What is on earth the difference between NAND Flash and NOR Flash? （2）
To overcome or reduce the limitation of the slower read speed, the data is usually read in the form of a page in NAND flash memory, which is the smaller portion of the erasing block. The contents of a page are read sequentially using the address and command cycles only at the beginning of each read cycle. Sequential access durations for NAND flash are generally lower than random access durations for NOR flash devices. When using the random access architecture of NOR flash, address lines need to be switched at each read cycle to accumulate random access for sequential reads. As the size of the data chunk to read increases, the cumulative latency in NOR flash becomes greater than that of NAND flash. As a result, sequential reads of NAND flash can be faster. However, the performance difference between the two types is apparent only when transmitting large data blocks that are typically larger than 1 KB because the initial read access duration of NAND flash is much longer.
In both Flash technologies, data can only be written to a block when it is empty. The slow erase operation of NOR flash makes its write operation slower. In NAND flash, similar to reading, data is usually written or programmed as a page (usually 2KB). For instance, using NAND flash S34ML04G2 alone requires 300μS to write to a page.
In order to speed up write operations, modern NOR Flashes also use buffer programming similar to page writing. For instance, NOR flash memory S70GL02GT mentioned above supports buffer programming, which enables it to write word-like timeout multibyte programming. For example, the buffer programming of 512-byte data can achieve the throughput capacity of 1.14MBps.
NOR flash memory usually requires more current than NAND flash memory during initial power-up while the standby current of NOR flash is much lower than that of NAND flash. The instantaneous active power of the two flash types is equal. Therefore, the effective power is determined by the duration of memory activity. NOR Flash has advantages in random reads, while NAND Flash consumes relatively little power in erase, write, and sequential reads
The reliability of storing data is an important performance index of any storage device. A flash memory may encounter a phenomenon called bit flip, some of which can be reversed. The phenomenon is more common in NAND flash memory than in NOR flash memory. For yield reasons, NAND flash goes bad as it attaches to scattered bad blocks, and more memory units go bad as the erasing and programming cycles continue throughout the life of NAND flash. Therefore, bad blocks processing is a mandatory function of NAND flash memory. For another, NOR flash memory has zero bad blocks and a very low accumulation of bad blocks during the service lifetime of memory. Therefore, NOR flash is superior to NAND flash when it comes to the reliability of storing data.
Another aspect of reliability is data retention, where NOR flash again dominates. For example, NOR flash S70GL02GT provides data retention for 20 years with up to 1K programming / erase cycle while NAND flash S34ML04G2 provides typical data retention for 10 years.
The number of programming and erase cycles was an important feature to consider. This is because NAND flash is used to provide 10 times better programming and erase cycles than NOR flash. As technology advances, this is no longer applicable because the performance of these two types of memory in this respect is already very close. For example, both S70GL02GT NOR and S34ML04G2 NAND support 100,000 programming-erase cycles. However, the whole lifetime of NAND flash memory is longer than that of NOR flash because the block size used in NAND flash memory is smaller and the smaller area is erased with each operation.
Table 1 provides a summary of the main points discussed in this article.
Table 1: The comparison of main features, general and specific comparison data between NOR flash and NAND flash
Usually, NOR flash memory is an ideal choice for applications that require lower capacity, faster random read access and higher data reliability for example, such as code execution requirements. NAND flash memory is ideal for applications that require higher memory capacity and faster write and erase operations, such as data storage and so on.