Electronic Design

Hybrid Flash Memory Gaining Traction For Handheld Applications

Smartphones continue to grow in popularity as their feature-rich environment steadily expands. Smartphone designs, which have evolved significantly over the past several years, are the result of an unprecedented amount of multi-functional convergence. The evolution continues as designers look at smarter memory solutions to resolve a number of important smartphone design issues.

A smartphone’s popularity is not only gauged by the sophistication of the operating system, but also by the number of third-party software applications. The ideal memory solution for a smartphone platform would support these features by providing the reliability needed for critical code and the storage capacity required for user data, all in one single memory device. Hybrid memory will increasingly be the solution designers look to because it combines the performance bandwidth of single-level cell (SLC) NAND flash memory with the cost-efficiency of multi-level cell (MLC) flash.

In the early days, the memory needs for mobile handheld devices were satisfied with a small amount of NOR flash memory for booting and code storage. Actually, this works well even today for many low-feature code-centric phones that do not need to handle user data such as mp3 files.

However, as multimedia devices were forced to incorporate more features over the years to meet strong consumer demand, designers moved to low-cost, SLC NAND flash memory either as an alternative or complement to NOR. Compared to NOR, SLC NAND is more cost-effective in addition to writing data faster. But MLC is significantly more cost-effective than either one. Hence, some phones now also allow users to augment phone storage with a removable MLC-NAND-based memory card if desired, but this still does not resolve the previously mentioned design issues.

Today most smartphone designers prefer to embed a large amount of memory. This is not just to support high-end cameras, higher-quality audio/video, PC-like functionality such as e-mail, and GPS capabilities, but mainly to encourage greater platform extensibility by third-party software developers. Smartphones currently have up to 32 Gbytes of embedded storage and that is estimated to increase to 128 Gbytes by 2011. This, combined with the constraints of increasingly limited board space due to the use of larger batteries, has placed considerable pressure on the industry to provide a single memory solution that provides the benefits of SLC and MLC NAND flash. And the industry is now responding with a hybrid solution.

Such hybrid memories are already available in the market today for high-end feature phones such as music phones and navigation-based phones (see figure). Designers using them have the flexibility to set a boundary that apportions a specific amount of the memory as SLC, while the rest is defined as MLC. For example, in a 4-gigabit hybrid device, the designer may choose to partition one gigabit of the memory as SLC for optimally storing critical code. The remaining area will be two gigabits of MLC, which will be used for data storage.

Industry-wide, the need for hybrid efficiencies has sparked a comprehensive standards effort. A hybrid solution for smartphones has been recently defined by JEDEC as the eMMC4.4 (embedded Multimedia Card) standard. In the last couple of years, eMMC devices, such as Samsung’s moviNAND and Toshiba’s eMMC NAND have been gaining considerable popularity as storage memory in smartphones. An eMMC device provides a simple interface using the industry-standard MMC protocol, and a controller that insulates the designer from NAND-flash technology changes with respect to ECC requirements, page-size and block-size changes, wear-leveling, and so on.

The previous eMMC, version 4.3, defined a power-on boot mode to allow quick booting from the eMMC device. Still, since eMMC devices typically incorporate MLC-based NAND chips, designers were not comfortable storing boot code and other critical code in them because of concerns that doing so could impact reliability and performance. The new eMMC4.4 standard allows the designer to set and operate memory partitions in an enhanced performance mode (SLC mode).

In an eMMC4.4 device, the designer can completely control how much density to configure in an enhanced (SLC) mode, and how much in MLC mode. The SLC area and MLC area of the device can be specified with sharply different reliability and performance numbers. For example, while the SLC area may offer an endurance specification of up to 50,000 write cycles, the MLC area may offer less than 10,000 cycles. Hence, the SLC area can be used to store code that requires more reliability and speed, or data that is frequently updated, and the MLC area can be used for user data and general read-only code like that for the OS, which does not require a high endurance number.

This flexible partition management scheme, which is a key feature in the new eMMC4.4 protocol, improves the overall cost-effectiveness of the system. Flexibly-partitioned or hybrid memory devices with a 64 Gbyte density or higher are expected to reach the market next year.

Smartphone designers should consider a single memory solution for all code, data, and storage needs. The processor will be saved the burden of implementing and validating various kinds of flash controller interfaces. And, with a hybrid SLC + MLC flash solution, the smartphone designer can gain the best in memory performance, reliability, and space utilization at a highly attractive price.

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