Cool Means Small

Anyone who does their computing on the go recognizes first-hand the trade-off between size and performance. Notebook computers can perform much like a desktop machine, but even the tiniest ones may not be small enough to carry comfortably. Personal digital assistants, while pocket-sized, lack computing muscle and force users to synchronize their data. Antelope Technologies recently introduced a solution to this size-versus-performance dilemma.

The company has developed a modular computing system built around a 5 x 3 x 0.75-inch computing core. Encased in aluminum, the core contains a 20-Gbyte hard drive, 256 Mbytes of memory, a 1-GHz Transmeta Crusoe processor (See DN 04.05.04 page 61), and other system components. Think of it as a full-fledged computer minus the user interface. When used out of the office, the core slips into a variety of mobile "shells" which provide color, touch-sensitive displays, input devices, and various connectivity options. Back in the office, the core drops into a docking station to connect with a monitor, keyboard, and other peripherals. A single-point connector with a proprietary pin-out provides a quick hookup to both shells and docking stations.

To put such a fast processor in such a small space, Antelope faced a fundamental dilemma related to heat dissipation. It's a problem that all mobile computers face to some degree, but it's arguably worse for those who make "ultra personal computers," an emerging class of handheld computers that offer the performance of much larger machines (see sidebar). The small form factor of Antelope's core and other ultra personal computers makes cooling inherently more difficult, according to John Heinlein, director of strategic partner initiatives for Transmeta Corp, which supplies the processors for Antelope and other ultra personal computer makers. "There's simply less surface area to dissipate heat generated inside the case," he says. Making matters worse, the size of these handheld computers and their desired battery life preclude the use of fans. "They don't usually have room for active cooling," Heinlein adds.

Antelope's sealed core is no exception. Because it has no active cooling and runs such a fast processor, heat dissipation always loomed large as a potential problem. Tom Scott, Antelope's chief technology officer, says that the company's engineering team wanted to keep the core's exterior at less than 35C or so, even though the electronics inside "haven't even broken a sweat" at that temperature. "Anything above thirty five degrees would start to feel pretty warm to the human touch," he notes. Aided by some modular computing technology licensed from IBM, Antelope's engineers tackled the potential heat problem partly through mechanical design and good thermal engineering. For example, the company crafts the core's case from aluminum largely because of its heat transfer capabilities. "The case is basically a big heat sink," Scott says, noting that a series of cooling fins on one side of the case help out with the heat transfer.

But it turns out that Antelope's modular computing core owes much of its reduced form factor to electronic components and operating system software that manage power efficiently-and generate less heat in the first place.

A Cool Processor

"The processor is definitely the key," says Scott. "We couldn't do what we did with current Intel technology." Antelope's engineering team liked a couple of things about the 5800 Series Crusoe processor currently used in the core. With a maximum thermal design power of 7.5 watts, this processor inherently has a low-power architecture. For example, it employs relatively few logic transistors in the processor core. And it integrates the normally separate Northbridge controller, reducing the power requirements by another two to three watts. But Scott says that the Crusoe's power management feature, or LongRun^(R), made the biggest difference in this application.

Rather than simply toggling the processor off-and-on, LongRun acts as a throttle for the processor, continuously adjusting its clock speed and voltage in a series of steps matched to the computing demands. "It's a bit like a dimmer switch," Heinlein says. When plugged into the wall at its docking station, users can allow the Antelope to chug along near its full 1-GHz rating. "They have all the power they want, so why not take advantage of the processor?" Scott adds.

Out in the field in a handheld shell, when heat and battery life matter more, the processor can taper off to as little as 300 MHz with a corresponding reduction in voltage. Scott concedes that this throttling down the processor trades off a bit of performance in some circumstances, "but not a lot." He can't say exactly how much power Long Run saves, since the savings depends on individual usage patterns. But the amounts would typically be substantial, given that power dissipation falls dramatically with decreasing voltage and frequency-power equals the product of capacitance, voltage squared, and frequency.

For his part, Heinlein describes the overall efficiency of the processor in terms of power and performance. He reports that the Crusoe typically consumes less than one watt while running most software applications and under two watts in nearly all cases. Only "aggressive uses" such as games might push the processor over two watts. Put another way, the difference between typical power consumption and "processor running at full bore," at its maximum thermal design power, comes to about six watts.

And UPCs require every bit of that efficiency. Heinlein notes that in larger notebook computers over the past few years, processor improvements have them "much less of a factor in system power." A typical 14-inch notebook computer screen, for example, might consume 3.5 watts, and its hard drive could eat up about five watts. The small displays and hard drives in a UPC take just a fraction of that power, which ironically throws the requirements of even the lowest power processor once again into sharper focus.

Software Helps Manage Power

Making the core so small didn't come down entirely to hardware. Software played an important role in the miniaturization too, by contributing to the system's ability to manage power efficiently-and thereby stay cooler. Scott explains that Antelope engineers made good use of the Window XP's Advanced Configuration and Power Interface (ACPI)-a specification that defines the interface between the computer's basic input output system (BIOS) and the system board. The spec allows Antelope engineers to coordinate power management features among the hardware, the operating system, the application software, and any peripherals.

This standard proved particularly important given the unique demands imposed by modular computing, whose central notion is that the same computing core will have to work in different environments. One minute it may be running full bore in a desktop cradle. Another minute it could be humming along in a handheld shell. "The device may go to sleep in one environment and wake up in another," Scott says. "But it always has to know what environment it's in." Developing the BIOS that can handle these ever-changing uses from a power management standpoint turned out to be "one of our toughest engineering challenges," says Scott.

Scott notes other software-related factors also proved important in improving the computing core's efficiency. Antelope engineers found that utility software associated with some of the computer's components tended to drive the processor needlessly. "They triggered quite a lot of heating in the handheld configuration," he reports, citing the signal strength meter for the wireless card as an example.

For the Future: Smaller Yet

Since every little bit of board real estate counts when computers get this small, Antelope also went with the Crusoe for its size. This 5800 processor currently used in the core measures just 29 x 32.4 mm and integrates the Northbridge logic, saving the space and power taken up by a separate chip. Heinlein notes that the 5900 series processor, which came out after Antelope developed the core, has an even more svelte 21 x 21 mm form factor. Scott says the next generation of the core will likely use Transmeta's Efficeon processor, which has a 29 x 29 mm package size and outperforms the Crusoe in part by executing twice as many instructions per clock cycle.

Scott would like to take advantage of the extra space afforded by a smaller processor in future versions of the core-though not necessarily to shrink its overall size. Instead, he would use the extra space on the board to add more features. "If we save 20% on board space, we could go from to 256 to 512 Mbytes or even to one gig of memory," he says. "Everyone always wants more memory." Or Antelope could use the space to put other features on the board-things like additional video memory, GPS, or Bluetooth.

Reach Senior Editor Joseph Ogando at[email protected].