Comparison between nor flash and NAND Flash

1. Nor features in-chip execution (xip, execute in place), so that applications can run directly in flash memory without having to read the code into system Ram. The advantage is that the program can be run directly from flash and stored as code, but the process is complicated, the price is relatively expensive, and the transfer efficiency of nor is very high ~ The small size of 4 MB has high cost efficiency, but the low write and erase speeds greatly affect its performance.

 

2. The NAND structure provides an extremely high unit density, achieves a high storage density, and the write and erase speeds are fast. The difficulty of using NAND lies in Flash management and special system interfaces. Advantages: large storage capacity and low cost. The disadvantage is that you cannot directly run programs with addressing and can only store data. In addition, NAND Flash is prone to bad areas, so verification algorithms are required.

 

Write operations on any flash device can only be performed in an empty or erased unit.

 

(1) It is very easy for the NAND device to perform the erasure operation. Nor requires that all the bits in the target block be written to 1 before the disk is erased.

 

(2) When the nor device is erased ~ The write/Erase time of a block of KB is 5 S. The erased time of norflashsector varies depending on the brand and size, for example, 4 mflash, some sector erasure time is 60 ms, while some require a maximum of 6 s. In contrast, the erased NAND device is composed of eight ~ For 32 KB blocks, it takes up to 4 ms to perform the same operation

 

(3) When selecting a storage solution, the designer must weigh the following factors.

 

● Nor reads faster than NAND. (In fact, the unit is page, and the read Speed of NAND is also good)

● NAND writes much faster than nor.

● NAND's 4 Ms erasure speed is far faster than nor's 5s. (Now the erased time of norflash should not be in seconds)

● Most write operations require erasure first.

● NAND has fewer erased units and fewer erased circuits.

 

(4) interface differences

 

Norflash comes with an SRAM interface and has enough address pins to address it, making it easy to access every byte inside it.

Nand devices use complex I/O ports to access data in a serial manner. Different products or vendors may use different methods. Eight pins are used to transmit control, address, and data information. The read and write operations on NAND are measured in blocks (commonly used in the small page mode 512bytes and big page Mode 2 Kbytes). This is a bit like hard disk management. Therefore, nand-based memory can replace hard disks or other Block devices.

 

(5) Capacity difference:

 

Norflash occupies 1 ~ Most of the 16 MB flash memory market, while nandflash is only used in 8 ~ Among the MB products, this also shows that nor is mainly used in code storage media, and NAND is suitable for data storage.

 

(6) Reliability and Durability

 

-Life (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 having a block erasure cycle of 10 to 1, the typical NAND block size is eight times smaller than that of the nor device, each NAND memory block is deleted less frequently within a given period of time.

 

-Bit Switching

All flash devices are plagued by bit switching. If the bit is changed, you must use the error detection/error correction (EDC/ECC) algorithm. The problem of bit inversion is more common in nand flash memory. When using nand flash memory, the EDC/ECC algorithm should be used. Using NAND to store multimedia information is not fatal. 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

The Bad blocks in the NAND device are randomly distributed. The NAND device needs to initialize and scan the media to find the bad blocks and mark them as unavailable. In an existing device, a high failure rate may occur if this processing is not performed in a reliable way.

 

(7) ease of use

 

Nor-based flash memory can be used very directly. When using the NAND device, you must first write the driver to continue other operations. Writing information to a NAND device requires considerable skill, because designers must not write information to Bad blocks, which means virtual ing must be performed from beginning to end on the NAND device.

 

(8) software support

 

Running code on the nor device does not require any software support. When performing the same operation on the NAND device, the driver is usually needed, that is, the memory technology driver (MTD ), the NAND and nor devices require MTD for write and erase operations.

Fewer MTDS are required to use the nor device. Many vendors provide more advanced software for the nor device, including the trueffs driver of the M-system, this driver is used by windriversystem, Microsoft, qnxsoftwaresystem, 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.

 

(9) To use NAND Flash to store data and programs on the handheld computer, 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, you must first start the machine with a small nor flash, and load OS and other software from the NAND flash into the SDRAM for running.

 

 

 

How to enable nor and NAND Flash!

 

This article Reprinted from carefree storage Forum; original address: http://forum.51stor.net/read-htm-tid-3826.html

 

1. Nor Flash has an independent data bus and address bus, which can be accessed randomly like Ram. Therefore, it can run programs directly on nor flash, so nor flash can be used to start, execute in place is called xip.

 

2. The NAND Flash adopts the I/O mode to read data. It only has eight-bit data and the bus shared by the address. The read time series must be controlled by software, therefore, the address and Data Bus cannot be directly connected like nor flash, so random access as memory is not allowed, and xip cannot be used directly for startup.

 

The reason why a chip like at91sam9260 can be started on NAND is that when the 9260 internal ROM is started, it will check whether there is valid code in dataflash or nandflash, if yes, use the ROM driver to copy the code of the specified size to the internal sram0, and then jump to sram0 for execution.

 

In addition, you can burn the loader code in another ROM and directly run the loader in the Rom to copy the code from NAND to the SDRAM for startup.

 

In general, it is better to use nor flash if there is nor flash. xip can be used. Nor flash is suitable for code storage and xip, with fast reading speed. NAND is suitable for storing large amounts of data.