Flash is an EEPROM that does not need Vpp voltage signal and belongs to volatile memory. There are two types of flash memory, nand flash memory and nor flash memory, which are classified according to the arrangement of memory cells in the matrix.
Basic principles:
Floating gate
Flash memory cells are based on floating gate technology. MOS transistor consists of two overlapping gates CG and FG. FG is isolated in the middle, like an island floating in the air, which is the origin of the name "floating gate". FG is surrounded by oxides, and electrons can stay in it for a long time without external force. The operations of injecting and removing electrons from the floating gate are called programming and erasing respectively. These operations modify the threshold voltage Vth of the memory cell, which is a special type of MOS transistor. By applying a fixed voltage to the CG terminal, two storage levels can be distinguished: when the gate voltage is higher than the battery's Vth, the battery is turned on, otherwise it is turned off.
Main differences between NOR flash memory and NAND flash memory:
1, physical structure:
NOR Flash adopts parallel array architecture, and each unit can be accessed directly through contacts, which is also the reason why NOR Flash has excellent random performance.
Or a flash memory structure.
NAND adopts serial structure, with 32 or 64 memory cells connected in series, as shown in the figure. Two selection transistors are placed at the edge of the row to ensure the connection with the source line and the bit line. Each NAND row shares a bit line contact with another row. The doors are connected by word lines.
NAND flash memory structure
2. Storage unit area, unit cost, reading speed and power consumption
Due to the different physical structures, the differences between the two flash memories in area, unit cost, reading speed and power consumption:
NOR Flash: large storage unit area, high unit cost, fast reading speed and high power consumption;
NAND Flash: small storage unit area, low unit cost, slow reading and low power consumption;
The cell size of NAND flash is almost half that of NOR device. Because the manufacturing process is simpler, the NAND structure can provide higher capacity under a given size, which correspondingly reduces the price. NOR flash occupies most of the flash memory market with the capacity of 1 ~ 16mb, while NAND flash is only used for products with the capacity above 8MB, which also shows that NOR is mainly used for code storage media and NAND is suitable for data storage.
3. Interface and use
NOR Flash interface is similar to SRAM interface, which has enough address pins for addressing and can easily access each byte. Since the NOR flash interface directly uses the NOR-based flash memory, it can be connected like other memories and can run code directly on it.
Flash devices use complex I/O ports to access data serially, and the methods of each product or manufacturer may be different. Eight pins are used for time-sharing transmission of control, address and data information. When using NAND devices, you must write drivers or hardware controller support before you can continue to perform other operations. So it's not as convenient and easy to use as NOR Flash.
4, bit exchange
Bit swapping is also called bit rotation/bit flip, that is, the stored value changes from 0 to 1 or from 1 to 0. All flash memory devices are plagued by bit swapping. Therefore, an error detection/correction algorithm must be adopted. The problem of bit inversion is more common in NAND flash memory. When using NAND flash memory, NAND vendors suggest using EDC/ECC algorithm at the same time. This problem is not fatal when using NAND to store multimedia information. Of course, if local storage devices are used to store operating systems, configuration files or other sensitive information, EDC/ECC systems must be used to ensure reliability.
NOR flash has no error correction system because of its different physical structure and low probability of bit inversion.
5. Bad block handling
Bad blocks in NAND devices are randomly distributed, so it is necessary to scan the media to find them and mark them as unusable. NOR Flash has few or no bad blocks, so there is no bad block processing module.
Flash device:
At present, all flash memories conform to PCMCIA standards and can be used in various devices conveniently. At present, there are two kinds of cards: one is flash memory card, and there is only one memory bank composed of flash memory chips, which needs special software to manage when used. The other is called Flash driver card, which has a control circuit composed of microprocessor and other logic circuits besides Flash chip.
1. flash card:
Flash card, also known as flashcard, is a kind of memory that uses flash technology to store electronic information. Generally, it is used as a storage medium in digital cameras, palmtop computers, MP3 and other small digital products. So it looks as small as a card, so it is called a flash card. According to different manufacturers and different applications, there are probably some flash cards, such as SmartMedia, CompactFlash, MultiMediaCard, SecureDigital, MemoryStick, XD-PictureCard and MICRODRIVE. Although these flash cards are different in appearance and specifications, their technical principles are the same.
Compact flash lamp
Compact flash memory card is the first flash memory card introduced by Sandisk in 1994. It completely changed the use of flash memory technology. For the stored data, CF card has higher security and protection than the traditional disk drive. Most network and telecom equipment and digital cameras, such as routers and switches, still use CF cards as the main external storage devices.
SM card
SM card is a flash memory card that Toshiba withdrew from 1995 1 1 in June. Samsung bought the production and sales license at 1996, and the two companies became the main SM card manufacturers. SmartMedia card is a common micro memory card in the market, which was once very popular on MP3 players. Smart media cards are considered as a substitute for floppy disks. It used to be a storage format widely supported by digital cameras, but now it is declining. Compared with other formats, the biggest advantage of this format is that SM cards with any capacity can be used in a standard 3.5-inch floppy disk drive through a converter called FlashPath.
MMC card
MMC card was jointly launched by Siemens and SanDisk, who founded CF, in 1997, claiming to be the smallest flash memory card in the world. In recent years, MMC card technology has been almost completely replaced by SD card, but MMC card can still be read by SD card compatible devices, so it still has its functions.
MS card
MS card, usually called memory stick, is a flash memory card developed by Sony. It was put on the market in 1998+ 10. It adopts Sony's appearance, protocol, physical format and copyright protection. The specifications of MS card are very similar to those of MMC listed at the same time.
Sd card
SD card was jointly launched by Panasonic, Toshiba and Sandisk. It was first issued in August 1999, and it is as big as a stamp. SD card reader is similar to USB floppy drive of computer. The card reader with SD card has the same function as the U disk and the same size as the ordinary U disk. The USB interface is used for the connection between the card reader and the host. This product was shot with a digital camera. There are two kinds: external and internal. Many new personal computers have built-in multifunctional card readers.
TF card
The TF card invented by SanDisk Company is a miniature flash memory card, which is mainly used for mobile phones. In 2004, it was renamed MicroSD. Almost the size of a fingernail, there are direct slots on mainstream desktops and notebooks, which can read and write data after being connected by SD card reader.
Xd picture card
XD is a flash memory card for digital cameras. In July 2002, it was jointly released by Fuji Rubber Roller and Olympus to replace SM.
Mini secure digital memory card
MiniSD is a miniature standard SD card issued by SanDisk in 2003, specially designed for mobile phones. The minSD adapter is connected to the card and is compatible with all devices equipped with standard SD card slots.
md/microdrive
Micro-hard disk MD is a very small hard disk data storage device developed by IBM, and 1999 is listed to counter the mainstream flash memory products in the market. IBM sold the hard disk division to Hitachi, so since 2003, the technology and patents of MicroDrive have been owned by Hitachi. The advantages of micro hard disk are large storage capacity and fast reading and writing speed, but the disadvantages are power consumption, easy heating, short service life and poor seismic performance.
2. Flash card
EMMC card
EMMC adopts a unified MMC standard interface, and encapsulates high-density NAND Flash and MMC controller in a BGA chip. According to the characteristics of flash memory, the product has included flash memory management technology, including error detection and correction, average flash memory erasure, bad block management, power failure protection and other technologies. Users don't need to worry about the changes of flash technology and internal technology of products. EMMC single chip saves more internal space of the motherboard.
UFS card
Ufusufs is a high-performance interface protocol, which also represents storage devices using this protocol. It is designed for applications that need to minimize power consumption, including mobile systems such as smartphones and tablets, and automotive applications. Its high-speed serial interface and optimized protocol can significantly improve throughput and system performance.
USB flash disk
U disk, according to its homonym, is also called "USB flash drive". U disk is a kind of flash memory, so it is sometimes called flash disk. The biggest difference between U disk and hard disk is that it does not need physical drive, plug and play, storage capacity far exceeds that of floppy disk, and it is very convenient to carry. It integrates U disk storage technology, flash memory technology and universal serial bus technology. Compared with other portable storage devices, Flashu disk has many advantages: it occupies less space, usually runs faster, can store more data and has reliable performance. You can disconnect during reading and writing without damaging the hardware and only losing data. This kind of disk uses USB mass storage device standard, and Linux, Mac OS X, Unix and Windows all have built-in support.
Solid state disk
SSD usually consists of a storage unit and a control unit. SSD is the most mainstream hard disk at present, and it will hardly be replaced by other hard disks in the foreseeable future. SSD has the advantages of fast reading and writing speed, earthquake resistance, low power consumption, wide working temperature range and light weight.
There are many kinds of flash memory, which is widely used in daily life, but its price is still at an expensive stage, which also limits the use of flash memory. I hope that flash memory can get rid of the disadvantage of high price and really enter people's lives completely.
Heart of the King 2 Click on the demo.