Analyzing NAND flash memory in depth

Source:   Editor: Lucky Update Time :2019-2-5

For many consumer audio-video products, NAND flash memory is a better storage solution than a hard drive, especially for those applications whose capacity don't exceed 4GB. As people continue to pursue products with lower power consumption, less weigh and better performance, it is proved that NAND is extremely attractive.

The NAND flash memory array is divided into a series of 128kB blocks, which are the smallest erasable entities in the NAND device. Erasing a block is to set all bits to "1" (and all bytes to FFh). It is necessary to programmatically change the erased bits from "1" to "0". The smallest programming entity is a byte. Some NOR flash memory can perform read and write operations simultaneously (as shown in figure 1). Although NAND cannot carry read and write operations at the same time, it is able to use a method called shadowing to achieve the two operations at the system level. This approach, which has been used on personal computers for years, is to load the BIOS from a low-rate ROM into a comparatively high-rate RAM.

The rate of NAND is comparatively high, because there are no metal contacts in the string of NAND. The reason why the size of NAND flash memory units is smaller than NOR (4F2:10F2) is that each unit of NOR requires independent metal contacts. NAND, like hard disk drives, is based on sector(page), and is suitable for storing continuous data, such as images, audio, or personal PC data. Although random access can be achieved at the system level by mapping data to RAM, achieving this requires additional RAM storage space. In addition, like hard disks, NAND devices have bad sectors so that require error-correcting codes (ECC) to maintain the integrity of data.

The smaller area storage unit accounts for, the smaller area of the bare chip. In this case, NAND can provide flash memory products, which have larger storage capacity for current low-cost consumer market. NAND flash memory is used in almost all erasable memory cards. The multiplexing interface of NAND provides a similar pin output for all the latest devices and densities. This pin output allows design engineers to migrate it from smaller densities to larger densities without changing the hardware design of the circuit board.

A comparison between NAND and NOR flash memory

The advantage of NAND flash memory is the rate of write (programming) and erase operations is fast, and the advantage of NOR is the ability to achieve random access and carry out write (programming) operations on bytes (as shown in figure 2 below). NOR’s random access capacity allows it to directly execute code in place(XiPexecute in place), which is often required for embedded applications. The disadvantage of NAND is the low rate of random access, and it is limited by the low rate of reading and erasing. NAND is more suitable for storing files. Nowadays, more and more processors are equipped with direct NAND interfaces and can import data directly from NAND (without NOR).

The real benefits of NAND are a fast programming speed and a short erasing time. NAND supports the continuous write operations with