Non-Volatile Memory: NAND, NOR and Beyond

Peter Sommerfeld
Design Engineer
Nuvation
The plethora of choices for non-volatile memory in new designs means that the various aspects of these technologies should be investigated from the point of view of cost, features, power, and performance before deciding on a given device. For large non-volatile memories, NAND and/or NOR flash are typically used in current designs. There a major functional differences between NAND and NOR which must be understood in selecting the appropriate memory devices. There are also numerous design tricks to better enhance the performance and reliability of the system. Finally, there are new technologies emerging which will also be explored. This article will reference Nuvation’s IP Camera as an illustration with requirements for small form factor, high performance, reliability, easy of use, and low cost.


NAND vs. NOR

The table below shows the major functional differences in NAND and NOR memory.

	NAND	NOR
Cost (100 kU)	< $0.01/Mb	$0.06 - $0.15/Mb
Read Speed	~ 15-25 MB/s	~ 50-100 MB/s
Write Speed	~ 2-8 MB/s	~ 0.15 MB/s
Access Method	Block-based serial	Random
Block size	16 kB-128 kB kB	128 kB
Reliability	1,000,000 cycles	100,000 cycles
Table 1: Major Functional Comparison of NAND vs NOR memory

Cost-wise, NAND has an advantage over NOR because the cell size is inherently smaller. NAND is also scaling to ever-smaller densities. It was believed that the lithography cut-off would be around 45 nm, but Micron has devised 25 nm in the lab (EE Times, Feb 9, 2007). In addition to cost advantages, NAND also has roughly an order of magnitude greater reliability than NOR. NOR has advantages in that it is inherently XIP (eXecute In Place) and is very fast to read. The XIP property means code can be run directly from the device, whereas with NAND must be accessed serially and in a block fashion.

As can be seen in the following chart, NAND sales are eclipsing NOR, with the NOR market barely expected to grow. Part of the momentum is the massive depreciation in NAND pricing. Due to competition and aggressive die shrinks by the big three (Samsung, Micron, and Hynix) and an over-supplied market, prices may drop as much as 65% on 2007, however due to better supply/demand management, pricing has recently stabilized.

Figure 1: Sales of NAND vs NOR Source: EE Times

While it would be nice to have the random access properties of NOR in Nuvation’s IP Camera, the low cost and high densities available in NAND products were some of the deciding factors to using this flash technology.

Practical Firmware Issues

In practice, flash (both NAND and NOR) has some peculiarities that must be dealt with effectively in software. Like a hard disk, flash can come with bad blocks from factory, or bad blocks can develop over time since the cells wear out over many write cycles. For this reason, some form of BBTs (Bad Block Tables) must be generated and managed for the life of the device. In MMC, SD, USB mass storage and similar form factors, a controller chip inside the unit performs this bad block management. On the IP Camera, a bare NAND chip is used and the filesystem software must effectively perform this duty.

Specially designed filesystems must be used on flash, and these have been in use for some time now. With the IP Camera running Linux, JFFS2 (Journalled Flash File System ver. 2) is an excellent choice. Besides bad block management, it provides features such as wear leveling, journaling, and compression. Wear leveling moves blocks around in the background to approximate all blocks on the device being accessed the same amount, so that one block doesn’t wear out significantly earlier than any other. Compression makes better use of this often precious resource, transparently compressing data transfers to/from flash. Journaling provides for a safer filesystem in that modification of existing data is written to a new part of the flash, and the old copy is marked invalid. With this approach, power failure during a write does not cause catastrophic failure because the old copy is still available, which is critical in an embedded design such as the IP Camera.

Newer NAND and NOR devices

To bridge the gap between disparate architectures, form factors, and interfaces, the industry continues to push new devices. Of interest are products like OneNAND from Samsung, which provides a NOR-like execute-in-place structure, NAND-like write speeds, NOR-like read speeds, and an SRAM cache, providing for best-in-class performance from various technologies. For the IP Camera, we determined that the read performance was more than sufficient with a NAND flash, and that XIP could be handled by the small ROM on the TI DaVinci DSP, so OneNAND was not providing significant benefit in this application.

On the Horizon

Semiconductor manufacturers have always been looking for a single unified memory technology, a Holy Grail of sorts, which can supplant a combination of other memory technologies (both volatile and non-volatile). It appears there are at least two contenders: PRAM (phase-change memory) and MRAM (magnetoresistive RAM).

Phase-change memory is gathering steam now. This technology works by heating and cooling chalcogenide materials between amorphous and crystalline states. The operation and materials are similar to what is used in optical disks, but on a much smaller scale. Active players in this market include Intel, IBM, STMicroelectronics, and Ovonyx. Phase-change has cost advantages over DRAM and flash by virtue of its simpler manufacturing process and smaller cell size, and has vastly greater read/write reliability over flash. Its inherent rad-hard property also makes it suitable for space applications, whereas flash storage can be affected by radiation. Intel is expecting volume production of its first 128 Mb parts in Q2 2007 and Samsung has 512 Mb prototypes.

MRAM stores information in magnetic storage elements. It offers an SRAM-like interface, with read and write speeds comparable to SRAM, and random access capability. It uses much less power than DRAM, in part because no refreshing is required, and less power than flash because no charge pump is required for writes. The limitations for MRAM include scalability, with methods currently not scaling beyond 90nm, whereas flash is heading towards half that. Freescale is a major player in MRAM, with a 4 Mb part in full production. Unfortunately, at the present time MRAM is still costly and only produced in low density version.


Conclusion

The non-volatile memory market is an interesting mosaic of competing technologies with a delicate balance between features, performance, and cost. Whatever your non-volatile memory needs, Nuvation will remain on the leading edge, continuing to provide the highest performance, lowest cost, and best TTM system solutions for you. After looking at the available options for our IP Camera, we chose NAND for non-volatile memory and used the built-in ‘execute-in-place’ memory inside the DaVinci DSP to allow the shadowing of NAND bootloaders and OS into DDR2, thereby avoiding the necessity for an additional NOR flash. The read/write bandwidth is sufficient for the application, the NAND footprint is manageable in the IP Camera’s very compact footprint, and it even allows future upgradeability to higher densities.

For more information please contact Nuvation at sales@nuvation.com.


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