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 minimize 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 < 1 unit in an hour of operation. Focusing only on the clock source for this SSD assembly identifies a potential issue. As an example, the mean time to failure (MTTF) for a common crystal oscillators exhibit a failure rate higher than the MTBF of the SSD. Common crystal oscillators from Vecton and FOX show MTTF rates of 1.6M hours and 1.75M hours respectively at an operating temperature of +85ºC. Fortunately, MEMS oscillators are available from vendors such as Discera that have MTTF rates in excess of 5.6M hours at 85ºC.
More about MEMS-based oscillators
In "An overview of MEMS based timing devices," John Williamson, contributing editor on the MEMS Investor Journal writes:
"MEMS based oscillators are increasingly being used as an alternative to quartz in a growing number of electronic systems. According to our estimates, the global market for MEMS oscillators was $21.4 million in 2010 and is expected to reach $312 million by 2014, with consumer products representing nearly half of the market. The gains are based on manufacturers' successful efforts to overcome earlier limitations of MEMS based oscillators in specific market segments while directly targeting the drawbacks of quartz based devices. . . . Companies such as Discera, Sand 9, Silicon Labs, SiTime, and Vectron are addressing these issues by providing MEMS based alternatives to quartz oscillators in high-volume applications such as consumer electronics, telecommunications and automotive."
Read the full article here.
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 <2 ppm frequency shift.
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 µm x 450 µm 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.
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.