Nor flash and NAND Flash

Nor flash and NAND Flash

Nor flash and NAND Flash are two major non-loss flash technologies on the market. Intel first developed nor Flash technology in 1988, which completely changed the original programmable read-only memory (EPROM) and EEPROM (electro-erasable read-only memory) in the world. Next, in 1989, Toshiba published the NAND Flash structure, emphasizing lower costs per bit, higher performance, and easy upgrade like a disk through interfaces. Nor flash 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. Nor transmission efficiency is very high, in 1 ~ The small size of 4 MB has a high cost efficiency, but the low write and erase speed greatly affects its performance. The structure of NAND can provide a very high unit density, which can reach a high storage density, and the write and erase speeds are also very fast. The difficulty of using NAND lies in Flash management and special system interfaces. Generally, the read speed of the nor statement is a little faster than that of the NAND statement, and the write speed of the NAND statement is much faster than that of the nor statement. You should consider these situations in the design.

Detailed description of nor and NAND

Although nor and NAND are widely used, there are still a considerable number of hardware engineers who cannot tell about nor and NAND Flash. For example, "Flash Memory" can often be used with "nor memory. Many insiders do not know the advantages of NAND Flash compared with nor. Because in most cases, flash memory is only used to store a small amount of code, nor flash memory is more suitable for some. Nand is an ideal solution for high data storage density. 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. Nor transmission efficiency is very high, in 1 ~ The small size of 4 MB has high cost efficiency, but the low write and erase speeds greatly affect its performance. The NAND structure provides a very high unit density, achieves a high storage density, and writes and erases quickly. The difficulty of using NAND lies in that flash management requires 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. Write operations on any flash device can only be performed in an empty or erased unit. In most cases, the flash device must be erased before writing. 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 0 before the erasure. Since the nor device is erased with 64 ~ For blocks of KB, the time for performing a write/erase operation is 5 S. In contrast, the erased NAND device is 8 ~ For 32 KB blocks, 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 for a given 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 weigh the following factors.
1) Nor reads faster than NAND.
2) nand writes much faster than nor.
3) nand's 4 Ms erasure speed is far faster than nor's 5s.
4) Most write operations require erasure first.
5) 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 every byte in 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. Nand read and write operations use 512 bytes of blocks, which is a bit like hard disk management. Naturally, NAND-based memory can replace hard disks or other Block devices.

Capacity and cost

The unit size of NAND Flash is almost half the size of the nor device. Because the production process is simpler, the NAND structure can provide a higher capacity within the given mold size, thus reducing 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 MB products, this also shows that nor is mainly used in code storage media. NAND is suitable for data storage, while NAND is in compactflash, secure digital, PC cards, and MMC (Multi Media card) the largest share of the memory card market.

Reliability and Durability

When Using Flash Media, reliability is a key issue to be considered. Flash is an ideal storage solution for systems that need to expand MTBF (mean time between failures. The reliability of nor and NAND can be compared in terms of life (durability), bit switching and bad block processing.
Lifetime (durability) in nand flash memory, the maximum number of writes per block is 1 million, and the number of nor writes 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. In some cases (rarely, Nand occurs more often than nor), a bit is reversed or reported to be reversed. One-bit changes may not be obvious, but if it occurs on a critical file, this small fault may cause system downtime. If there is only a report problem, it may be solved by reading multiple times. Of course, if this bit changes, you must use the error detection/error correction (EDC/ECC) algorithm. The problem of bit inversion is more common in nand flash memory. The NAND supplier recommends using 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 I 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 an existing device, a high failure rate may occur if this processing is not performed in a reliable way.

Easy to use

The nor-based flash memory can be used very directly, and can be connected like other memory, and code can be directly run on it. 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. 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.

Software Support

When discussing software support, we should distinguish basic read/write/erase operations from high-level software for disk simulation and flash 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 usually needed, that is, the memory technology driver (MTD ), memory technology devices (MTD) is required for writing and erasing NAND and nor devices ). 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 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.

The main vendors of NAND Flash are Samsung and Toshiba. flash is used in USB flash drives, various memory cards, and MP3 players. due to technological differences, NAND Flash has 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.