NAND flash benefits multimedia handsets
Within mobile phones, the use of NAND flash for improved data storage has grown in three areas:
- Multichip packages (MCP) and package-on-package (PoP);
- High-density embedded storage in addition to MCPs/PoPs;
- Card slots in handsets.
The early MCP solution for basic talk-only mobile phones was low-density RAM + NOR. This solution is still widely used today, driven primarily by low-end cell phones targeting emerging markets. Multimedia phones, however, have adopted NAND-based MCP solutions because of the density, cost and write performance advantages of NAND flash.
One of these NAND-based MCP solutions is simply to add NAND flash for data storage to a traditional NOR-based MCP. An example would be pseudostatic SRAM (PSRAM) + NOR + NAND, where code is executed directly out of the NOR; NAND is used for data storage; and PSRAM is for working memory.
Another NAND-based MCP solution is to replace NOR altogether with DRAM + NAND (in which the DRAM is low-power SDRAM). In this case, both code and data are stored in the NAND flash. When the mobile phone is turned on, the code is then shadowed (copied) from NAND to DRAM, and thus executed out of the DRAM. On one hand, the trade-off takes additional boot up time when the phone is turned on. On the other hand, there are multiple advantages related to cost and simplification, and DRAM is excellent at fast code execution. This solution eliminates the need for NOR, which is more expensive per bit than NAND. Also, NAND flash and DRAM have to their advantage economies of scale as the two most widely used memory technologies. For these reasons, DRAM + NAND combinations are the fastest growing of the NAND-based MCP solutions.
DRAM + NAND demand
Toshiba forecasts that over 50 per cent of MCPs worldwide will incorporate NAND flash in 2008, up from roughly one-third in 2006. DRAM + NAND MCPs are projected to account for two-thirds of the NAND-based MCP demand by 2011.
PoP technology is essentially an MCP stacked on top of a processor to save board space. This requires that the top memory PoP be compatible with the bottom processor. The memory alternatives described for MCP also apply to PoP. However, because of potentially tighter height restrictions, the maximum memory density that can be supported within PoP is typically less than that of MCP.
For many of today's multimedia phones and smart phones, storage densities supported within MCP are not enough. In addition to MCP, a separate high-density NAND storage device is also embedded within the phone. While MCPs typically incorporate single-level cell (SLC) NAND densities ranging from 512Mbit to 4Gbit, the additional embedded storage chips support NAND densities ranging from 1GB to 16GB today using multi-level cell (MLC) NAND, which stores two or more bits per cell.
For these embedded storage chips, there are a variety of memory interfaces supported in the market, but these solutions all follow a basic concept. MLC NAND die are used to achieve large density storage, cost-effectively, in a small space. A controller is then incorporated with the MLC NAND device to manage error-code correction (ECC), wear leveling and bad block management requirements of the MLC NAND die, relieving the host processor of this task.
The interface is that of a standard SLC NAND, even though the device is using MLC NAND, thus simplifying design.
Other popular embedded storage interfaces include a high-speed MMC interface, or an SD interface, which has been widely used to support removable SD cards.
The embedded storage concept can also be incorporated within MCP for a one-chip solution, rather than two separate devices, although the maximum density supported is less than that of the two-chip solution. There are two basic approaches. The first solution is to simply add MLC NAND + controller to the die of an MCP. Typically, up to 2GB of storage can be incorporated within the MCP one-chip approach today. The second solution is to use a NAND die + controller, where the NAND die can be partitioned between SLC NAND and MLC NAND.
This approach is used in Toshiba's mobileLBA NAND. The SLC NAND die area is used for storage of application programs that require high reliability and performance, and the MLC NAND die area for data storage, all in one chip. The flexibility of this approach to partition SLC and MLC on the same die means that a separate SLC NAND die is not required. This solution can be cost-effectively implemented not only for MCP with large density storage, but also as a replacement to traditional lower density MCP using SLC NAND.
One misconception is that there will be a total system cost savings if the host processor takes the task of managing the MLC NAND die, rather than using integrated flash + controller solutions from memory suppliers who essentially have this task. Why pay for the added controller incorporated with the memory?
The challenge is that the complexity of managing the MLC NAND die is increasing with each generation of die shrinks. This is not simply a matter of the ECC requirements increasing, but of rising complexity of NAND flash with successive design rule shrinks to smaller lithographies. Each supplier's MLC NAND has its own unique characteristics, and each supplier can optimise its own controller to harness the best performance out of their own MLC NAND die. With the speed how NAND die shrinks are occurring, the developer of a host processor would have a great challenge to keep up with each suppliers' shrink and with optimised performance.
Essentially, customers are faced with the option of purchasing a host processor that can manage MLC NAND directly. But they would likely be stuck purchasing the prior generation's MLC NAND die since that would be what the host controller could support.
Generally, this means higher cost and lesser performance than simply purchasing from memory suppliers with their own controller-optimised MLC NAND solutions for the current generation MLC NAND die.
Another NAND flash growth area and one of the largest in terms of bit growth within mobile phones, is the use of card slots to enable additional removable storage. About 25 per cent to 30 per cent of handsets had card slots in 2006, over 50 per cent are expected to include them in 2008, and projections reach 70 per cent to 75 per cent by 2010. MicroSD is the most popular format so far, and currently up to 8GB is supported in the market.
Today's multimedia phones require the density, cost and write performance advantages that NAND flash offers. Because of these, NAND flash has become mainstream within mobile phones, including MCPs, through the growing use of additional high-density embedded storage chips, and growth of card slots.
The use of MLC NAND as opposed to SLC NAND is already pervasive within high density embedded storage solutions such as those supporting an LBA, MMC or SD interface, or within microSD cards. So too, will MCPs start to incorporate more MLC NAND as the storage density requirements within MCPs continue to increase. Managing the MLC NAND will fall upon memory suppliers to achieve optimum performance with timely introduction to coincide with the latest MLC NAND die shrink technology.
|Related Articles||Editor's Choice|
|Related Articles||Editor's Choice|