Comparison between norflash and nandflash

Flash Memory, also known as flash memory, has two main types: norflash and nandflash. We will give a comparison between them from multiple perspectives. In actual development, designers can make reasonable selection of Flash Memory Based on Product requirements.

1. Interface comparison

Norflash has a common SRAM interface, which can be easily attached to the CPU address and data bus. It has low requirements for CPU interfaces. Norflash 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. For example, the RO segment in uboot can be directly run on norflash. You only need to copy the RW and Zi segments to ram to run them.

The nandflash device uses a complex I/O port for serial data access, and eight pins are used to transmit control, address, and data information. Because the timing is complex, it is recommended that the CPU be integrated with the NAND controller. In addition, because nandflash is not attached to the address bus, if you want to use nandflash as the boot disk of the system, the CPU must have special functions, for example, when the nandflash startup mode is selected, the S3C2410 will automatically read 4 k Data from nandflash to the SRAM at address 0 during power-on. If the CPU does not have this special feature and you cannot directly run the code on nandflash, you can use other methods. For example, many nandflash development boards use nandflash, except nandflash, A small norflash is used to run the startup code.

2. Comparison of capacity and cost

Compared with nandflash, norflash has a smaller capacity, generally ranging from 1 ~ About 16 Mbyte, some new processes use the chip superposition technology to increase the norflash capacity. In terms of price, norflash is relatively higher than nandflash, for example, the current market for a 4 Mbyte am29lv320 norflash retail price is about 20 yuan, and a Mbyte k9f1g08 nandflash retail price is about 30 yuan.

The nandflash production process is simpler. The NAND structure can provide a higher capacity within the given mold size, thus reducing the price accordingly.

3. Reliability Comparison

Bad blocks in NAND devices are randomly distributed. They have previously been used to eliminate bad blocks. However, they are not cost-effective because the yield rate is too low and the cost is too high. The NAND device needs to initialize the media to detect 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. The bad block problem does not exist in norflash.
In the flash bit flip (a bit is flipped) phenomenon, the probability of NAND appears much greater than norflash. This problem is fatal when storing key files in flash. Therefore, we recommend that you use EDC/ECC and other validation algorithms when using nandflash.

4. Life comparison

In nand flash memory, the maximum number of writes to each block is 1 million, while that of nor is 100,000. The service life of flash memory is also related to the file system mechanism, requiring the file system to have the loss balancing function.

5. Upgrade comparison

The upgrade of norflash is troublesome because the address line of norflash varies with the capacity, so it is inconvenient to replace the norflash chip of different capacity. Usually we will solve this problem by making some jumper Resistors on the address line of the circuit board, for norflash of different capacities.

The nandflash interfaces of different capacities are fixed, so the upgrade is simple.

6. read/write Performance Comparison

Write operation: Any flash device write operation can only be performed in an empty or erased unit. It is very easy for the NAND device to perform the erasure operation, and nor requires that all the bits in the target block be written to 1 before the erasure. When the nor device is erased ~ For blocks of KB, the time for performing an erasure/write operation is about 5 s. The erased NAND device is composed of 8 ~ For 32 KB blocks, it takes up to 4 ms to perform an erasure/write operation.

Read operation: Nor reads faster than NAND.

7. File System Comparison

In Linux, MTD is used 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 a read-only file system. To implement read/write operations on flash, we usually select the jffs and jffs2 file systems on norflash, and select the yaffs or yaffs2 file systems on nandflash. The yaffs2 file system supports nandflash memory with large pages (more than 512 bytes/page.