Nand and nor flash

Nor and NAND are two major non-loss flash technologies on the market. Intel first launched nor in 1988
Flash technology has completely changed the previous integration of EPROM and EEPROM
The world. Next, in 1989, Toshiba published the NAND Flash structure, emphasizing the reduction of the success rate of each bit.
Higher performance, and the interface can be used like a disk
Easy to upgrade. However, after more than a decade, a considerable number of hardware engineers are still confused about nor and NAND Flash.

Flash memory can often be used with nor memory. Many insiders are confused.
Compared with nor, Chu nand flash memory is superior
In most cases, flash memory is only used to store a small amountCodeAt this time, nor flash memory is more suitable for some. While NAND is high
Ideal solution for data storage density.
Nor features in-chip execution (xip, execute in place ).ProgramYou can directly
It runs in flash memory and does not have to read the code into the system Ram.
Nor transmission efficiency is very high, in 1 ~ 4 MB of small capacity has high cost efficiency, but very low write and erase
The speed greatly affects its performance.
The NAND structure provides an extremely high unit density, achieves a high storage density, and the write and erase speeds are also high.
Soon. The difficulty of using NAND lies in the management and needs of flash.
Special system interfaces.

Performance Comparison
Flash flash memory is a non-loss-prone memory. It can be used to erase and re-program memory unit blocks called blocks. Any
Write operations on flash devices can only be performed on empty or erased units.
So in most cases, you must erase the data before writing. The NAND device performs erasure.
The operation is very simple, and nor requires that
First, all bits in the target block must be written to 0.
Since the nor device is erased with 64 ~ The write/Erase time of a block of KB is 5 s.
In contrast, the erased NAND device is ~ 32 KB
The number of rows. It takes up to 4 ms to perform the same operation.
When the block size is erased, the performance gap between nor and nadn is further extended. Statistics show that
Given a set of write operations (especially when updating small files ),
More erasure operations must be performed in the nor-based unit. In this way, when selecting a storage solution, the designer must
The following factors must be weighed.
● Nor reads faster than NAND.
● NAND writes much faster than nor.
● NAND's 4 Ms erasure speed is far faster than nor's 5s.
● Most write operations require erasure first.
● NAND has fewer erased units and fewer erased circuits.

Interface differences
Nor Flash has an SRAM interface and enough address pins to address it, so it can easily access each
One byte.
Nand devices use complex I/O Ports for serial data access. Different products or vendors may use different methods.
. Eight pins are used to transmit control, address, and data information.
Nand read and write operations use 512 bytes, which is a bit like hard disk management.
Instead of hard disks or other Block devices.

capacity and cost
the unit size of NAND Flash is almost half the size of nor devices. As the production process is simpler, the NAND structure can be
to provide a higher capacity within the given mold size, which reduces the price accordingly.
nor flash occupies 1 ~ Most of the 16 MB flash memory market, while NAND Flash is only used in 8 ~ Among the 128 M
B products, this also shows that nor is mainly used in code storage
storage media, and NAND is suitable for data storage, nand has the largest share in the market of compactflash, secure digital, PC cards, and m
Mc memory cards.

reliability and durability
when using the flahs media, reliability is a key concern. For systems that need to expand MTBF
, Flash is a very suitable storage solution. The reliability of nor and NAND can be compared from three aspects: lifetime
(durability), bit switching and bad block processing.
lifetime (durability)
in nand flash memory, the maximum number of writes to each block is 1 million, while that of nor is 100,000. In addition to the block erasure cycle advantage of the NAND
memory, the typical
NAND block size is 8 times smaller than that of the nor device, each NAND memory block is deleted less than once in a given period of time
.
bit switching
all flash devices suffer from bit switching. In some cases (rare, the number of NAND occurrences is
more than nor), a bit is reversed or reported to be reversed
.
the change of the first digit may not be obvious, but if it occurs on a critical file, this small fault may cause the system to stop.
. If you only report a problem, read it several times
.
Of course, if this bit is changed, you must use the error detection/error correction (EDC/ECC) algorithm .
the inversion problem is more common in nand flash memory. The NAND supplier builds
to discuss the use of the EDC/ECC algorithm when using nand flash memory.
this problem is not fatal when using NAND to store multimedia information. Of course, if you use a local storage device
to store operating systems, configuration files, or other sensitive information,
you must use the EDC/ECC system to ensure reliability.
Bad Block Processing
Bad blocks in NAND devices are randomly distributed. I have tried to eliminate bad blocks before, but it is found that the yield rate is too low
, the cost is too high, and it is not cost-effective at all.
the NAND device needs to initialize the media to detect Bad blocks and mark them as unavailable. In a
manufactured device, if this
cannot be processed using a reliable method, the failure rate is high.

Easy to use
The nor-based flash memory can be used very directly, and can be connected like other memory, and can be directly
Run the code.
Due to the need for I/O interfaces, Nand is much more complicated. The access methods for various NAND devices vary from manufacturer to manufacturer.
When using the NAND device, you must first write the driver to continue other operations. Write to NAND Device
Entering information requires a lot of skill, because designers must never break
Block write, which means that virtual ing is required from beginning to end on the NAND device.

Software Support
When discussing software support, we should distinguish basic read/write/erase operations from high-level disk simulation and
Software for flash memory management algorithms, including performance optimization.
Running code on the nor device does not require any software support. When performing the same operation on the NAND device
The driver is required, that is, the memory technology driver (MTD
), The NAND and nor devices require MTD for write and erase operations.
Fewer MTDS are required when using nor devices. Many vendors provide more advanced software for nor devices.
Including the trueffs driver of M-system.
By Wind River system, Microsoft, QNX software system, Symbian, Intel, and other vendors
.
The driver is also used to simulate diskonchip products and manage nand flash memory, including error correction, Bad Block Processing, and
Loss balancing.

(Correct one point: During the nor erasure, all data is written as 1 rather than 0. Moreover, the erasure time of the nor flash sector varies depending on the brand and size. For example, 4 M Flash, some sector erases 60 ms, while some require a maximum of 6 s .)

Nor flash is mainly supplied by Intel, micro, and other vendors. It was once a mainstream flash product, but it is hard to squeeze it by NAND Flash. Its advantage is that it can run programs directly from flash, but the process is complicated and the price is relatively high.

The main vendors of NAND Flash are Samsung and Toshiba. They all use this flash in oil disks, various memory cards, and MP3 players. Due to different technologies, they have a larger storage capacity than nor flash, and cheap. However, there are also disadvantages: you cannot directly run programs with addressing, and you can only store data. In addition, NAND Flash is prone to bad areas, so verification algorithms are required.

In the handheld computer, you must use NAND Flash to store data and programs, but nor flash must be enabled. In addition to the Samsung processor, other mainstream processors used in handheld computers do not support programs directly started by NAND Flash. Therefore, it is very troublesome to start the machine with a small nor flash, and load OS and other software from the NAND flash into the SDRAM.

Nor flash:

1) Serial nor flash (Serial nor flash) is used for the motherboard, graphics card BIOS chip, etc.

2) parallel nor flash (parallel nor flahs) is used for PDA, mobile phone and other communication devices.