Higher resolution cameras, music, video and games are all driving the need for higher density, low cost data storage. NAND flash memory is excellent for high-density, low-cost and fast write speed data storage. As a result, the adoption of NAND in mobile phones continues to grow at an accelerated pace.
Within mobile phones, the use of NAND flash for improved data storage has grown in three areas:
- Multi-chip Packages (MCP) and Package-On-Package (POP)
- High-density embedded storage in addition to MCPs/POPs
- Card slots in handsets
Multi-chip packages
The historical MCP solution for basic talk-only mobile phones was low-density random access memory (RAM) + NOR. This solution continues to be used today, driven primarily by low-end mobile phones targeting emerging markets. Multimedia phones, however, have adopted NAND based MCP solutions due to 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 Pseudo SRAM (PSRAM) + NOR + NAND, where code is executed directly out of the NOR, NAND is used for data storage, and PSRAM 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. The tradeoff is that this 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.
As reflected in the following figure, Toshiba forecasts that over 50 percent 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.
Figure 1. NAND MCP growth within mobile phones
Package-On-Package
Package-On-Package (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, due to potentially tighter height restrictions, the maximum memory density that can be supported within POP is typically less than that of MCP.
High-density embedded storage
For many of today's multimedia phones and smartphones, storage densities supported within MCP are not enough. Thus, in addition to MCP, a separate high-density NAND storage device is also embedded within the phone. Whereas MCPs typically incorporate Single Level Cell (SLC) NAND densities ranging from 512 megabit (Mb)1 to 4 gigabit (Gb)2, the additional embedded storage chips support NAND densities ranging from 1 gigabyte (GB)3 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 as reflected in the following figure. 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.
In the figure below, 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 MultiMediaCard interface (HS-MMC) interface, or an SD interface, which has been widely used to support removable SD cards.
