Rethinking Good Enough

By Ed Sperling
Power has been elevated from an afterthought to one of the top considerations and tradeoffs in SoC design, edging out performance and area in many cases and in some cases even cost and features.

Tradeoffs in design always change, depending upon what the most pressing concern is among consumers at any time. For decades, performance was always the top of anyone’s list, followed closely by cost. The MIPS and GHz wars made for great competitive marketing. But as devices become more mobile, and as even the largest enterprises focus on energy costs, the reigning king is power. How long does a battery last between charges on a smart phone or a laptop given a normal use case? How may kilowatt hours does it take to run a server?

This isn’t always a clean tradeoff, however. For one thing, some design features require more power, forcing changes in other parts of a design. And in other cases, the lack of any single use model makes it almost impossible to guess how a device will be used. One consumer may rely on voice calls, while another focus on text and still another may play games and stream video.

What stays, what goes
Decisions about what to keep aren’t always simple. Consider an LED TV design, for example. Flattening the screen requires audio enhancement because it’s impossible to get good enough sound out of a TV without playing tricks with the sound. That typically means more post-processing, more codecs, and more energy consumed.

“There are lots of things that can be done to enhance the audio experience,” said Larry Przywara, senior director of multimedia marketing at Tensilica. “The TV designers are space constrained. That requires various volume boosts, equalization and sound widening techniques just to do what they used to do. That’s doable, though, because as algorithms have gotten more complex the SoCs have gotten more powerful.”

Overall, they also use less energy to drive the SoC and the complete system. But sometimes that requires increasing power budgets in one place and decreasing them in another. “The issues in the mobile space are now finding their way into home entertainment,” said Przywara. “With post-processing you need slight modifications in other places to keep a limited power budget.”

In televisions, that energy can come from a variety of places. For example, the current design on some TVs relies on brighter pixels in the middle, where most people focus their eyes, and dimmer pixels on the corners where viewers don’t look.

Making tradeoffs
For both video and audio, the real change is a combination of improved technology and what consumers are willing to live with. Fifteen years ago most audiophiles wouldn’t touch a CD, and even several years ago the focus on quality in DVDs was considered the competitive edge. More people have migrated to the center of the spectrum as CD quality improved and streaming offers vast convenience even if it isn’t high-definition.

“Audio, from a technology standpoint, is not a big deal,” said Cary Chin, director of director of technical marketing for low-power solutions at Synopsys. “The real focus is on video, and today the real question is how you trade off storage with communications. Do you spend more time and energy to compress it or store it? And do you store it locally or in the cloud? As we focus more on portable devices, power and cost are the main factors.”

The other question is just how much power efficiency is enough. A smart phone uses basically the same technology as a tablet, yet the tablet gets significantly longer battery life between charges, while the smart phone needs to be charged every day.

“Tradeoffs are a great way to define area where the technology is evolving,” Chin said. “In digital video you can improve the resolution, but most of the computation and power is spent in compression and decompression. Even with printers, you can print with finer technology but it’s usually more important to lower the cost. Low power is one of the areas that will become critical to all of these decisions over the next 5 to 10 years.”

But even the technology that can command a premium—products from companies such as Apple, high-end graphics from Nvidia, and laptops from Lenovo—haven’t skimped when it comes to saving power.

“The tradeoff is how much energy you use at any time and how much energy you need to accomplish a task,” said Barry Pangrle, solutions architect for low-power design and verification at Mentor Graphics. “In general there will be more dark silicon and more functionality on a chip, but it won’t all be running at the same time.”

One of the more interesting tradeoffs has to do with which processors are used for what functions. Nvidia’s Tegra3 graphics chip, for example, has a four-core graphics engine and a fifth, lower-power and lower-performance chip for less data-intensive tasks.

Features or function
Perhaps the hardest thing to determine is whether to cut features, cut performance, or live with more power consumption when it’s needed. Will Ruby, senior director of RTL power product engineering at Apache Design, said what’s changed is that power is a fundamental requirement, along with features and functions. Engineers have to meet the power spec, even if that means some tradeoffs.

“There are two aspects to a tradeoff,” he said. “One is at the spec level. What features can you add for what performance and power. As more and more people learn how to do low-power design, they will meet or beat specs. Of course that usually means even more aggressive specs in the next design. The second is a spe-level tradeoff. How much time does it take to switch to a different application, for example? If it’s one-tenth of a second that will be a big difference from two-tenths of a second.”

Some tradeoffs also occur on the process side. Do you use older low-power process technology, or do you use the fastest general-purpose process technology and turn a block off as quickly as possible? Or do you dope the channel or swap to fully depleted silicon on insulator substrates?

Conclusion
None of these tradeoffs are fixed. They can be tweaked and tweaked again, because what may be good enough for one market, one group of users or at any point in time may be different somewhere else six months later.

What is significant, though, is just how integral a part power has become in all of these decisions. “The real key is how you can exploit all the possibilities of what you can get with relatively low power,” said Pete Hardee, marketing director at Cadence. “If you’re trying to freeze frame a golf swing in video, you may want to go completely the other way—all the way up to 60 frames per second. If power is the issue, you may want a slower frame rate. And it’s not just about battery life. Reliability is a big headache for customers. The ability of low-power techniques to control the performance profile can increase reliability, too.”