Comparison of NAND Flash and NOR Flash for Embedded Learning

ROM and RAM refer to semiconductor memories, ROM stands for Read Only Memory, and RAM stands for Random Access Memory. ROM can still maintain data when the system stops Power Supply, and RAM usually loses data after power-off. Typical RAM is the memory of the computer.

There are two kinds of RAM, one is called Static RAM/SRAM, and SRAM is very fast. It's the fastest storage device ever read and written, but it's also very expensive, so it's only used in very demanding places. , For example, a level of CPU buffer, two buffer. The other is called dynamic RAM (Dynamic RAM / DRAM), DRAM retains data for a very short time, slower than SRAM, but it is still faster than any ROM, but DRAM is cheaper than SRAM in terms of price Many, computer memory is DRAM.

DRAM is divided into many kinds, the common one mainly includes FPRAM/FastPage, EDORAM, SDRAM, DDR RAM, RDRAM, SGRAM and WRAM,etc., introduce a kind of DDR RAM here.

DDR RAM (Date-Rate RAM) is also called DDR SDRAM. This improved RAM and SDRAM are basically the same, except that it can read and write data twice in one clock, which doubles the data transmission speed. . This is the most used memory in the current computer, and it has a cost advantage, in fact defeated Intel's other memory standard - Rambus DRAM. On many high-end graphics cards, high-speed DDR RAM is also equipped to increase the bandwidth, which can greatly improve the pixel rendering capabilities of the 3D accelerator card.

How memory works:

Memory is used to store data and programs that are currently in use (that is, in execution). The memory of the computer we usually refer to refers to dynamic memory (ie, DRAM). The so-called “dynamic” in dynamic memory refers to When we write data to DRAM, the data will be lost after a period of time, so we need an additional circuit to perform the memory refresh operation.

The specific working process is as follows: whether a memory cell of a DRAM stores 0 or 1 depends on whether the capacitor has a charge, a charge represents 1 and no charge represents 0. However, over time, the capacitance of the representative 1 will discharge, and the capacitance representing 0 will absorb the charge. This is the reason for the data loss. The refresh operation periodically checks the capacitor. If the power is greater than 1/2 of the full charge, it is considered to represent 1 The capacitor is fully charged; if it is less than 1/2, it is assumed to represent zero and the capacitor is discharged to maintain the continuity of the data.

There are also many kinds of ROM, PROM is a programmable ROM, PROM and EPROM (Erasable Programmable ROM) is the difference between the two, PROM is a one-time, that is, after the software into the can not be modified, this is early The product is now impossible to use, and the EPROM is a general-purpose memory that erases the original program by ultraviolet light irradiation. Another type of EEPROM is erased electronically. The price is high, write time is long, and writing is slow.

For example, mobile phone software is generally placed in EEPROM. We call. Some of the last dialed numbers are temporarily stored in SRAM. We do not immediately write to the record (the call history is stored in EEPROM), because there is an important job at that time. (Call) to do, if written, long wait is to make the user tolerant.

FLASH memory, also known as flash memory, combines the advantages of ROM and RAM, not only with electronically erasable and programmable (EEPROM) performance, but also can not lose power while losing data and can quickly read data (NVRAM advantage), U disk And this memory is used in MP3. In the past 20 years, embedded systems have been using ROM (EPROM) as their storage device. However, in recent years, Flash has completely replaced the status of ROM (EPROM) in embedded systems and used as a storage bootloader and operating system or program. Code or directly used as a hard disk (U disk).

Flash mainly has two kinds of NOR Flash and NADN Flash.

The reading of NOR Flash is the same as the reading of our common SDRAM. Users can directly run the code loaded in NOR FLASH, which can reduce the SRAM capacity and save the cost.

NAND Flash does not adopt the random read technology of the memory, its reads are carried out in the form of reading one piece at a time, usually read 512 bytes at a time, the flash that adopts this kind of technology is cheaper. The user cannot directly run the code on the NAND Flash, so a lot of development boards using NAND Flash not only use NAND Flah, but also make a small NOR Flash to run startup code.

NOR Flash is generally used for small capacity because it has a fast read speed and is used to store important information such as the operating system. For large-capacity NAND FLASH, the most common NAND FLASH application is the DOC (Disk On Chip) used in embedded systems. And we usually use the "flash drive", you can erase it online. The current FLASH market mainly comes from Intel, AMD, Fujitsu and Toshiba, and the main manufacturers of NAND Flash are Samsung and Toshiba.

Comparison of NAND Flash and NOR Flash

NOR and NAND are the two main non-volatile flash memory technologies on the market today. In 1988, Intel first developed the NOR flash technology, completely changing the situation in which the original EPROM and EEPROM dominated the world. Then, in 1989, Toshiba released a NAND flash architecture, emphasizing lower cost per bit, higher performance, and easy upgradeability via interfaces, just like disks. But after more than a decade, there are still quite a few hardware engineers who can't distinguish between NOR and NAND flash.

The phase "flash memory" can often be used interchangeably with the phase "NOR memory". Many industry players are also confused about the advantages of NAND flash memory technology over NOR technology, because in most cases flash memory is used to store small amounts of code, and NOR flash memory is more suitable. NAND is an ideal solution for high data storage densities.