Ultra-low power followed by ultra-small size are the two most important aspects of microelectromechanical system (MEMS) sensors used in wearables. MEMS manufacturer mCube Inc. (San Jose, California) discovered this in its research and has set out to meet that not-so-tiny goal. Since mCube already had the industry's smallest MEMS+CMOS die at 1-by-1 millimeters — allowing room for larger batteries in wearables — they decided to concentrate on technologies that would help extend the battery life of fitness, health monitoring and activity tracking wearables from hours or days, to weeks or months, according to chief executive officer (CEO) of mCube, Ben Lee.
“In a nutshell we already make the world's smallest accelerometer — a 1-by-1 die in a 2-by-2 package,” Lee told EE Times in advance of their wearables announcement. “And we wanted to be the first to design an accelerometer specifically for the wearables market, so we redesigned our ASIC to fulfill their needs.”
As an example, Lee described how their wearable MEMS chips saves power for a Bluetooth headset. When you set it down it goes into “sniff” activity-mode, which consumes only 0.6 microAmps by only sampling the accelerometer often enough to sense it being picked back up. When the user does pick it back up, mCube's redesigned ASIC is fast enough to turn the headset back on and pair before the user even has it in their ear, according to Lee.
In fact, mCube has designed into its new ASIC three low-power modes for different functions — sniff at 0.6 microAmps, single-sample mode at 0.9 microAmps and a 25Hz sample rate, and a 4.7 microAmps mode with a 50Hz sample rate and full operation. It also allows the designer to adjust the sample rate from between 0.4-and-400 Hz as well as adjust its resolution for 8-, 10- or 12- bits (with a 32-sample first-in first-out, FIFO, buffer) or at 14-bits for single samples, giving wearable designers a full spectrum of options for saving the maximum amount of power while meeting the specifications of their application, thus extending the battery life of the wearable, according to mCube.
How's it work?
The way mCube achieves its 1-by-1 millimeter die size is by using a special 8-inch wafer fabrication facility at Taiwan Semiconductor Manufacturing Company Limited (TSMC, Hsinchu, Taiwan) whose front-end fabs standard CMOS chips, but then switches to MEMS processing steps in the middle of the line thereby enabling a MEMS+CMOS chip to be created on a single fab line in 3-D with normal vias. As a result, there are no bonding wires to pick up RF interference or come loose, and much less parasitic capacitance than with the usual two-die solution, according to mCube.
As the self-proclaimed maker of the world's only monolithic MEMS+CMOS single-chip sensors, mCube offers accelerometers, magnetometers and soft gyroscopes (combining its accelerometer chip with an Alps magnetometer) and holds a portfolio of 122 patents filed, and 48 granted, to prevent copy-cats from following their example. So far, it has shipped 100 million of these MEMS devices into the smartphone and tablet market. And it hopes its newly redesigned ultra-low-power MEMS chips will capture an equally significant portion of the emerging wearables market.