Flash memory is a non-volatile computer memory storage medium that allows multiple erases or writes during operation. This technology is mainly used for general data storage, as well as the exchange and transmission of data between computers and other digital products, such as memory cards and USB flash drives.
Catalog
| Ⅰ Introduction |
| Ⅱ Principle of storage |
| Ⅲ Technical characteristics |
| Ⅳ Application of flash memory |
Ⅰ Introduction
Flash memory is a kind of non-volatile memory, that is, data will not be lost even if the power is turned off. Flash memory is a special type of EPROM that is erased in macroblocks. The two main types of flash memory are named NAND flash and NOR flash. Flash memory can retain data for long periods without current supply. Its storage characteristics are equivalent to hard disks. This characteristic is the basis for flash memory to become a storage medium for various portable digital devices. Because flash memory does not rewrite data in bytes like RAM (random access memory), it cannot replace RAM. Flash memory has the advantages of small size, low power consumption, and is not easily damaged by physical damage. It is an ideal storage medium for mobile digital products. The core memory of all USB flash drives and memory cards of various digital devices is flash memory.
Flashcard is a memory that uses flash memory technology to store electronic information. It is generally used as a storage medium in small digital products such as digital cameras and computers. According to different manufacturers and different applications, flash memory cards are probably SmartMedia (SM card), Compact Flash (CF card), MultiMediaCard (MMC card), Secure Digital (SD card), Memory Stick, XD-Picture Card (XD card), and micro hard drive (MICRODRIVE). Although the appearance and specifications of these flash memory cards are different, the technical principles are the same.
Ⅱ Principle of storage
To explain the storage principle of flash memory, let’s start with EPROM and EEPROM.
EPROM means that the content can be erased by special means and then rewritten. The basic unit circuit (memory cell) is often a floating gate avalanche injection MOS circuit, referred to as FAMOS. It is similar to the MOS circuit, in which two high-concentration P-type regions are grown on the N-type substrate, and the source electrode S and the drain electrode D are respectively led out through ohmic contact. There is a polysilicon gate floating in the SiO2 insulating layer between the source and the drain, and there is no direct electrical connection with the surrounding. This circuit indicates whether the floating gate is charged to store 1 or 0. After the floating gate is charged, a positive conductive channel is induced between the source and the drain, and the MOS tube is turned on, which means that 0 is stored. If the floating gate is not charged, a conductive channel is not formed, and the MOS tube is not conductive, that is, 1 is stored.
EPROM basic storage unit
The working principle of the EEPROM basic storage unit circuit is shown in the figure below. Similar to EPROM, it generates a floating gate above the floating gate of the EPROM basic cell circuit. The former is called the first floating gate, and the latter is called the second floating gate. An electrode can be drawn to the second-stage floating gate to connect the second-stage floating gate to a certain voltage VG. If VG is a positive voltage, a tunnel effect is generated between the first floating gate and the drain. So electrons are injected into the first floating gate, that is, programming and writing. If VG is made a negative voltage, the electrons of the first-stage floating gate are strongly lost, that is, erased. It can be rewritten after erasing.
EEPROM basic storage unit circuit
The basic unit circuit of flash memory is similar to EEPROM, which is also composed of a double-layer floating gate MOS tube. However, the gate dielectric of the first layer is very thin and serves as a tunnel oxide layer. The writing method is the same as that of EEPROM. A positive voltage is applied to the second-stage floating gate to make electrons enter the first-stage floating gate. The reading method is the same as the EPROM. The method of erasing is to use the tunnel effect between the first-stage floating gate and the source to attract the negative charge injected into the floating gate to the source. Due to the use of source plus positive voltage erasure, the source of each cell is connected together. In this way, the flash memory cannot be erased byte by byte but erased in full or block. With the improvement of semiconductor technology, the flash memory has also realized the design of a single transistor (1T), which is mainly to add a floating gate and a selection gate to the original transistor to form a unidirectional conduction current between the source and drain on the semiconductor Floating shed to store electronics. The floating gate is wrapped with a layer of silicon oxide film insulator. Above it is the selection/control gate that controls the conduction current between the source and drain. The data is 0 or 1 depending on whether there are electrons in the floating gate formed on the silicon substrate. With electrons is 0, without electrons is 1.
The flash memory is initialized by deleting data before writing. Specifically, electrons are derived from all floating gates, that is, all data will be returned to “1”. When writing data, it works only when the data is 0, and does nothing when the data is 1. When writing 0, a high voltage is applied to the gate electrode and the drain, increasing the energy of electrons conducted between the source and the drain. As a result, electrons will break through the oxide film insulator and enter the floating gate. When reading data, a certain voltage is applied to the gate electrode. When the current is large, it is set to 1. And the small current is set to 0. In a state where the floating gate has no electrons (data is 1), a voltage is applied to the gate electrode and a voltage is applied to the drain. In this case, a large number of electrons move between the source and the drain to generate a current. In the floating gate with electrons (data is 0), the conduction electrons in the channel will decrease. Because the voltage applied to the gate electrode is absorbed by the floating gate electrons, it is difficult to affect the channel.
Ⅲ Technical characteristics
NOR and NAND are the two main non-volatile flash memory technologies on the market.
NOR flash memory
In 1984, the inventor of Toshiba Co., Ltd. Fujioka first proposed the concept of flash memory. Unlike traditional computer memory, the feature of flash memory is NVM, and the recording speed of flash memory is also very fast.
Intel is the first company in the world to produce flash memory and put it on the market. In 1988, the company introduced a 256K bit flash memory chip. It is the size of a shoebox and is embedded in a recorder. Later, this type of flash memory invented by Intel was collectively called NOR flash memory. It combines EPROM and EEPROM technologies and has an SRAM interface.
NAND flash memory
The second type of flash memory is called NAND flash memory. It was developed by Hitachi in 1989 and is considered an ideal replacement for NOR flash memory. The write cycle of NAND flash memory is 90% shorter than that of NOR flash memory, and its save and delete processing speed is relatively fast. The storage unit of NAND is only half of NOR, and NAND obtains better performance in smaller storage space. Given the excellent performance of NAND, it is often used in memory cards such as CompactFlash, SmartMedia, SD, MMC, xD, and PC cards, USB sticks, etc.
The storage unit of the NAND flash memory adopts a serial structure, and the reading and writing of the storage unit are performed in units of pages and blocks. A page contains several bytes, and several pages form a storage block, and the storage block size of NAND is 8 to 32KB. The biggest advantage of this structure is that the capacity can be made very large. NAND products with a capacity of more than 512MB are quite common. The cost of NAND flash memory is low, which is conducive to large-scale popularization.
The disadvantage of NAND flash memory is that the read speed is slow, and it has only 8 I/O ports, which is much less than NOR. The 8 I/O ports in this area can only complete the data transmission in the form of signal transmission, and the speed is much slower than the parallel transmission mode of NOR flash memory. Also, the logic of NAND flash memory is an electronic disk module structure. There is no special storage controller inside. Once a bad data block occurs, it cannot be repaired, and its reliability is worse than that of NOR flash memory.
Since erasing a NOR device is performed in 64-128KB blocks, the time to perform a write/erase operation is 5s. In contrast, erasing a NAND device is performed in 8-32KB blocks, performing the same operation only takes up to 4ms. The difference in block size when performing an erase further widens the performance gap between NOR and NADN. Statistics show that for a given set of write operations (especially when updating small files), more erase operations must be performed in NOR-based units. In this way, when choosing a storage solution, the designer must weigh the following factors.
● The read speed of NOR is slightly faster than NAND.
● NAND write speed is much faster than NOR.
● NAND 4ms erase speed is much faster than NOR 5s.
● Most write operations require an erase operation first.
● The erase unit of NAND is smaller, and the corresponding erase circuit is less.
Ⅳ Application of flash memory
NAND flash memory is widely used in digital devices such as mobile storage, digital cameras, and handheld computers. Driven by the strong development of digital equipment, NAND flash memory has been showing exponentially high-speed growth.
“Flash memory” can often be used interchangeably with “NOR memory”. Many people in the industry are not sure about the advantages of NAND flash technology over NOR technology. Because in most cases flash memory is only used to store a small amount of code, then NOR flash memory is more suitable. NAND is an ideal solution for high data storage density.
The characteristic of NOR is the on-chip execution (XIP, eXecute In Place) so that the application can be run directly in the flash memory without having to read the code into the system RAM. The transmission efficiency of NOR is very high, and it is very cost-effective in the small capacity of 1 to 4MB, but the very low write and erase speed greatly affects its performance.
NAND structure can provide extremely high cell density and can achieve high storage density. Its write and erase speed is also very fast. The difficulty of applying NAND is the management of flash and the need for special system interfaces.
“USB flash drive” is the most obvious portrayal of flash memory in daily life. Flash memory has appeared in many electronic products before the U disk. The traditional way of storing data is the volatile storage of RAM, and the data will be lost when the battery runs out. Products using flash memory overcome this problem and make data storage more reliable. In addition to flash disks, flash memory is also used in electronic products such as BIOS, PDAs, digital cameras, voice recorders, mobile phones, digital TVs, and game consoles in computers.
Flash Card