Employ the proper flash memory in your design

To NAND or NOR. That is the question. Different applications and functions should be handled with different types of flash memory.

In their search for the perfect "universal memory," designers of embedded systems are like characters from the play Waiting for Godot. They're waiting for an off-stage character, "Godot," to come on stage, meanwhile speculating which of the current or new characters appearing on stage are Godot.

From it's inception in the 1970s, the embedded systems industry as we know it as always been waiting for an off-stage semiconductor-based character named "universal memory" to come along and replace the memory hierarchy inherited from mainframes, minicomputers, and desktop computers: nonvolatile hard disk drives for long term mass storage and backup, dynamic RAM for local fast memory access, and SRAM and ROM for extremely fast access and code storage.

This desire has been heightened and intensified as computing as become more embedded, mobile, and portable and candidates for the role of "universal memory" have come on stage. Some--such as EEPROMS, EPROMs, UV-EPROMs, ferroelectric RAMs, and various pseudo-RAM combinations--have been turned down. Others such as magnetic RAM are being considered, but are in doubt for a number of economic and technical reasons.

However, some of the current characters on the stage--particularly the various flavors of NAND and NOR flash EPROMs--are being touted by their vendors as that offstage character "universal memory," or at least a relative and "close personal friend" including OneNAND, OrNAND, iNAND, GBNAND, moviNAND, ManagedNAND and NANDrive. Giving the diversity of actors trying out for the role, choosing the right memory subsystem is much more complicated now, especially if you're adding more multimedia functions to mobile and embedded systems while shrinking physical size and reducing overall system cost. Not only could the code and data storage needs have increased in these systems, but you've got to do it all more reliably with less of everything.

Flash memory is the most practical solution, but knowing which type of flash fits best in a system is the key. Is NAND, NOR, managed NAND, or some hybrid the best choice?

The use of NAND flash, an inexpensive and high-density nonvolatile memory requiring defect management, to satisfy these growing code and data storage needs makes the memory subsystem even more complex. Add to that the need to support different memory types, interfaces, vendors, and vendor-specific features, and the memory subsystem even more complex.

A completely managed memory subsystem solution can be designed that uses an industry-standard RAM (PSRAM or SDR/DDR SDRAM) interface. This managed memory subsystem would provide seamless integration with the host chipset/processor and eliminate the need for the host system to manage the complexity and deficiency of built-in memory devices.

Unlike NAND flash, NOR flash is one of the oldest and the most widely used memory types in current embedded systems. It's used for both code and data storage. Its main advantage is that the code is executed directly (execute-in-place) from the NOR flash memory. Also, NOR flash can directly interface with the host processor, which enables easy design-in and fast time-to-market.

With the increased deployment of multimedia functions in embedded systems, the need for code and data storage is also increasing. For these applications, using higher density NOR flash for code and data storage becomes more expensive when compared with alternative solutions like NAND flash. In addition, the highest density NOR flash available today is 1 Gbit. Moreover, multimedia data storage requires both high read and write performance. As a result, system designers have turned to NAND flash for storing multimedia files as well as application code in many embedded applications, including high-end cell phones.