The difference between NAND Flash and NOR Flash is detailed

We use a smart phone in addition to a usable space (such as Apple 8G, 16G, etc.), there is a ram capacity, many people are not very clear, why need two such a chip to do storage, this is what we want to talk about. These two kinds of storage devices we are collectively referred to as "flash", Flash is a memory chip, all called Flash EEPROM memory, through the program can modify the data, that is usually said "flash". Flash is also divided into NAND flash and nor flash two kinds. This kind of memory is used in USB and MP3.

The phase "flash memory" can often be used interchangeably with the phase "nor memory". Many in the industry are also confused about the superiority of NAND flash technology to nor technology, because in most cases flash memory is used only to store a small amount of code, and nor flash memory is more appropriate. NAND is an ideal solution for high data storage densities. Nor Flash read the same as our common SDRAM read, the user can directly run the code loaded in NOR Flash, which can reduce the capacity of SRAM and thus save costs. NAND Flash does not take the memory of random read technology, its reading is in a single read a piece of the form, usually read 512 bytes at a time, the use of this technology Flash relatively inexpensive. The user cannot directly run the code on NAND flash, so a lot of the development boards using NAND flash use a small nor Flash to run the startup code in addition to the NAND Flah.

Nor Flash is an Intel company that developed nor Flash technology in 1988. Nor is characterized by in-chip execution (XIP, execute in place) so that the application can run directly in Flash flash memory without having to read the code into the system RAM. Nor is highly cost effective in 1~4MB small capacity, but very low write and erase speeds greatly affect its performance, as well as its high transmission efficiency.

Nand-flash memory is a kind of flash memory, in 1989, Toshiba Company published the NAND flash structure. The non-linear macro element model is adopted in the system, which provides a cheap and effective solution for the realization of the solid-state large-capacity memory. Nand-flash memory has the advantages of large capacity, fast rewriting speed, and is suitable for storage of large amount of data, so it has been widely used in the industry, such as embedded products including digital camera, MP3 Walkman memory card, compact U disk, etc.

NAND Flash and NOR flash principle
First, the principle of storing data
The two types of flash memory are three-terminal device as a storage unit, respectively, the source, drain and gate, and the field effect tube works the same principle, mainly using the effect of the electric field to control the source and the leakage between the drain, the gate of the current consumption is very small, the difference is that the FET is a single gate structure, and FLASH is a A floating gate is added between the gate and the silicon substrate. [attach]158 [/attach]
The floating gate is composed of a nitride sandwiched between two layers of silica material, and the middle nitride is a charge potential well that can store the charge. The upper and lower layers of oxides are thicker than 50, to avoid breakdown.
Second, the heavy discharge of the floating gate
The process of writing data to a data unit is the process of injecting charge into the charge potential well, two techniques for writing data, a hot electron injection (injection) and F-n tunnel effect (Fowler Nordheim tunneling), the first of which is the source The pole is charged to the floating gate, and the latter is charged to the floating gate through a silicon base. NOR FLASH is charged to the floating gate by means of a hot electronic injection, while NAND is charged to the floating gate through the f-n tunnel effect.
Before writing the new data, the original data must be erased, which is different from the hard disk, that is, the charge of the floating gate is dropped, both FLASH through the F-n tunnel effect discharge.
III, 0 and 1
In this regard, the two flash, the charge to the floating gate is written to the ' 0 ', there is no injected charge to indicate ' 1 ', so the flash erase data is write 1, which is the opposite of the hard disk;
In the case of an electric charge in a floating gate, a positive space charge area will be formed between the source and the drain due to the inductive action of the floating gate, and the transistor will be in the conduction state regardless of the bias voltage applied to the control pole. In the case of transistors with no charge in the floating gate, the source and drain can be switched on only when the control pole is applied with an appropriate bias voltage, and the electrode is induced on the silicon base, which means that the transistor is cut off when no bias voltage is applied to the control pole. If the source of the transistor is grounded and drain the pole line, in the absence of bias voltage, the detection transistor conduction state can obtain data in the storage unit, if the level on the bit line is low, indicating that the transistor is on the state, read the data is 0, if the bit line is high, then the transistor is in the cut-off state, The data read is 1. The read operation does not change the data in the FLASH because the control gate exerts little or no voltage at all during the reading of the data, which is insufficient to change the amount of charge in the floating gate.
Iv. connection and Addressing methods
Two Flash has the same storage unit, the same principle, in order to shorten the access time is not for each unit to separate access operation, but a certain number of access units for collective operation, NAND flash memory units are in series between, and NOR type flash units are parallel , in order to effectively manage all the storage units, the storage unit must be uniformly addressed.
All of the NAND storage units are divided into blocks, each block is divided into several pages, each page is 512byte, is 512 8 digits, that is, each page has 512 bit lines, each bit line has 8 storage units, so the data stored on each page is exactly the same as the data stored in one sector of the hard disk, this is the design time In order to facilitate data exchange with the disk deliberately arranged, then the block is similar to the hard disk cluster, the capacity is different, the number of blocks different, the number of pages that make up the block is also different. When reading the data, when the word line and the bit line lock a transistor, the transistor's control pole does not bias voltage, the other 7 plus bias voltage, conduction, if the transistor's floating grid charge will be conductive to make the bit line low level, read out the number is 0, the reverse is 1.
NOR's each storage unit connected in parallel to the place line, convenient for each one random access, with a dedicated address line, can achieve a one-time direct addressing, shortening the FLASH on the processor instructions to execute. Five, performance

The difference between NAND Flash and NOR flash
One, NAND flash and NOR flash performance comparison
Flash Flash memory is non-volatile memory, can be called Block of memory unit block for Erasure and re-programming. The write operation of any flash device can only be done within an empty or erased unit, so in most cases the erase must be performed before the write operation. The erasure of NAND devices is straightforward, and nor does it require that all bits in the target block be written to 0 before erasing. Since the erase nor device is carried out in a 64~128kb block, the time to perform a write/erase operation is 5s, in contrast, the erasure of the NAND device is carried out in a 8~32kb block, and the same operation is performed for up to 4ms. The difference in block size when performing the erase further widens the performance gap between nor and NADN, and the statistics show that for a given set of writes (especially when updating small files), more erase operations must be done in the nor-based unit. This way, when choosing a storage solution, the designer must weigh the following factors.
1, nor read a bit faster than NAND.
2, NAND write speed is much faster than nor.
3, NAND 4ms erase speed is much faster than nor 5s.
4. Most write operations require a wipe operation first.
5, the NAND erase unit is smaller, the corresponding erase circuit less.
Second, NAND Flash and NOR flash interface differences
NOR Flash has an SRAM interface with enough address pins to be addressable, which makes it easy to access every byte inside of it.
NAND devices use complex I/O ports to sequentially access data, and the methods of each product or vendor may vary. 8 pins are used to transmit control, address, and data information. NAND read and write operations take a 512-byte block, which is a bit like a hard drive to manage such operations, and it is natural that NAND-based storage can replace hard disks or other block devices.
Iii. capacity and cost of NAND flash and NOR flash
NAND Flash unit sizes are almost half the size of nor devices, and because of the simpler production process, NAND structures can provide higher capacity within a given mold size and correspondingly lower prices.
Nor Flash occupies the bulk of the 1~16MB flash market, while NAND flash is used only in 8~128MB products, which also shows that nor is it primarily used in code storage media, NAND is suitable for data storage, NAND in CompactFlash, The largest share of Secure Digital, PC cards and MMC memory cards is in the market.
Iv. reliability and durability of NAND flash and NOR flash
One of the key issues to consider when using flahs media is reliability. Flash is a very suitable storage solution for systems that require extended MTBF. The reliability of nor and NAND can be compared from three aspects of longevity (durability), bit switching and bad block handling.
V, NAND Flash and NOR flash life (durability)
The maximum number of erase times per block in the NAND flash memory is 1 million, and nor is 100,000 times the number of erase writes. In addition to the 10:1 block erase cycle advantage of NAND memory, the typical NAND block size is 8 times times smaller than nor, and each NAND memory block is removed less frequently within a given time.
Six, Bit Exchange
All flash devices are subject to the phenomenon of bit switching. In some cases (rarely seen, NAND occurs more often than nor), a bit is reversed or reversed. One change may not be obvious, but if it happens on a critical file, this small glitch can lead to system downtime. If you just report a problem, read it a few times and it can be solved. Of course, if this bit really changes, then the error detection/error correction (EDC/ECC) algorithm must be used. Bit reversal problem is more common in NAND flash, NAND vendors recommend using NAND flash while using
Seven, EDC/ECC algorithm
This problem is not fatal when storing multimedia information in NAND. Of course, if you are using a local storage device to store operating systems, configuration files, or other sensitive information, you must use the EDC/ECC system to ensure reliability.
Eight, bad block processing
The bad blocks in the NAND device are randomly distributed. There have been efforts to eliminate the bad block, but found that the yield is too low, the price is too high, it is not cost-effective.
The NAND device requires an initial scan of the media to discover the bad block and mark the bad block as unavailable. In devices that have been made, failure to do so through a reliable method will result in high fault rates.
Nine, easy to use
You can use nor-based flash memory very directly, you can connect like other storage, and you can run code directly on it.
NAND is much more complex due to the need for I/O interfaces. Access methods for various NAND devices vary by manufacturer. When using a NAND device, you must write to the driver before you can continue with other operations. Writing information to NAND devices requires considerable skill, as designers must never write to bad blocks, which means that virtual mapping is required throughout the NAND device.
Ten, software support
When discussing software support, you should distinguish between basic read/write/erase operations and high-level software for disk emulation and flash management algorithms, including performance optimizations.
No software support is required to run code on nor devices, and in the same operation on NAND devices, drivers are often required, i.e. memory technology drivers (MTD), and NAND and nor devices require MTD for both write and erase operations.
There is a relatively small amount of MTD required to use nor devices, and many vendors offer more advanced software for nor devices, including the M-system TrueFFS driver, which is driven by Wind River System, Microsoft, QNX software Used by vendors such as System, Symbian, and Intel.
The drive is also used to simulate and manage NAND flash for diskonchip products, including error correction, bad block handling, and loss balancing.

The difference between NAND Flash and NOR Flash is detailed