Follow+Star Public Number, don’t miss wonderful content
Compiled by | strongerHuang
WeChat Official Account | Embedded Column
FLASH memory, also known as flash memory, is mainly divided into:NorFlash and NandFlash.In practical development, designers can make reasonable choices for flash memory based on product requirements. Below, we will compare and introduce from multiple perspectives.
Embedded Column
1
NorFlash has a universal SRAM interface, which can be easily connected to the CPU’s address and data bus, requiring low interface demands from the CPU.
NandFlash devices use complex I/O ports to serially access data, with 8 pins used to transmit control, address, and data information. Due to the more complex timing, it is generally best for the CPU to integrate a NAND controller. Additionally, since NandFlash is not connected to the address bus, if you want to use NandFlash as a system boot disk, the CPU needs to have special functions, such as the s3c2410 which automatically reads 4k data from NandFlash into address 0’s SRAM when selected for NandFlash boot mode upon power-up. If the CPU does not have this special function, users cannot directly run code on NandFlash. Other methods can be adopted, for example, many development boards using NandFlash also use a small NorFlash to run boot code.
Embedded Column
2
Compared to NandFlash, NorFlash has a smaller capacity, generally around 1~16MByte. Some new processes have adopted chip stacking technology to increase the capacity of NorFlash. In terms of price, NorFlash is generally higher than NandFlash, for instance, a 4Mbyte AM29lv320 NorFlash retails for around 20 yuan, while a 128MByte k9f1g08 NandFlash retails for around 30 yuan.
The production process of NandFlash is simpler, and the NAND structure can provide higher capacity within a given mold size, thereby reducing the price.
Embedded Column
3
Bad blocks in NAND devices are randomly distributed. Previous efforts to eliminate bad blocks found that the yield was too low and the cost too high, making it not worthwhile.NAND devices require an initialization scan of the medium to discover bad blocks and mark them as unusable.In manufactured devices, if this process cannot be reliably performed, it will lead to a high failure rate.However, the bad block issue does not exist in NorFlash.
Regarding the phenomenon of bit flipping (a bit changing state), the occurrence rate in NAND is much higher than in NorFlash. This issue is critical when storing important files in Flash, so it is recommended to use verification algorithms such as EDC/ECC when using NandFlash.
Embedded Column
4
In NAND flash memory, the maximum erase cycles for each block isone million times, while NOR can be erasedone hundred thousand times.The lifespan of flash memory is also related to the file system mechanism, which requires the file system to have wear leveling functionality.
Embedded Column
5
Upgrading NorFlash is more troublesome because different capacities of NorFlash require different address lines, making it inconvenient when replacing NorFlash chips of different capacities.Typically, we solve this issue by using jumper resistors on the address lines of the circuit board for different capacities of NorFlash.
In contrast, the interface of different capacities of NandFlash is fixed, making upgrades simple.
Embedded Column
6
Write Operation:Any flash device’s write operation can only be performed in empty or erased cells.Erasing NAND devices is very simple, whereas NOR requires all bits in the target block to be written as 1 before erasing.Erasing NOR devices is done in 64~128KB blocks, taking about 5 seconds for an erase/write operation.Erasing NAND devices is done in 8~32KB blocks, taking at most 4ms for an erase/write operation.
Read Operation:NOR’s read speed is faster than NAND.
Embedded Column
7
The Linux system uses MTD to manage different types of Flash chips, including NandFlash and NorFlash.Common file systems that support running on Flash include cramfs, jffs, jffs2, yaffs, and yaffs2.The cramfs file system is read-only.If you want to perform read and write operations on Flash, we typically choose jffs or jffs2 file systems for NorFlash and yaffs or yaffs2 file systems for NandFlash.The Yaffs2 file system supports large pages (greater than 512 bytes/page) for NandFlash memory.
Disclaimer:Some materials in this article are sourced from the internet, and copyright belongs to the original author. If there are copyright issues, please contact me for deletion.
Reply in the backend with ‘Embedded Software and Hardware Comprehensive Content’ to read more related articles.
Click ‘Read Original’ to see more shares, and feel free to share, save, like, and view.