NAND flash and NOR Flash

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The difference between NOR flash and NAND flash

1) The basic unit of flash memory chips to read and write different
The application of NOR chip operating in the "word" as the basic unit. In order to facilitate the management of the large capacity NOR flash, usually divided into logical block NOR memory size is 128KB or 64KB, sometimes block is further divided into sectors. Read and write at the same time the specified logical block number and offset. The application of NAND chip operation is to "block" as the basic unit. NAND flash memory block is relatively small, generally is 8KB, then each is divided into a page, the page size is 512 bytes. To modify a byte in the NAND chip, we must rewrite the entire data block.
2)NOR flash memory is random storage medium, for the small amount of data applications; NAND flash memory is continuous storage medium, suitable for storing large data.
3) The separate NOR address line and the data line, so the NOR chip can be connected to the data line like SRAM. The use of NOR chip and like memory chips; in general, its transmission efficiency is high, the executable program can be executed in the chip (XI P, eXecute In Place), so the application can be run directly on the flash flash memory, don't have to read the code in RAM system. Due to the characteristics of NOR, embedded systems are often NOR chip chip used to start. And NAND share the address and data bus, the need for additional connection control input and output, so the NAND chip chip to start.
4) N AN D flash memory chip because of shared address and data bus, does not allow for a byte or a block of data clear, can only be reset operation on a fixed size region; and the NOR chip can operate on word. So when dealing with small data quantity of I/O operation is faster and the speed of NAND. For example, a NOR chip usually write a word takes 10 microseconds, then in 32 bit bus to write 512 bytes to 1280 milliseconds; while NAND flash write 512 bytes need time including: 512 * per byte 50 nanosecond + 10 microsecond page time + 200 microsecond piece erase time = 234 seconds.
5)NAND flash memory capacity is larger, the largest capacity has reached 8G bytes. In order to facilitate the management, storage space using a NAND block and page two level storage system, that is to say the storage space of flash memory is two-dimensional, such as K9F5608UOA flash memory block size is 16K, the page size is 512 bytes, 16 bytes per page also spare area for storing error checking code space (sometimes referred to as out-of-band, OOB space); in the write operation of the NAND flash every time a byte of data in the cache within the District, and then issued a "write" write operation. Due to the NAND operations are based on the block and page as a unit, so the data to the NAND flash memory read and write, NAND faster than NOR flash.
6)The reliabilities of NOR flash memory is higher than that of NAND flash memory, this is mainly because the NOR type flash memory interface is simple, less data operation, a switching operation is little, so high reliability, little bad block, which is generally used in high demand on the reliability. On the contrary, NAND flash memory interface and operation are relatively complex, a lot of key exchange operation, more data is required an error detection / correction (EDC/ECC) algorithm to ensure the integrity of the data, so the risk of problems is much greater, bad block is inevitable, but due to the bad block is random distribution, even the correction could not do.
7)Common NAND Flash address line and the data line, have a certain effect on the speed of reading and writing; and the NOR Flash flash memory data and address lines separately, so relatively fast speed to read and write some.
NAND and NOR chip common first performance in writing data to the chip must be content to empty the corresponding chip, and then write, is usually said "erase before write". But NOR chip only write a word, while NAND need to erase the entire block. Secondly, the number of erase flash memory is limited. When the flash is used to approach the use life, often write operation failed; reach the service life of flash memory, although the internal storage of data can be read, but not write operation. So in order to prevent the above problems, not to some specific regions. Repeated write operation. Usually NAND erasable times higher than that of NOR chip, but since the NAND is block erasable, block in the page if there is a failure of the entire block will fail, and the erasing process is complex, the probability of failure is relatively high, so the longer life on the whole NOR.

Another common flash memory read and write operation is not only a physical operation, actually store the data in the flash memory must use algorithm, this module in the driver for MTD'(Memory Technology Drivers) module or in the FTLZ (Flash Translation Layer) layer, the specific algorithm and chip manufacturers and chip type of relationship.
From the perspective of use, NOR flash and NAND flash memory is different characteristics: (1) the storage density of NOR is low, so the store a byte cost is higher, and NAND flash memory density and storage capacity are relatively high; (2) NAND type flash memory wipe, write documents (especially large files continued even) very quickly, very suitable for sequential reads occasions, and NOR reading speed is very fast, has a good performance in the application of random access. NOR and NAND Each one has his good points., but the two advantages that can not be reflected in a chip. So, the designer in the selection of chip, can only hasten their profits and avoid the harm, the appropriate choice of in between in accordance with the purpose and main functions.

Comparison of NAND and NOR Technology

The general principle is: use NAND type flash memory in multimedia application, large capacity, while in the data / program storage applications using NOR type flash memory. According to this principle, the designer can also put two kinds of flash memory chips used in combination with NOR chips, stored procedures, using NAND chip to store data, the complementary advantages of two kinds of flash memory. In fact, this clever design has been widely used in mobile phone, PocketPC, PDA and electronic dictionaries and other equipment.

In the choice of a storage solution, the designer must balance among the various factors, in order to obtain a higher price. Taking mobile phone as example, to support the XIP technology of NOR flash memory to run OS, very fast, not only simplifies the design, but also reduce the cost, so many mobile phone use NORRAM design. Deficiency of NOR flash memory storage density is low, so there is a design using NAND + RAM. For the two kinds of schemes, it is hard to say which one is better, because we cannot leave the specific product from a simple to evaluate. The pursuit of neat mobile phone will need NOR Flash support; the pursuit of large storage capacity of the mobile phone will be more selective NAND flash; design and pursue both functional and speed of the mobile phone will be the use of NOR + NAND + RAM, this design can play NOR and NAND from their respective advantages.

In addition to speed, storage density, designers in the choice of flash memory chips, also need to consider the design of interface, plug and play many problems in design and driver, because the two types of flash in the above several also have many different. For example, in terms of drivers, NOR devices running the code does not need any software support, and to perform the same operation on the need to store technology driver in NAND device (MTD) support. Although NAND and NOR devices in the write and erase operations are needed for MTD, but for NAND driver development is more difficult, because the NAND flash memory error correcting and bad block management functions are needed to achieve through the driver.

Using the difference

Architecture design in difference reflects the use on and NOR and NAND's own inseparable, first in the interface, the design of NOR has an obvious characteristic of traditional flash, so the actual application is relative to the new complex I/O design NAND much easier. But, in the use of NAND flash memory, you must first write drivers, can continue to perform other operations.

Secondly, in the capacity of the rewritable, each NAND block erasable times in 100000 to 1000000 between, NOR is just its 1/10, and each erase block the capacity of NAND is only NOR of 1/8 to 1/2, this indicates that, each block erase frequency to less than NOR of flash memory, thus helping to extend the service life. In the data storage time, both are almost the same, level for 10 years.

However, due to the series structure, between the NAND transistors are more likely to affect, the logic of the 0 into the 1 transistor logic, and it is difficult to find the problem, this phenomenon can be called the bit flipping (Bit-Flipping), which requires the use of the EDC/ECC (error detecting codes / error correction code) for correction, this the problem of NOR is less.

In addition, NAND there are a bad block management problem, in NAND flash memory, because of the bad blocks are randomly distributed, so the need for scanning and the bad block mark, as with the hard disk bad sectors. The current products, can allow a maximum of 80 bad blocks. Bad block makes the written information requires considerable skill to the NAND flash memory, because the designer can't write to the bad block, which means that in the NAND flash from first to last must be virtual mapping.

In the degree of software support, should distinguish the basic reading / writing / erasing operation and high level for disk emulation and flash memory management algorithm software, including performance optimization.

Code support does not need any software that runs on NOR flash memory, in the NAND flash memory on the same operation, usually requires the driver, also is the memory technology driver (MTD), NAND and NOR flash memory write and erase operations are needed MTD.

The need to use NOR flash memory when MTD are relatively less, many manufacturers have to provide for more advanced software NOR flash memory, which includes the M-System TrueFFS driver, the driver is Wind River System, Microsoft, QNX Software System, Symbian and Intel used by manufacturers.
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Posted by Sampson at December 31, 2013 - 7:17 AM