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Breakthrough Silicon Oscillator Eyes Mobile Market

Manchester, U.K.: A British startup is, perhaps ambitiously, targeting the mobile phone handset market with crystal-replacement technology built entirely on a standard 180-nm CMOS process.

The first device from eoSemi, a 32-kHz timing reference circuit, draws just 8 µA of supply current in active mode and delivers accuracy down to ±30 ppm, complying with requirements from handset manufacturers (see the figure).

The first device from eoSemi, a 32-kHz timing reference circuit, draws just 8 µA of supply current in active mode and delivers accuracy down to ±30 ppm, complying with requirements from handset manufacturers.

Breakthroughs in battery and display technology have driven explosive growth in portable consumer devices, but crystals have remained largely unchanged since they were invented in 1917, said Ian Macbeth, CEO of eoSemi.

Crystals, which provide essential frequency references to ICs, represent the last mechanical part in an otherwise electronic circuit. So, finding a silicon replacement for them is therefore extremely desirable. An all-silicon frequency device would be physically smaller, potentially even die stacked, and would eliminate the PCB-track exclusion zone under a crystal.

“Crystals are a bit of a dinosaur inside a modern mobile phone,” Macbeth said. “There is significant customer pull to remove quartz for cost and size reasons.... Most tier-1 semiconductor companies have teams to try and do this [in silicon] and most have failed.”

“One mobile handset company has had a spec out for a silicon oscillator for the last seven years,” adds Steve Cliffe, vice president of sales and marketing at eoSemi. “It has to run off microamps, not milliamps, be accurate to less than 100 ppm, and have less than a picosecond of jitter. We are the first to meet this spec.”

While making an oscillator from silicon is a low-cost process that has the potential for integration, the material does not perform well over temperature and stress.

The company’s Accurate Timing Oscillator Circuit (ATOC) uses proprietary techniques to sense and compensate for changes in temperature as well as physical stress within the device from, for example, printed-circuit board (PCB) flexure. The all-silicon oscillator circuit can be tuned instantaneously in real time. Plus, it compensates for drift over the lifetime of the device.

The company says that though the chip intellectual property (IP) is important, a great deal of the company’s IP lies in how to calibrate the individual devices, a secret process that will apparently prevent the chips from being reverse engineered.

After testing each individual device, a unique 18-bit calibration code is produced and written to a nonvolatile memory inside the device. Then in the field, data from stress and temperature sensors on the die recalibrates the oscillator periodically.

The period can be set between 0.25 and 30 seconds and can be adjusted when a big temperature gradient is experienced, for example, when receiving a call.

The power supply current quoted is based on a 4-second recalibration period. The company’s oscillators will be calibrated as standard across the –40°C to 85°C range, but Cliffe said that devices could be calibrated for higher temperatures in the future.

Breaking into the handset market is notoriously difficult for small and medium enterprises. Yet eoSemi says it has developed a partnership strategy, whereby established partner companies will give major customers access to eoSemi’s technology through trusted supply channels.

“Tier-1 partners will be taking it to market on our behalf,” Cliffe says. “We are not on approved vendor lists. [Our partners] will manufacture it themselves and deliver it as their product.” However, Cliffe did say that the company is also making its own chips to build a reference design flow.

Commercial production, in a 1.5- by 1.5-mm chip-scale package, will begin in the third quarter of 2012.


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