The Age Of No-Spin Doctors

NAND flash is replacing DRAM, but it’s hardly a new technology when it comes to storage; NOR continues to grow.

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By Pallab Chatterjee
Solid-state flash memory still isn’t cheap, but performance, reliability and power have transformed it from a niche market into a mainstream one. And it’s about to get even more popular.

At the recent flash memory summit, the majority of the sessions focused on the further penetration of NAND flash into the consumer electronics product segment. NAND technology already has become the process technology driver for semiconductors, replacing DRAM after some 30 years. Current NAND technologies have moved from single-bit cells (SLC) to double-cell (MLC) and now triple-cell (TLC) architectures. The advancements in these architectures and the process geometries are driving the base densities of flash memory in mobile product up from 4Gb to 8G to now 16G and 32G as nominal configurations. The price differential is still high on a per-bit basis to compete with rotating media for the 256G and higher applications in the consumer marketplace.

The NAND Flash technology is being utilized with three primary interfaces: eMMC, SATA and PATA. The PATA interface is the original parallel interface for the ATA disk drives, and the format is supported in SSDs for a retrofit basis. Most industrial systems that included storage (rotating media-based) since the mid ’80s had PATA interfaces until the wide availability of lower-cost SATA went mainstream in the early ’00s. The interface is supported by standard form factors with a wide cable connect, a mini cable and slot connector, and in small-space applications by a BGA format connection. This allows for industrial design change, while keeping the integrity of the current design architectures and operation.

eMMC is the preferred embedded solution for new mobile designs. The architecture of memory, controller and package in a single unit works well. Products like Sandisk’s iNAND are driving the cusp of the cost vs. performance curve, along with the performance metrics of the light vs. heavy multi-tasking application tradeoffs. These range from the standard device (iNAND) at 15MB Read/5MB Write, the iNAND Ultra which is a mid-performance Android phone targeted product at 40MB R/20MB W, and the iNAND Extreme which is a high-performance tablet and gaming technology targeted product at 80MB R/50MB W. The eMMC interface is designed for internal memory use, and does not support user changeable memory (no memory card version).

eMMC designs support using the memory as both data store and cache applications. The control supports ECC and is improving the BER. The maximum densities are with these products, which are on their fourth generation and showing 3 bits/cell resulting in 64GB die. The interface is power-aware, so the application has to take care of the total power cycle used, the partitioning, and the wear leveling characteristics of the memory. The interface is designed to resolve multiple host request streams, so it is well optimized for multitasking as is needed for gaming and multi-core processing from tablets. This feature, however, adds some overhead to single-core, single-tasked applications.

On the standard drive interface side, SATA is the dominant interface. It allows for easy board-level development and substitution into the correct form factor for the final product. The NAND flash used is currently both SLC and MLC depending on application. Standard SSDs use a cable or edge connector based form factor. There is a new SATA approved uSATA interface, which standardizes the BGA pinout for single-chip embedded SATA SSD function. Sandisk introduced an iSSD product in this defining format. These are high-power and high-speed interfaces and support up through SATA3.0 (6.0Gb/sec) throughput. These parts support high temperature operation due to the package dimensions (1.2mm thick for 32GB to 1.85mm thick for 128GB), being compatible with tablets. The products can operate in low-power mode down to 10mW if properly designed in.

While NAND flash dominates the market at high densities, NOR flash is still growing. NOR is the dominant system memory in sizes under 1GB, for STB, FPTV, software-controlled radio, automotive and auto-tainment systems, DVRs, avionics, and most industrial control embedded designs.