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Speed Matters

Storage at the Center: eMMC Memory and Its Place in Cutting-Edge Mobile Devices
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Why does eMMC’s architecture matter? Simply put, it’s usually faster to have memory and storage management in the CPU. Tom’s Hardware provided a glimpse of this last November when comparing3 the performance of three different tablet platforms. Reviewer Andrew Ku showed that the eMMC-based Motorola Xoom made USB file transfer to built-in storage at 13.18 MB/s. The same transfer test on the iPad 2, which uses a direct NAND interface and a CPU with integrated storage management, showed a transfer rate of 19.19 MB/s. The maximum bus speeds of the two approaches are 104 MB/s and 133 MB/s, respectively.

Fortunately, help is coming. In March of 2011, the Open NAND Flash Interface (ONFI) Working Group published its version 3.0 standard, based on the DDR2 interface. Two months later, Samsung revealed its own NAND specification called Toggle DDR2. Both designs target a maximum data rate of 400 MB/s and should be arriving in consumer devices soon. In practical use, Samsung expects Toggle DDR2-based eMMC chips to deliver sequential read speeds of up to 80 MB/s and sequential writes of up to 40 MB/s—roughly triple the throughput of today’s products. The difference in transfer times when, say, moving a movie from a PC to the mobile device, should be impressive.

This change in bus speeds illustrates another reason why it could prove advantageous to keep the controller in the memory chip. With innovation happening so quickly in the memory world, being dependent—arguably hamstrung—on an external controller in another component could hamper platform improvements. Keeping the controller in the eMMC with the NAND can lead to more flexible platform designs and ultimately faster time to market for the final mobile device. This way, device vendors are free to change their OS or chipset without having to requalify the NAND, because eMMC handles flash management independently from those elements. This simplifies the development and platform updating process.

While this article doesn’t aim to dive overly deep into technical matters, hardware enthusiasts may be interested in examining some of the fine differences in today’s fourth-generation eMMC specifications. Most devices using eMMC today adhere to the 4.41 specification.