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Why flash storage is important to MEMS

Posted: 30 Apr 2013 ?? ?Print Version ?Bookmark and Share

Keywords:Flash storage? solid state drive? NAND-flash memory? MEMS?

Flash storage has evolved from a specialised solution for mobile devices with limited memory capacity to a standardised technology serving mobile, client and enterprise computing. The performance requirements imposed on flash storage are not trivial, and these requirements flow down to the electronic components used to assemble the device: CPUs, memory and clocks. The demands for small size, low power, high shock immunity and multi-million hour mean time between failures (MTBF) reliability are not simple to achieve, but are a good fit with today's MEMS clock generators capabilities.

In the 1990s solid state storage (SSS) technology was only used for specialised small-size storage applications such as digital cameras or USB data sticks. Today, the cost per gigabit of the technology continues to decrease to the point that SSS is now competing with and replacing hard disc drive (HDD) storage devices for both client and enterprise computing memory solutions.

SSS devices use nonvolatile memory as the storage media, which removes all moving parts, such as the magnetic drive that HDD storage has. SSS device architecture consists of an embedded processor controlling read, write, erase, encryption, and error detection functions on a network of memory blocks. The result is a memory storage unit that features faster access time, smaller size, lower power consumption, reduced cooling requirements, improved shock and vibration immunity and increased reliability.

SSS drives use NAND-flash memory technology to execute read/write functions to the storage media. The functionality of the NAND-flash memory is similar to the electronically erasable programmable read-only memory (EEPROM). The floating-gate transistor of the NAND-flash memory is the key component, as the floating gate minimises the layout requirements of the memory cell, allowing a significant density improvement over other configurations.

NAND-flash performs read/write functions at a block level, with the typical block size being 4 kb (4,096 bits). This key SSS metric is referred to as the program-erase cycle (P-E cycle) and is used to compare the operational life of varying memory cells. NAND-flash configurations include single-level cells (SLC) 1 bit of information per memory cell, and multi-level cells (MLC) 2, 3 or 4 bits per memory cell. Memory cells can be configured to either maximise memory density, or P-E Cycle lifetime, the two specifications have an inverse relationship. Typical SLC performance is > 100K P-E Cycles/memory cell, two level MLC or MLC-2 performance is > 3k to 10k P-E Cycles / memory cell. A third configuration Enterprise MLC (eMLC) is now being offered which has the MLC-2 density but an improved P-E Cycle performance > 20k to 30k/memory cell.

Solid state storage
The Solid State Storage Initiative (SSSI) has identified four major configurations:

???The solid state drive (SSD) is configured as a form, fit and function drop-in replacement for existing HDD units.
???The solid state card (SSC), a printed circuit that interfaces via a standard PCIe bus.
???The solid state module (SSM), which resides in dual-inline memory module (DIMM) or the small outline DIMM (SO-DIMM). DIMM may interface as a parallel ATA bus such as a compact flash device, or via a Serial ATA bus such as the C/Fast devices.
???Portable USB memory sticks. Devices are classed by the end user's requirements.

SSS technology's "no moving parts" architecture eliminates the fans that would typically be used for forced-air cooling. Without forced-air cooling, however, SSS devices operate at elevated temperatures. MEMS-based oscillators excel in this higher-temperature environment as the device features a compensated phase locked loop (PLL) architecture. The Discera MEMS device measures the internal temperature of the MEMS resonator and numerically compensates the PLL divider operation to maintain a constant, compensated frequency across a wide operating range, -40 ºC to +85ºC, or -55ºC to +125ºC if required. An additional feature of MEMS clock generators is low-power operation to minimise temperature rise within the SSS. MEMS-based clocks are available with power consumption from 200 mW down to 20 mW depending on output format configuration. The newest low-power devices that will operate at 2-mW consumption will be introduced to the market in 2013.

One specification that requires further explanation is the MTBF > 2M Hours. The specification does not mean that a single unit operates in excess of 228 years between failures, it refers to a population of >2 Million units has a failure rate Mechanical shock is detrimental to crystal resonators performance and failure rates. The acceleration causes a short-term shift to the crystal resonance frequency. With prolonged exposure to the crystal and the mounting structure, a permanent frequency shift to the resonator. High levels of shock can induce failure by damaging the mounting plates. In contrast MEMS resonators have a high immunity to mechanical shock, due to the very low mass of the MEMS resonator. The resonator portion of MEMS designed by Discera has a mass of 7.2 nano-grams. Due to the low mass, oscillators assembled with this resonator have been tested to shock accelerations of 10 kilo-G and exhibited Mobile SSS devices such as a USB data stick can be quite small and compact. Crystal oscillators become more expensive as the size of the resonator and the associated packaging is shrunk. MEMS devices use the world's smallest resonator, a 450?m x 450?m MEMS device mounted on a small CMOS ASIC device containing the oscillator electronics. Discera MEMS oscillators are available in plastic packages down to 2.5x2.0 mm, or in chip scale packages at 1.6 x 1.2 mm. The smaller packages do not impose reliability or performance penalty for MEMS devices as the identical die are to those used for all packages.

Flash storage shock and MTBF requirements may be the primary reason leading SSD and NVRAM card manufacturers are already designing with MEMS clock generators vs. crystal-based solutions. Additionally the small size and low cost are equally important for consumer products such as the USB data stick. The low power capability is an added advantage for any system without forced-air cooling. MEMS clock generators are a critical component making SSS technology viable.

About the author
Scott Griffith is the director of applications engineering at Discera, a MEMS-based oscillator and clock generator semiconductor manufacturer located in San Jose California. He brings 29 years of product development experience to the position including senior management positions at Rockwell Semiconductor, Conexant, WiSpry, Comarco and ASCOM, AG. He holds a BSEE from the University of Virginia.

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