NAND-NOR convergence trend

I see a new trend emerging in Flash designs. The designer has traditionally seen NAND and NOR as competitors. However, now there seems to be efforts at converging the two in the same design to get the best of both the worlds. In the traditional NAND vs NOR scenario, the designer carefully balances the advantages and disadvantages of both before selecting one. Say, for instance, his primary need is speed. NAND accesses each cell through adjacent cells while NOR allows for individual access to each cell. As a result, NOR can allow a code to execute directly from within a NOR chip, not requiring that code to be copied to RAM for execution, thus offering faster boot times. NOR has been widely used for low density environments needing fast reads. NAND grew from EEPROM specifically with the design aim of low cost per bit. Its availability in high densities with fast write access makes it ideal for storage. But complexities such as its pin configuration using data paths shared by both address information and data creates design problems in ensuring the signals are correctly delivered.

While it is common to use both NAND and NOR in a system design, the system suffers from the disadvantages of both while trying to balance the advantages of both. For instance, the system may have the limited density of NOR and poor life expectancy of NAND. If NAND reaches the end of its life, the entire system has to be discarded. An innovative though not very effective solution to the problem was using NAND controllers. Discarding NOR whenever possible and going for NAND controllers greatly reduce complexities of NAND by having software send programming and read information to a predefined set of registers as against software accessing NAND directly.

However, over the past many years both NAND and NOR have not seen any major technological evolution. It is in this context that I see a great Flash technological advance in the new technologies introduced in Samsung OneNAND and Spansion OrNAND. These products have leveraged the advantages of each technology and minimized their disadvantages in a single solution, thus beginning the era of NAND-NOR convergence. OneNAND is a single-die chip with a standard NOR Flash interface using a NAND Flash Array. OrNAND delivers the NAND interface with the reliability and fast read performance of NOR Flash memory.