October 2001

It's Time for Clockless Chips

Continued from page 2

By Claire Tristram

Clockless to the Rescue

By throwing out the clock, chip makers will be able to escape from this bind. Clockless chips draw power only when there is useful work to do, enabling a huge savings in battery-driven devices; an asynchronous-chip-based pager marketed by Philips Electronics, for example, runs almost twice as long as
competitors' products, which use conventional clocked chips.

Like a team of horses that can only run as fast as its slowest member, a clocked chip can run no faster than its most slothful piece of logic; the answer isn't guaranteed until every part completes its work. By contrast, the transistors on an asynchronous chip can swap information independently, without needing to wait for everything else. The result? Instead of the entire chip running at the speed of its slowest components, it can run at the average speed of all components. At both Intel and Sun, this approach has led to prototype chips that run two to three times faster than comparable products using conventional circuitry.

"Look at it this way," says Intel's Ebergen. "You give me a folder, I work on it, I give it back to you, and the fact that I give it back indicates I'm done. We don't have to communicate every five seconds. We might do the job much faster by agreeing between the two of us when to get things started and when to get things done and not worry about synchronizing our work every step along the way."

Another advantage of clockless chips is that they give off very low levels of electromagnetic noise. The faster the clock, the more difficult it is to prevent a device from interfering with other devices; dispensing with the clock all but eliminates this problem. The combination of low noise and low power consumption makes asynchronous chips a natural choice for mobile devices. "The low-hanging fruit for clockless chips will be in communications devices," starting with cell phones, says Yobie Benjamin, a technology strategist for the consulting firm Ernst and Young. So convinced is Benjamin of the technology's promise that he has personally invested in Asynchronous Digital Design, a clockless startup out of Caltech.

Two other new firms, Theseus and Manchester, England-based Self-Timed Solutions, are focusing on clockless chips for smart cards. Fant maintains that a key problem holding back smart cards is that conventional chips make it easy to crack the chip's security codes by watching the signals. "The clock is like a big signal that says, Okay, look now,'" says Fant. "It's like looking for someone in a marching band. Asynchronous is more like a milling crowd. There's no clear signal to watch. Potential hackers don't know where to begin."

Speed, energy efficiency and stealth sound like important goals for any chip, not just those used in a few niche applications. But while Sun, IBM and Intel all have small research groups working on asynchronous designs for specialty applications, neither they nor anyone else has announced work on a general-purpose clockless microprocessor. This seems an odd oversight. An industry that considers the improvement of processor speed to be an almost sacred goal has forsaken one of the most promising avenues for making chips go faster. You just have to ask why.

Why, for example, did Intel scrap its asynchronous chip? The answer is that although the chip ran three times as fast and used half the electrical power as clocked counterparts, that wasn't enough of an improvement to justify a shift to a radical technology. An asynchronous chip in the lab might be years ahead of any synchronous design, but the design, testing and manufacturing systems that support conventional microprocessor production still have about a 20-year head start on anything that supports asynchronous production. Anyone planning to develop a clockless chip will need to find a way to short-circuit that lead.
"If you get three times the power going with an asynchronous design, but it takes you five times as long to get to the market-well, you lose," says Intel senior scientist Ken Stevens, who worked on the 1997 asynchronous project. "It's not enough to be a visionary, or to say how great this technology is. It all comes back to whether you can make it fast enough, and cheaply enough, and whether you can keep doing it year after year."

Philips's asynchronous chip has given the company's pagers the ability to last almost twice as long, on the same battery power, as clocked alternatives. But its debut in 1998 followed a decade of dedicated research. Asynchronous researchers from the beginning understood that their task wasn't just to build another chip, but rather to build a way to design, test and manufacture that chip. And that wasn't easy.