Just a few short years ago, the expectations of cell-phone users centered around one capability: being able to reliably conduct voice calls while they were away from a wired telephone set. Since then, impressive technological advances have been achieved. The brick-sized, transportable phone sets of yesteryear have morphed into soon-to-be-available, 6+-megapixel camera phones. Such devices offer DVD-quality video and a high-fidelity music player. They also flaunt a high-end, 3D-graphic gaming console and high-speed wireless-Internet-connection combinations.
To put it simply, cellular phones have stopped being just telephones. These "Swiss army knives" of the personal communications system are now the most impressive example of the convergence of computing, communications, and personal entertainment. With these devices, one can generate, share, enjoy, and store multimedia information as part of today's on-the-move lifestyle.
For the microelectronics industry, however, these supercomputing organisms invite a complex problem: Keeping these products running on very small batteries for longer periods of time is a major challenge. As a result, special emphasis is now being dedicated to extending the battery life of such systems. Designers are expanding their operating time from a few minutes to many hours while standby times rise from hours to weeks.
In doing so, designers are stressing a multi-level matrix of theoretical and practical limits—from the fundamental laws of physics to system-level-optimization techniques. These techniques cannot be added to an existing system later on. They are major components of both application hardware and software. For optimal results, designers must therefore make use of energy-reduction-enabled, foundation-semiconductor structures. Such structures include logic elements, input/output cells, and memory blocks. At the same time, designers must implement dynamic power-consumption-management procedures.
This article will explore some of the most advanced system-level energy-management techniques that are currently in development. It also will analyze how these techniques are enabled at the foundation-semiconductor-structure level. A quantitative estimate of the resulting battery-life extension will then be calculated for a system. This estimate will make smart usage of all of the techniques that are now available in a commercially available, semiconductor Intellectual-Property (IP) product.
To better visualize the power-management problem faced by a next-generation cell-phone designer, consider the high-level block diagram shown in FIGURE 1. For a device of such complexity, the only strategies that will effectively optimize energy consumption must include run-time-behavior analysis. Such analysis translates into a continuous monitoring of all of the requested services and performance levels at every individual block of the system. It selectively turns off or adjusts the performance of those system elements.
Some of the early, pioneering energy-management schemes simply turned selected areas of a system on and off. These schemes were based on rudimentary utilization algorithms. It's now understood that an energy-waste cost is associated with turning off and on a section of an electronic device. Depending on how frequently a block is turned off and on, this cost may exceed the benefit of powering it off in the first place.
In addition, designers now know that smoothing the discharge profile over time—rather than having dramatic discharge-rate changes—results in longer times between battery recharging. A meaningful run-time-behavior analysis has to consider many aspects, including block workloads, energy-consumption rates, the usage environment, and requested services. Based on a block-workload analysis, adaptive-predictive shutdown techniques can put inactive system areas in power- or shut-down mode. This step occurs as a result of previous usage patterns.
Among the examples of energy-consumption-based power-management techniques are communication-based power management (CBPM) and heat management with passive cooling. CBPM exercises dynamic and proactive control over system components. It delays components that are executing less performance-critical operations even if they're not idle. By dynamically blocking the execution of certain components, CBPM can regulate the system's power profile. In contrast, heat management with passive-cooling procedures monitors the system's temperature. It limits the power consumption of certain sections and even disables services if necessary.
Based on the usage environment, some of the system parameters are adjusted. Such adjustments include compulsory system shutdown. The usage environment could be, for example, the strength of the received RF signal, the intensity of the ambient light, or the remaining battery charge. To reduce battery discharging, some services also may be disabled. Even if the user requests those services, they won't be performed.
As mentioned previously, implementing a comprehensive energy-management strategy imposes new system-level requirements with both hardware and software implications. The software component of such a strategy contributes to its flexibility and adaptability. The hardware aspect includes additional functions and blocks like timers, sensors, and power and clock controllers. It also incorporates very specific transistor-level features, which enable battery-life optimization. Among these system-level requirements are the following:
Although cost isn't included in this article's analysis, a smart system designer will select the optimal mix in order to achieve the best cost/battery-life combination for the intended cellular-phone device. By carefully selecting the basic elements that will be used in a design, the designer can keep cost within budget while implementing a wide range of energy-management techniques. The semiconductor components that are included in a cellular-phone design are responsible for a major share of the power that's dissipated. Aside from being developed with features that must support system-level energy-management techniques, these components must also be inherently low-power-consuming structures.
Most of the semiconductor devices that are used in systems like wireless cellular phones are fabricated in CMOS processes. The power that's dissipated by CMOS structures is composed of two factors: dynamic and static power dissipation. Dynamic power dissipation results from the active switching of the transistors' logic state. In contrast, static power dissipation occurs because of the current that leaks from the transistors while they're powered.
The main contributors to the dynamic power dissipation of CMOS structures are the applied voltage, operating frequency, and switching-structure capacitance. For static power dissipation, the main contributors are the applied voltage and the threshold voltage (Vt) of the used transistors. Of course, the silicon manufacturing process also has a huge influence on the overall power that is dissipated.
Recently, numerous innovative power-dissipation-reduction techniques have been used in semiconductor devices. Some of these are listed below:
Cellular-phone users probably have some of the most demanding expectations out of the whole consumer market. These users want to receive and make voice calls; send messages; take, send and receive photos and videos; listen to music; and play games for extended periods of time before recharging their batteries. For these reasons, cellular-phone developers need to implement every possible and available energy-reduction technique into their designs.
Providers of foundation-semiconductor products, such as memory blocks, input/output pads, and standard cell libraries, are working to support this goal. They have integrated support for the most advanced, innovative power-dissipation-control techniques. An example is Virage Logic's IPrima Mobile Platform. To address the requirements of consumer and wireless personal communications products, this platform significantly minimizes and manages power dissipation without paying a performance penalty. Its secret is that it supports all of the advanced power-dissipation-reduction techniques that were mentioned here (FIG. 2). As a result, it claims that power reductions are now achievable in the range of 20X for static and 80% for dynamic dissipation.
All of the energy-conservation techniques that were described here are in the process of becoming embedded in soon-to-be-released, feature-rich cellular phones. Using these techniques, it's possible to estimate the resulting battery-life extension between recharges. Of course, some techniques are already available in existing devices. Those techniques weren't included as part of the energy-savings estimates for this article. Instead, the analysis focuses on the savings that were gained by applying DVFS and voltage-island-support techniques as well as ABB for standby-power-dissipation enhancements. Note that this analysis assumes the use of a Lithium-Ion (Li-ion), 1100-mA/hr. battery pack. The cost impact of these techniques wasn't included in the analysis.
In addition, two user profiles were used. The first is a typical, low-profile user (me) who essentially uses a cellular phone to make voice calls. Per day, my phone doesn't get more than 2 hrs. of active use. It is only on for around 14 hrs. per day. The second, heavy-duty user (my daughter) logs 6 hrs. of active usage. Her usage consists of a lot of calls, data downloading, listening to MP3 files, some picture taking, and some video watching. The phone is always kept on.
My current phone needs to be recharged every three to four days. My daughter has to charge her phone every day (roughly every 12 hrs.). Now, assume that when we get our new cellular phones, they will incorporate all of the energy-management and saving techniques that were mentioned in this article. I would be able to go through an entire workweek without recharging. At around 5.8 days, that's a 60% improvement. My daughter would need to recharge roughly every 30 hrs.—a very impressive improvement of 130%.
Cell-phone users of all varieties are calling for longer battery life. Yet next-generation, feature-rich cellular phones are going to further increase the pressures on battery life. Luckily, new techniques and semiconductor IP platforms are now available to help designers address this challenge head on.