be erased and written repeatedly, which solves the drawback that PROM chips can only be written once. EPROM chips have a very obvious feature. On the ceramic package, there is a glass window through which you can see the integrated circuit inside. The internal chip can be erased by irradiating ultraviolet rays through the hole. The EPROM eraser is needed to complete the chip erasing operation. The writing of data in EPROM should use a dedicated programmer, and a certain programming voltage (VPP=12~24V, depending on different chip models) must be added when writing content to the chip. The model of EPROM starts with 27. For example, 27C020 (8*256K) is an EPROM chip with 2M Bits capacity. After the EPROM chip writes the data, the window must be sealed with an opaque sticker or tape to prevent the data from being damaged by the surrounding ultraviolet rays. When the EPROM chip is in a blank state (after being erased with ultraviolet light), the data of each memory cell inside is 1 (high level).

1. Features

EPROM programming needs to use a programmer. The programmer is a device used to generate high-voltage pulse signals required for EPROM programming. When programming, the data of EPROM is sent to the random access memory, and then the programming program is started, and the programmer writes the data into EPROM row by row.

A piece of programmed EPROM can retain its data for about 10-20 years and can be read unlimited times. The erasing window must remain covered to prevent accidental erasure by sunlight. The BIOS chips of old computers are generally EPROM, and the erasing window is often covered by a label printed with the BIOS issuer's name, version, and copyright notice. EPROM has been replaced by EEPROM (electrically erased read-only register).

Some microcontrollers produced before the advent of flash memory use EPROM to store the version of the program to facilitate program development. For example, the use of one-time programmable devices will cause serious waste in debugging.

2. Working principle

PROM is a programmable device. The mainstream product adopts a double-layer gate (two-layer poly) structure, including EPROM and EEPROM. The working principle is roughly the same. The main structure is shown in the figure:

Double-layer grid structure 

When there is no electron injection in the floating gate, and the voltage is applied to the control gate, the electrons in the floating gate move to the upper layer, and holes appear in the lower layer. Due to induction, electrons are attracted and the channel is opened. If electrons are injected into the floating gate, the threshold voltage of the tube is increased, and the channel is closed. In this way, the switching function is achieved.

EPROM writing process

As shown in Figure, this is the EPROM writing process. A high voltage is applied to the drain, and electrons flow from the source to the drain channel to fully open. Under the action of the high voltage, the tension of the electrons is strengthened, and the energy makes the temperature of the electrons extremely rise, so the electrons become hot electrons. This kind of electrons is hardly scattered by the vibration of atoms. When the high voltage is applied by the control gate, the hot electrons can jump over the SiO2 barrier and be injected into the floating gate.

Ⅱ EEPROM

EEPROM (Electrically Erasable Programmable Read-Only Memory), a memory chip that does not lose data after power failure. EEPROM can erase the existing information and reprogram it on the computer or special equipment. EEPROM is a user-changeable read-only memory (ROM) that can be erased and reprogrammed (rewritten) by a higher voltage than normal. Unlike EPROM chips, EEPROM can be modified without removing it from the computer. In an EEPROM, when the computer is in use, it can be reprogrammed frequently. The life of the EEPROM is a very important design consideration parameter.

1. Background knowledge

In the early development of microcomputers, BIOS was stored in ROM (Read Only Memory). The data inside the ROM is burned in by a special method in the factory during the ROM manufacturing process. The content can only be read and cannot be changed. Once burned in, the user can only verify whether the written data is correct. No further changes can be made. If you find any errors in the information, you have to discard it and make a new copy. ROM is produced on the production line. Due to the high cost, it is generally only used in mass applications.

Because of the inconvenience of ROM manufacturing and upgrading, people later invented PROM (Programmable ROM). There is no data in the PROM that was originally made in the factory. Users can write their own data with a dedicated programmer, but this opportunity is only one time. Once written, it cannot be modified. If an error occurs, the chip can only be scrapped. PROM has the same characteristics as ROM, but its cost is higher than that of ROM, and the speed of writing data is slower than that of ROM mass production. Generally, it is only suitable for occasions with small demand or verification before the mass production of ROM.

2. Working Principle

It is inconvenient to operate by EPROM. Most of the BIOS ROM chips on the motherboard later use EEPROM (Electrically Erasable Programmable ROM). The erasure of EEPROM does not need to resort to other equipment. It uses electronic signals to modify its content and uses Byte as the minimum modification unit. It does not need to wash all the data to write, completely getting rid of the EPROM Eraser and programmer. EEPROM still needs to use a certain programming voltage when writing data. At this time, the content can be easily rewritten with the special refresh program provided by the manufacturer. Therefore, it is a dual-voltage chip. With the dual voltage characteristics of the EEPROM chip, the BIOS can have a good anti-virus function. When upgrading, turn the jumper switch to the "ON" position, that is, add the corresponding programming voltage to the chip, and you can easily upgrade. In a normal situation, turn the jumper switch to the "OFF" position to prevent CIH viruses from illegally modifying the BIOS chip. Therefore, many motherboards still use EPROM as the BIOS chip and as a major feature of their own motherboards.

In the absence of other external forces, the electrons will be well maintained. When electrons need to be eliminated, ultraviolet rays are used for irradiation to give the electrons enough energy to escape the floating gate. The EEPROM writing process uses the tunneling effect, that is, electrons with energy less than the energy barrier can cross the barrier to reach the other side. Quantum mechanics believe that when the physical size is equivalent to the free path of electrons, the electrons will exhibit volatility. From the point of view of the PN junction, when the impurity concentration of p and n reaches a certain level and the space charge is very small, the electrons will migrate due to the tunneling effect. The energy of the electron is at a certain level of allowable level. The range is called the "band". The lower energy band is called the valence band, and the higher energy band is called the conduction band. When electrons reach the higher conduction band, they can move freely between atoms, and this movement is current.

EEPROM writing process

The EEPROM writing process is shown in Figure 3. According to the tunneling effect, the SiO2 surrounding the floating gate must be extremely thin to reduce the potential barrier. The source and drain are grounded and in a conducting state. A high voltage higher than the threshold voltage is applied to the control gate to reduce the effect of the electric field and attract electrons to pass through.

eliminate the electron

In order to eliminate the electron, EEPROM is also achieved by the tunneling effect. As shown in Figure, apply high voltage to the drain, control the gate to 0V, flip the pull direction, and pull the electron out of the floating gate.

Ⅲ Difference between EEPROM and EPROM

1. The performance difference between EEROM and EPROM

EPROM is a rewritable memory chip, and its content will not be lost when power is off, which is non-volatile. It is programmed by an EPROM programmer, which can provide a higher voltage than the normal working voltage to program EPROM. Once programmed, EPROM can only be erased under strong ultraviolet radiation. In order to erase, the ceramic package of EPROM has a small quartz window. This quartz window is usually covered with opaque adhesive tape. When erasing, remove the adhesive tape and place it under strong ultraviolet light for about 20 minutes.

EEPROM is an electrically erasable programmable read-only memory, and its content will not be lost when power is off. Under normal circumstances, EEPROM is read-only like EPROM. When writing is required, a high voltage can be applied to the designated pin to write or erase, and the erasing speed is extremely fast. Generally, EEPROM chips are divided into two types: serial EEPROM and parallel EEPROM. The input/output of data in serial EEPROM is done through a 2-wire, 3-wire, 4-wire, or SPI bus interface while reading and writing. Data input/output is carried out via a parallel bus.

2. The application difference between EEPROM and EPROM

For a small system, switch setting parameters such as baud rate, synchronous and asynchronous selection, data length, and other parameters can be stored in EEPROM, and a data sequence or table can be formulated so that users can easily complete the selection. Each time the CPU is powered on, the specified working mode is set according to the parameters set in the EEPROM. For large-scale computers and communication networks, control information such as channel configuration, data rate, and terminal endurance can be displayed on the CRT in the form of a table. The system can be easily configured on the CRT. These data and tables are stored well, which solves the problems of stability and flexibility of the communication network.

EPROM is a commonly used memory. It is generally used as program memory in the development and application of single-chip microcomputers. In fact, it is also a programmable logic device. In addition to using it to implement logic functions, it can also be used to implement some complex measurements and control. The usual digital voltmeters use a liquid crystal display, which displays low brightness and low numbers.

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