Experts At The Table: Mobile Design Challenges

By Ed Sperling
Low-Power Engineering sat down to discuss the increasing challenges of designing for mobile devices with Qi Wang, technical marketing group director at Cadence; Cary Chin, director of technical marketing for low-power solutions at Synopsys; Bernard Murphy, CTO of Atrenta; and Dave Reed, senior director of marketing at SpringSoft. What follows are excerpts of that conversation.

LPE: What effect does the wireless spectrum have on design?
Murphy: Spectrum availability is getting squeezed. That’s why things like MIMO (multiple input multiple output communications) are becoming interesting. But to support MIMO you need multiple radios or complex radios, and radios suck power. You have to deal with that problem to take advantage of whatever spectrum is left.
Chin: I’ve been doing a lot of experiments with power lately, and it turns out that the screen is not always the biggest power hog in the system. In general it is, but if you’re in an area with poor communications signals the radio draws a lot more power than the screen. This is why some people with the first iPhones were surprised that they were getting 1.5 hours of battery life. With multiple radios in a device today, it’s a big deal. My phone gets much hotter when I’m streaming something while driving around in my car with the screen off than if I’m at home in a WiFi environment. We’ve been working on cutting a number of cords in mobile. The power cord is the most recent. But the one we cut previous to that was Ethernet. It turns out that was a big deal. There are new standards. Bluetooth was notorious for being a power hog. The new Bluetooth standard improves that. But power is really a global issue. When I worked in synthesis, we thought a lot about divide and conquer. But the ramification of power is global. A picowatt in one quarter of the chip affects the whole chip. That’s one thing we need to think differently about. It’s not just physically adjacent blocks or logically connected blocks.

LPE: But some of these effects are also outside the chip, like multiple towers handling a signal instead of just one. How you deal with that in the design? There are lots of disconnected pieces that aren’t even part of the SoC ecosystem.
Murphy: There is a lot of architectural power modeling. They’ll look at the SIM card and the battery and what’s around. Then you have to start thinking about where your transmission sources are, and as you move around between these things how much power are you using as you move around.
Chin: More and more we need to talk about this holistic approach for low-power design. You really need to think about everything in a global sense. It’s a difficult problem, and the solutions are moving toward more standardization and more IP. If we can take communications standards and have reasonably re-usable chunks, that will simplify things. But then having all of those things work together and making sure the low-power methodologies are consistent is not simple. We’re not going to run out of things to do.
Murphy: A lot of design teams thought they could handle power in the same way they handle test. The problem now is there is a much stronger coupling between power management and the design, so you can’t finish the design and then do the power management. They’re too interrelated. But you still need a high level of expertise to do the power management.
Chin: Power goes even above the chip-design process. It might even be something you need to think about before you think about the rest of your chip. And chip designers are pretty uncomfortable with that.
Wang: Most of our leading customers in the mobile market have come to the conclusion that they cannot address this from the same point of view. They have to combine software engineers, hardware engineers and system engineers all the way to the signoff. They need to work holistically doing modeling, estimation, create a spec, and at every stage check for the spec. We’re seeing more and more real customers adopting that approach. What’s ironic here is that EDA companies often get blamed for being behind, but in this case the EDA companies are ahead. They want to know why their power came up at 11 (volts) when then put down 5 (volts).
Reed: EDA is always a little bit behind the leading edge and ahead of everyone else. The people who have had to deal with power seriously for awhile have been doing just that. We’ve learned from them. But before test could be thought of separately it was part of everything else. It was great when you didn’t have to include test. It was liberating because you went from a small handful of people designing 7,000-transistor chips to tens of thousands of people designing millions of transistors. I have no doubt power will come to that stage.
Chin: I’m not so sure of that. I used to work in test and we created a methodology to make that separate. But in the case of power, it’s so closely intertwined that I can see the opposite happening. It may be power that’s most important, and whatever performance comes out is what we get. Design for power may become the biggest design parameter and everything else may fall beneath that.
Reed: Power debug has become a new thing you have to worry about, whether you’re using UPF or CPF.
Wang: Power has become an inseparable part from concept to silicon. Power is a requirement. It is not a technology or technique. If people want more functionality, it all boils down to energy needed to do the computation. In some ways, people have invented the problem. One CEO came to Cadence to talk about moving from desktops to mobile. He is trying to find a new market for his processors, so he is looking at medical and gaming. But what’s interesting is they’re not building games. They’re building simulators for physical effects. We need to worry about demand for bandwidth and power. And performance does not just mean speed. Bandwidth is part of the performance. Response time of downloading and communicating can be a bottleneck.
Murphy: And it still has to be within the cost envelope. Power has probably gone slightly beyond performance in importance, but it hasn’t gone beyond cost. Consumers are forever going to be cheap. If I could buy something last year for $200, it should be $50 next year.
Chin: But the iPhone is a $600 device that costs $200, so we’ve figured out different ways of paying for that to amortize the cost over time.
Wang: Cost is a constant, but that doesn’t solve the problem. The success of Apple is not just the device. They create a value chain so they don’t just try to extract the value from the hardware device. There’s too much competition for that. Another angle we need to consider is that it’s not just about consumer mobile devices. A much larger market for mobile applies to medical. In a hospital you will have thousands of sensors in one room. If you multiply that by the number of sensors you will have in your body, it’s a very large number. People want entertainment and they want to live longer. You don’t need a different methodology to low power and low cost. The application-driven business model will be the underlying baseline.

SLD: What changes if power becomes the starting point?
Murphy: FinFETs will have an effect. Intel thinks they’ve solved the leakage problem.
Wang: That creates a new problem, though, gate sizing. Previously you could increase the channel width to make the cell bigger. Now you have to make it higher.
Chin: As power becomes the most important factor for design, the design methodology and tools will change. You can envision that over the tools we use will be very different. Over the past 30 years we’ve defined a methodology that gives us the highest-performance devices for the lowest cost and the smallest area. But we’re optimizing for performance. If that changes, there’s a lot of room for new methodologies, new tools and new ways of designing. If you look at simulation tools, we’ve been doing logic simulation for at least 30 years. It won’t be long before someone introduces a simulator that simulates power as well as logic. Today we have lots of approximations that help to solve the problems we have today, but we go back to device-level simulation to figure out what’s really going on with power. With UPF and CPF, that’s a temporary thing. In five years design intent and power intent will be the same thing. I believe these things are going to change radically.
Murphy: But everything we worry about in mobile power is predicated on the fact that you are removed for a long period of time from charging sources. If energy scavenging becomes really effective the problem goes away.
Reed: In terms of things affecting designers, there are a number of them. One thing people have had to care a lot about lately is leakage. It’s been an ongoing challenge—whether it’s subthreshold leakage, so we turn these off harder, or the dynamic power. For 3D (stacking) we can reduce the power several ways if we shorten the connection. You also can widen the I/O and add more memory and memory channels.