The reason for the curiosity is that IBM's claim calls for a re-examination of what constitutes a computer. Previous systems, including the 2 x 2 x 4mm Michigan Micro Mote, retain their programming and data even when they are not externally powered.
Unplug a desktop computer, and its program and data are still there when it boots itself up once the power is back. These new microdevices, from IBM and now Michigan, lose all prior programming and data as soon as they lose power.
"We are not sure if they should be called computers or not. It's more of a matter of opinion whether they have the minimum functionality required," said Professor David Blaauw of Michigan.
In addition to the RAM and photovoltaics, the new computing devices have processors and wireless transmitters and receivers. Because they are too small to have conventional radio antennae, they receive and transmit data with visible light. A base station provides light for power and programming, and it receives the data.
The team explain that one of the big challenges when creating this ‘computer’ was how to run at very low power when the system packaging had to be transparent. The light from the base station—and from the device's own transmission LED—can induce currents in its tiny circuits.
"We basically had to invent new ways of approaching circuit design that would be equally low power but could also tolerate light," Prof. Blaauw said. For example, that meant exchanging diodes, which can act like tiny solar cells, for switched capacitors.
According to the Michigan team, another challenge was achieving high accuracy while running on low power, which makes many of the usual electrical signals (like charge, current and voltage) noisier.
Designed as a precision temperature sensor, the device converts temperatures into time intervals, defined with electronic pulses. The intervals are measured on-chip against a steady time interval sent by the base station and then converted into a temperature. As a result, the computer can report temperatures in minuscule regions—such as a cluster of cells—with an error of about 0.1 degrees Celsius, the team adds.
