Y switches set up low-power logic

By Eric Smalley, Technology Research News

The faster a computer runs, the hotter it gets. Eventually this will limit the speed of today's computer chip designs.

Among the alternatives researchers are looking into are Y-branch switches, which have the potential to use less energy because they turn circuits on and off by directing electrons in one of two directions rather than opening and closing the circuit. Computer chips heat up when electrons are blocked and their energy is dispersed as heat.

Another alternative is reversible computing. The general idea is that if a logic operation can be reversed, there is no net loss of energy.

A researcher from the Swedish Royal Institute of Technology has devised a scheme that configures Y-branch switches into reversible logic circuits that conserve information and therefore generate little heat. The scheme's low-power switches and reversible computing design have the potential to produce ultra low-power computer chips.

The key to reversible computing is designing logic gates that have the same number of inputs and outputs by configuring them to operate in both directions. "Throwing away information costs energy, and in conventional logic this is done all the time," said Erik Forsberg, a researcher at the Joint Research Center of Photonics of the Royal Institute of Technology and Zhejiang University.

For example, the Not AND, or NAND, gate, which is a common component of computer circuits, has two inputs but only output, said Forsberg. "You enter two bits of information into this gate but you only get one bit of information out, so effectively you've thrown away a piece of information," he said. "This surplus energy you get from discarding information will heat up your system."

Excess heat does more than make laptops too hot to put in laps. Thermal energy weakens the electronic and magnetic boundaries that define bits, potentially scrambling data and make circuits unusable. "With the ongoing scaling of the CMOS technology to smaller devices and more densely packed circuits, heat generation is one of the main design problems," said Forsberg.

Chipmakers have been able to steadily decrease the energy dissipation per gate operation, but they will eventually run out of room for improvement, said Forsberg. The use of cooling is an unattractive solution, he said. "We don't want to carry around jars of liquid helium to be able to run our laptops."

Y-branch switches can be used as a replacement for transistors if they are smaller than about 200 nanometers because electrons travel through them in an unbroken path rather than bouncing around as they do in ordinary circuits. This makes them easy to control and low-power. "In the Y-branch switch we only need to deflect the electrons whereas in a field effect transistor we need to be able to stop them," said Forsberg.

Y-branch switches called for in the scheme would have switching voltages of about 1 millivolt, which is at least 50 times smaller than switching voltages for field effect transistors, said Forsberg. A 1 millivolt switching voltage corresponds to about 0.6 milli electron volts of dissipated energy, he said. "The cost of throwing away a bit of information is at least 18 milli electron volts, 30 times larger than the switching energy," said Forsberg. "So by turning from conventional to reversible logic, the power dissipation could probably be reduced by several orders of magnitude in circuits based on Y-branch switches," he said.

Forsberg's ultra-low-power computing scheme implements reversible computing by building a controlled exchange gate from four Y-branch switches. The gate has three inputs and three outputs. The switches form the second and third inputs and second and third outputs, which are interconnected so that one fork of each input switch connects to the second output and the other fork connects to the third output.

The first input-output pair is designed to carry a control signal. If there is no control signal, signals at the second input travel to the second output and signals at the third input travel to the third output. If there is a control signal, the second and third signals cross, with a signal at the second input traveling to the third output and a signal at the third input traveling to the second output.

Controlled exchange gates can be combined to carry out all of the binary logic of computing. "This means that you can do computation without generating any heat in the system," said Forsberg. "Of course, this is in principle. Eventually some information has to be discarded, but you can design your circuit so as to do this as seldom as possible," he said.

Y-branch switches have to be operated at low currents, which results in low circuit speeds, according to Forsberg. To overcome this limitation, however, large numbers of the switches could be implemented in massively parallel architectures, he said.

Another consideration is working out the timing of the circuits so that control signals reach the switches before input signals, according to Forsberg.

The logic circuit could be built today, but the control gates of today's Y-branch switches generate more excess energy than is produced by the loss of information, said Forsberg. "The main challenge is that of fabrication techniques," he said. "Also, present-day Y-branch switches operate only at cryogenic temperatures," he said.

Fabrication techniques could be mature enough to produce Y-branch-switch-based reversible logic circuits in about 10 years, said Forsberg.

Forsberg published the research in the April 2004 issue of Nanotechnology. The research was funded by the Swedish Royal Institute of Technology.

Timeline:   10 years
Funding:   Government
TRN Categories:   Integrated Circuits; Logic
Story Type:   News
Related Elements:  Technical paper, "Reversible logic based on electron waveguide Y-branch switches," Nanotechnology, April 2004


May 5/12, 2004

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