Kill the Clock

By Tom R. Halfhill

Everyone knows AMD and Intel are turning away from stratospheric clock speeds and moving toward chips with multiple processor cores. But what if someone made a microprocessor that doesn’t have a fixed clock speed at all? Believe it or not, it’s been done, and a recent development has revived interest in this oddball technology. It’s called asynchronous logic, and it requires some explanation.

Almost all existing processors synchronize their internal operations to a regular heartbeat known as a clock signal. The signal is an electrical pulse that tells the transistorized gates when to perform their logic functions. A processor’s clock frequency is the number of ticks per second. It’s all very regulated, like musicians playing to the steady beat of a drum. This dominant type of logic circuit is called synchronous logic.

Asynchronous logic marches to the beat of a different drummer—actually, to no drummer at all. The logic circuits can operate as fast as they want, even if it’s faster or slower than other circuits on the same chip, and are not regulated by a global clock signal. Therefore, an asynchronous processor doesn’t have a fixed clock frequency. Parts of the chip might run at 1GHz while other parts run at 2GHz, or anything in between. The advantage of this approach is greater efficiency.

A processor’s instruction pipeline is like a factory assembly line: Program instructions enter at one end, pass through sequential stages of execution, and finally emerge with the calculated results. In a conventional processor, instructions move steadily through the pipeline, even if one stage is faster than another. Slower stages can delay faster stages. In an asynchronous processor, each stage can work at its own pace. Asynchronous logic works as fast as it can, and when it’s done, it’s done. Of course, it’s not quite that simple, or asynchronous logic would have caught on long ago. One challenge is orchestrating the untimed logic to keep a downstream circuit from activating before receiving the data it needs from an upstream circuit. (Imagine a factory worker trying to attach a wheel to a half-finished car without an axle.)

Two European companies, ARM and Handshake Solutions, recently announced the first commercially available 32-bit asynchronous microprocessor core. It’s designed for low-power applications, like cellphones. Other companies are watching closely. It could herald a better way to juice up performance without the power and heat problems afflicting today’s superspeed processors.