By Scott Beekman, Toshiba America Electronic Components, Inc.
Mobile Handset DesignLine (06/25/08, 12:28:00 PM EDT)
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
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.
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