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What's the best flash memory flavor for your design?

Posted: 16 May 2008 ?? ?Print Version ?Bookmark and Share

Keywords:NAND NOR flash? universal memory? flash for embedded systems?

The embedded systems industry has been waiting for "universal memory" to replace the memory hierarchy inherited from mainframes, minicomputers and desktop PCs. This desire has been heightened and intensified as computing has become more embedded, mobile and portable. Candidates for the role of "universal memory" have come on stage. Somesuch as EEPROMS, EPROMs, UV-EPROMs, ferroelectric RAMs and various pseudo-RAM combinationshave been turned down. Others such as magnetic RAM are being considered, but are in doubt for economic and technical reasons.

However, the various flavors of NAND and NOR flash EPROMs are being touted by their vendors as the "universal memory" or at least a relative, including OneNAND, OrNAND, iNAND, GBNAND, moviNAND, ManagedNAND and NANDrive.

Given the diverse options, 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 have to do it all more reliably with less of everything.

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

The use of NAND flash to satisfy growing code and data storage needs makes the memory subsystem more complex.

A completely managed memory subsystem solution that uses an industry-standard RAM interface can be designed. 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.

NOR flash is used for both code and data storage. Its main advantage is that the code is executed directly from the NOR flash memory. Also, NOR flash can directly interface with the host processor.

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 compared with alternative solutions like NAND flash.

NAND flash is well suited for applications requiring large code and data storage because it is inexpensive and is also available in high densities. Unlike NOR, NAND flash doesn't support execute in place (XIP) or random access. As a result, some systems that use NAND flash need a low-density NOR flash just for system boot-up and BIOS code execution. Although NAND flash is inexpensive and available in higher densities than NOR, NAND is less reliable and requires defect management, including error detection and correction, and wear-leveling to make it usable for many applications. These NAND flash management functions require complicated hardware and software.

Managed NAND
Due to the limitations of the embedded NAND controllers, many system designers are looking at managed NAND solutions. Managed NAND memory products (including iNAND, GBNAND, moviNAND, Managed NAND and NANDrive) are used mainly for data storage. They reduce system complexity by effectively managing the built-in NAND flash with a NAND controller and flash file system integrated into the same device. Using a managed NAND device eliminates the need for a complex NAND management function on the host. As a result, chipset vendors don't need to worry about coping with the evolving NAND technologies.

Hybrid boots
Because managed NAND flash doesn't provide boot capability, system designers must use a higher-cost NOR flash device for boot-up. However, hybrid productsincluding mDOC H3are now available. These hybrids use RAM and managed NAND in one device to simplify the conventional memory subsystem.

Hybrid products solve the boot issue associated with the managed NAND. They can boot directly from the NAND flash, eliminating the need for a higher-cost boot NOR flash device, which may reduce the overall system cost. Managed NAND hybrids also help reduce component count and save board space.

(a)

(b)

The All-in-OneMemory consists of a memory controller with built-in boot NOR flash, NAND flash and RAM in one package (a). The controller handles on-demand paging and other memory management functions (b).

On the down side, NAND hybrids have a longer boot time because they must copy the boot code from the NAND into the boot RAM after power-on. Also, NAND hybrids are complex, difficult to integrate and require an advanced OS that supports demand paging on the host.

Next-generation subsystem
Managed memory subsystem products are available today, offering all the features and benefits mentioned. One such configuration is shown in the figure. It consists of a memory controller with built-in boot NOR flash, NAND flash and RAM all in one package.

Using the RAM cache in front of the NAND flash, the controller handles on-demand paging and other memory management functions.

- Vijay Devadiga
Senior Staff Product Marketing Engineer
NAND and Smartcard Module Business
Silicon Storage Technology Inc.





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