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THE DUST FLIES

by Tom Cantrell

Start ı It's a Small World ı Cosmic COTS Motes ı That's Kode with a "K" ı Dust Buster ı Down and Dirty ı Sources and PDF

COSMIC COTS MOTES

Some of the Smart Dust gang got tired of waiting. Thereıs a lot of work that can and should be done (such as software) in the interim while the manufacturing challenges associated with the ultimate dust are overcome.

Hereıs where the so-called COTS Motes come in (see Figure 2). COTS highlights the use of commercial off-the-shelf technology, and Motes reflects the acceptance that a standard-chip solution doesnıt come close to achieving the minuscule form-factor goal of true dust. Nevertheless, the result is slick (and small and low-power) enough to be interesting.

Most of what I know about COTS Motes is taken from the Masters thesis of Seth Hollar. [2] Reading it, I couldnıt get over the feeling I was reading a Circuit Cellar article. It had the required theoretical analysis backed by elegant equations and such. But, it also had plenty of hands-on, under-the-hood action that made it a lot more interesting and relevant than the typical academic treatise.

I also appreciated the humility that comes along with actually getting something working. Theoretically perfect solutions rarely apply in the real world. Parts with the perfect specifications arenıt available, and the parts that are available donıt work perfectly anyway. Scientists whine about it; engineers design a way around it.

For instance, at first the plan was to use an SX52 MCU from Scenix (now Ubicom). On paper that seems fine, but at the time, only 5-V versions of the part were available, raising power consumption and the prospect of having to juggle multiple supplies. This real-world dilemma, much like those practicing designers face every day, prompted a switch to a 3-V Atmel AVR MCU.

Another example of reality-based design is seen with the radio subsystem. With all of the computing going on at Berkeley, it must have been tempting to jump on something like 802.11b or Bluetooth. But of course, from the deeply embedded perspective of Smart Dust, these alternatives are indisputably nonstarters.

Instead, Motes use a more mundane 900-MHz module, much like you would find in a low-cost portable phone (see Figure 3). With a home-brewed mini-me protocol (TCP/IP doesnıt fit in an 8-KB MCU), throughput may be measured in only hundreds of bytes per second, but thatıs more than enough for the task at hand.

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