The principle of Flash programming is that only 1 can be written as 0, and 0 cannot be written as 1. Therefore, before the Flash programming, the corresponding block must be erased, and the erasing process is the process of writing all bits to 1, and all the bytes in the block become 0xFF. Therefore, it can be said that programming is the process of writing the corresponding bit to 0, and erasing is the process of writing the corresponding bit, and the execution of the two is completely opposite.
(1) The basic units of flash memory chip reading and writing are different
The application operates on the NorFlash chip in "words" as the basic unit. In order to facilitate the management of the large-capacity NorFlash flash memory, the NOR flash memory is usually divided into logical blocks of 128 KB or 64 KB in size, and sometimes the sectors are also divided into sectors. When reading and writing, you need to specify both the logical block number and the intra-block offset. The application's operation on the NandFlash chip is based on "blocks". The blocks of NAND flash are relatively small, usually 8KB, and then each block is divided into pages, and the page size is generally 512 bytes. To modify a byte in the NandFlash chip, the entire block of data must be rewritten.
(2) NorFlash flash memory is a random storage medium for occasions with small data volume; NandFlash flash memory is a continuous storage medium suitable for storing large data.
(3) Since the NorFlash address line and the data line are separated, the NorFlash chip can be connected to the data line like the SDRAM. The use of the NOR chip is similar to the usual memory chip, the transmission efficiency is high, and the executable program can be executed on the chip (XI P, eXecute In Place), so that the application can run directly in the flash memory without having to read the code to the system RAM. Because of this feature of NorFlash, the NOR chip is often used as a boot chip in embedded systems. NandFlash shares the address and data bus, and needs to connect some control input and output, so it is more difficult to directly use the NAND chip as the boot chip.
(4) The NandFlash flash memory chip does not allow data to be erased for one byte or even one block because of the shared address and data bus. Only a fixed-size area can be cleared. The NorFlash chip can operate on the word. So the speed of dealing with small data I/O operations is faster than the speed of NorFlash. For example, a NorFlash chip usually needs 10us to write one word, 1280us to write 512 bytes on a 32-bit bus, and 512 bytes to write 512 bytes in NandFlash memory, including: 512×50ns+10us search time +200us Wipe time = 234us.
(5) The capacity of NandFlash flash memory is relatively large, and the maximum capacity has reached 8G bytes. In order to facilitate management, NandFlash's storage space uses a block and page two-level storage system, which means that its storage space is two-dimensional, such as K9F5608UOA The size of the flash block is 16K, the size of each page is 512 bytes, and each page has 16 bytes of free area for storing error check code space (also called out-of-band, OOB space). When writing, the NandFlash flash puts one byte of data into the internal buffer area and then issues a "write command" to write. Since the operation of the NandFlash flash memory is in units of blocks and pages, NAND is faster than NOR flash memory when reading and writing large amounts of data to the NandFlash flash memory.
(6) Reliability
The reliability of NorFlash flash memory is higher than that of NandFlash flash memory. Because NorFlash flash memory has simple interface, less data operation and less bit swapping operation, it has high reliability and few bad blocks. It is generally used for high reliability. local. NandFlash type flash memory interface and operation are relatively complex, bit swapping operation is also a lot, the key data is the need for error detection / error correction (EDC / ECC) algorithm to ensure data integrity, so the probability of problems is much greater Bad blocks are also inevitable, and because bad blocks are randomly distributed, even error correction cannot be done.
(7) NANDFlash general address line and data line share, have a certain impact on read and write speed; NORFlash flash data line and address line are separated, relatively speaking, read and write speed is faster.
Commonality between NANDFlash and NORFlash chips
First of all, the performance of writing data to the chip must first clear the corresponding content in the chip, and then write, that is, wipe and write first. Only the NORFlash chip only erases one word, and NAND needs to erase the entire block. Second, the number of flash erases is limited. When the flash memory usage is near the end of its life, the write operation often fails; when it reaches the end of its life, the data stored in the flash memory can be read, but it can no longer be written. Therefore, in order to prevent the above problem from occurring, it is not possible to repeatedly write to a specific area. Generally, the number of NANDFlash rewritable is higher than that of the NORFlash chip. However, since NANDFlash is usually a whole block, if one of the pages in the block fails, the entire block will be invalid, and the probability of failure is relatively high due to the complicated erasing process. Therefore, the overall life of NOR is longer.
Another commonality is that flash read and write operations are more than just a physical operation. In fact, storing data on flash memory must be implemented using algorithms. This module is usually in the driver's MTD' (Memory Technology Drivers) module or in FTLZ (Flash Translation). Layer) implementation within the layer, the specific algorithm and chip manufacturers and chip models have a relationship. By comparison, NAND is more suitable for complex file applications, but due to the relatively complicated use of NAND chips, there are high requirements for file systems.
(8) Interface comparison
NorFlash has a general-purpose SRAM interface that can be easily attached to the CPU's address and data bus, and has low CPU interface requirements. NorFlash features on-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 run directly on NorFlash, just copy the rw and zi segments to RAM and run it.
NandFlash devices use complex I/O ports for serial access to data, and eight pins for control, address and data information. Because the timing is more complicated, the general CPU is better to integrate the NandFlash controller. In addition, since NandFlash is not attached to the address bus, if you want to use NandFlash as the system boot disk, you need the CPU to have special functions, such as s3c2410 is selected. When the NandFlash boot mode is used, it automatically reads the 4k data of NandFlash to the SRAM of address 0 at power-on. If the CPU does not have this special function, the user cannot directly run the code on NandFlash, which can take other methods, such as many The development board using NandFlash uses a small NorFlash to run the startup code in addition to NandFlash.
(9) Comparison of capacity and cost
Compared with NandFlash, the capacity of NorFlash is small, generally around 1~16MByte. Some new technologies use chip stacking technology to make NorFlash's capacity bigger. In terms of price, NorFlash is higher than NandFlash. For example, the retail price of a 4Mbyte AM29lv320 NorFlash on the market is around 20 yuan, while the retail price of a 128MByte k9f1g08 NandFlash is around 30 yuan. The NandFlash production process is much simpler, and the NAND structure can provide higher capacity within a given mold size, which in turn reduces the price.
(10)
The bad blocks in the NandFlash device are randomly distributed. Previously, there have been efforts to eliminate bad blocks, but it has been found that the yield is too low and the cost is too high, which is not cost-effective. Nand devices require an initial scan of the media to detect bad blocks and mark bad blocks as unavailable. In a fabricated device, if this process is not possible by a reliable method, it will result in a high failure rate. The bad block problem does not exist on NorFlash.
In the phenomenon of Flash bit flip (a bit flip), NAND has a much higher probability of occurrence than NorFlash. This problem is fatal when Flash stores critical files, so it is recommended to use EDC/ECC when using NandFlash. Check algorithm.
(11) Upgrade comparison
The upgrade of NorFlash is more troublesome, because the address line requirements of NorFlash with different capacities are different, so it is inconvenient to replace NorFlash chips of different capacities. Usually we will solve this problem by making some jumper resistance on the address line of the board, for different sizes of NorFlash. The interfaces of different capacities of NandFlash are fixed, so the upgrade is simple.
(12) read and write performance comparison
Write operation: Any flash device write operation can only be performed in an empty or erased unit.
It is quite simple for a NAND device to perform an erase operation, while NOR requires that all bits in the target block be written as 1 before erasing.
When the NOR device is erased, it is performed in blocks of 64 to 128 KB, and the time for performing an erase/write operation is about 5 s. The erase NAND device is implemented in blocks of 8 to 32 KB, and it takes only 4 ms to perform an erase/write operation.
Read operation: The read speed of NOR is slightly faster than NAND.
(13) File system comparison
MTD is used in Linux systems to manage different types of Flash chips, including NandFlash and NorFlash. Common file systems that support running on Flash are cramfs, jffs, jffs2, yaffs, yaffs2, and so on. The cramfs file system is a read-only file system. If you want to implement read and write operations on Flash, we usually select jffs and jffs2 file system on NorFlash, and use yaffs or yaffs2 file system on NandFlash. The Yaffs2 file system supports large pages (greater than 512 bytes/page) of NandFlash memory.
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