It is difficult for the present generation to digest the fact that computers occupied a whole room when they were first built. What is more difficult is to accept the reality that computers as small as a cubic millimeter in size have already been developed. As unbelievable as it may sound the truth is that they exist and are called M3 Michigan Micro Motes. The press needs a name that is shorter and catchy and hence the nickname 'Smart Dust'.
The credit for developing these computers goes to a team of researchers led by Professor David Blaauw of the University of Michigan. Other than Professor David Blaauw the team consists of Dennis Sylvester, David Wentzloff, Prabal Dutta and several graduate students.
A single M3 Michigan Micro Mote is a sensor node. Sensor nodes gather information using sensors, store them, do some processing and provide an output. It can even communicate wirelessly with other motes. Thus, a smart dust can receive input, process the data, provide an output and even communicate wirelessly making it the smallest computer on earth. One of the key features of smart dust is a low power optical wake-up receiver. Wake-up receivers have been widely used to limit power consumption of main transceivers. Inter-layer communication in this device is achieved over I2C (Inter-Integrated Circuit). There are other features too that are related to power supply like a Power Management Unit, Brown-Out Detector, and Power On Reset.
The biggest source of concern when designing such a small device is the source of power. With the M3 Michigan Micro Mote things get even trickier. Like any small device the power source has to be very small. In the mote though, it has to be small but powerful enough to make wireless communication possible. The team behind smart dust did not just stand up to the challenge but won it convincingly. Not only did they manage to harness enough power from a miniature source but developed an excellent technique for level conversion too. The device draws its power from a solar cell of just a millimeter square in area that is capable of producing 20nW of power. This is enough for the sensors to work, all the processing to take place and do wireless transmission of data one bit at a time. Conventional methods employed for level conversions in ultra-low power voltage sources are based on the differential cascade voltage switch (DCVS) circuit. It requires proper two-sided pull-up and pull-down of voltage which is very difficult to meet. Disparity in pull-up and pull-down leads to failure of the level shift circuit. The smart dust team overcame this hurdle by introducing Limited Contention Level Converter (LCLC or LC2) which changes pull-up strength based on operation mode.
The Michigan Micro Mote is built of several layers stacked above one another. The first layer consists of an ultra-small solar cell which is the power source and a photo cell for optical communication. Then, there are layers for controlling solar energy harvesting and radio communication. They are followed by a layer for interfacing a temperature sensor and another layer for stabilizing power. Power regulation, memory and the processor constitute the next layer. The final layer is the battery. There is also provision for an optional layer to include other sensors depending on the application. These layers communicate with each other through a computer system bus designed by Sun Microsystems called MBus.
The smart dust devices are capable of engaging in wireless communication within a radius of about 2m at present. The team is working on improving this radius and they have been successful in testing smart dusts that have more than thrice the original radius for wireless communication.
It is highly necessary for wireless sensor nodes to have a stable oscillator. Their communication systems are activated only after certain time intervals to reduce power consumption. Instability in oscillator frequencies can cause irregular power consumption and shorter battery life. For increased stability quartz crystal oscillators are generally used. The use of quartz crystal oscillators is not feasible in an ultra-low power device like the smart dust due to their high power consumption. To overcome this hurdle the team came up with an idea to reduce power consumption by focusing on the driver, oscillation amplitude, and peripheral power. They designed a new circuit that applies different voltage levels as suitable for minimizing energy loss and eliminating static power consumption using a switched capacitor.
Low leakage memory is a very important factor in reducing static power consumption. However, existing devices are not suitable for smart dust due to size constraints. The low leakage 10T SRAM was proposed to tackle this problem which showed lowest leakage per bit value. A boosted supply implemented using switched capacitor networks and low-dropout regulator is used to reduce leakage power. Boosted supplies have the added advantage of providing high operation speeds.
One application of the smart dust can be monitoring temperature conditions of a room. It can sense the temperature and send the data wirelessly. The real scope of this device comes to light if we add further layers for other sensors and use the motes to implement a sensor network. An example is the use of smart dust devices in medicine by adding a pressure sensor layer. Another application is in security by adding a layer for motion detection.
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