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SMALL FORM FACTOR FORUM

The Future of Flash

COLIN MCCRACKEN

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What does a small form factor embedded system have in common with a smartphone, tablet, reader, Ultrabook, or even a data center? Not much. Or as little as possible, one might hope. These days, however, data and code storage technologies—namely, flash memory—are common across most of these device types thanks to the rapid flash advancements and commoditization over several years.

NAND flash has replaced NOR flash for most purposes, driven by cost deltas and bi-directional read/write usage far overshadowing read speed. Multi-level/layer cell technology, known as MLC, improves densities and costs with two (or more) bits per cell. Although shrinking geometries tend to challenge the write and erase times for floating gates, the industry chugs on unabated. Some OEMs favor single-level/layer cell technology (SLC) due to reliability concerns, but again the industry is very resilient and will improve designs where there is demand. Even if flash for consumers is deemed inadequate for the most discriminating embedded apps, there are suppliers who can go the extra mile.

What exactly does “embedded flash” mean? Historically, various disk-on-module solutions plugged into memory sockets on a board, or plugged into PCMCIA or CompactFlash connectors. These days, there are all sorts of standards with various parallel and serial interfaces as well, from SD/MMC to USB flash to 2.5” SATA SSDs that mechanically mount as replacements for rotating disk drives. Although most of these are known as consumer or enterprise devices, they are useful in embedded systems—for file transfers and booting OS images in development all the way to deployment as part of the systems.

While data center solutions range from stand-alone boxes with Ethernet connectors to large slot cards with PCIe interfaces that plug into standard backplanes for any rackmount chassis, embedded modules are chip-level or board-level in nature, optimized for size, weight, power and cost. Recently, single-chip ICs are even soldered on tiny CPU modules, although clearly these aren’t pluggable or removable. On the flip side, they are as rugged as soldered RAM vis-a-vis an SODIMM socketed RAM module. The chips have capacities that can hold even a large OS, application and Linux or Windows file system.

“Embedded flash” can also refer to customized hardware for applications that require very high security and data protection. Virtually any system connected to a network can be hacked, if not easily, at least theoretically by people intimately knowledgeable with the flash type being used. Flash modules usually come with a controller that is the unsung hero of the module. Inherent limitations of flash such as read/erase cycles and bad regions that cause bit errors are overcome by sophisticated wear-leveling algorithms that re-locate memory regions having data write traffic. Controllers even compress and decompress data on the fly. Each controller is optimized for a certain interface (hardware and protocol), whether serial / differential pair like USB 3.0 or SATA II, or slower and lower-power-consumption parallel SD card interface.

“Industrial flash” can mean a number of things at the module level, from wide temperature range or long lifecycle to SLC flash or even all three. In some ways, MLC and long lifecycle are pitted against each other. But you can expect to pay more, and rightfully so, for any of these industrial benefits since they go against the cost-focused grain of consumer media.

Where is it all going? The consumer and enterprise segments are on well-established trajectories. But the embedded market fosters additional degrees of freedom, which manufacturers can target for their various specializations. At the lowest level, firmware gurus can serve up nearly unbreakable security or wear-leveling that is optimized for even just one specific system. Some NRE money for the custom firmware tweaking is appropriate here. At the module level, we’ve only seen the tip of the iceberg in terms of new standards, since the fast bus interfaces are driving toward special signal integrity connector technology that hasn’t been used for solid state storage before. Small form factor modules and SBCs are hard-pressed for available connector space as it is, so new standards may need to combine other unrelated functionality to earn a seat at the SBC and COM (computer-on-module) table. This will take years to play out.

No matter which way flash evolves for consumer and embedded markets, together and differently, flash is here to stay and we will only become more addicted to it.