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Low-Power CPUs

Keeping Cool as Low-Power CPUs Bring Intelligence into Small Spaces.

More compute power is squeezing into ever smaller spaces. Even at lower power and smaller size, these CPUs still generate heat that must be dealt with.

CIARAN MACNEILL, VIA TECHNOLOGIES

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X86 processors run a huge variety of software, providing designers choices including Microsoft Windows, Linux and many real-time operating systems. These OSs are evolving and adding new features. The upcoming Windows 7 Embedded for example is expected to include Windows Vista and Windows 7 features such as Aero, SuperFetch, ReadyBoost, BitLocker Drive Encryption, Windows Firewall, Windows Defender, Address space layout randomization, Windows Presentation Foundation and Silverlight 2 among several other packages. On top of the OS there is often a third-party layer of software that comes between the platform and the developer such as Adobe Flash. Although Flash has recently added support for H.264, the video on almost all Flash Web sites currently requires an older generation decoder that is not implemented in mobile chips and must be run in software. This poses a problem for developers wishing to use low-power small form factor (SFF) embedded systems targeting small spaces and small applications where it is still necessary to manage the power consumption as well as the heat that is still dissipated by these low-power devices. 

A good rule of thumb for the power consumption of an x86 processor on a small form factor (SFF) board depends on what the design needs to achieve—its thermal design power (TDP). As the name implies, this is the wattage to which the thermal design is sized. Board technology, cooling techniques and power supply selection are driven by this figure. For SFF boards many designs have only passive cooling, and more power becomes problematic in fanless designs. Ten to fifteen watts is normally considered the limit to passively cool SFF systems.

Manufacturers of x86 processors have recognized this and are creating processors at lower power points specifically more suited for smaller designs with more processing power and better performance-per-watt than the previous generations. To address these problems Via released the Nano 3000E processor, the latest embedded version of its Nano Processor based on the 64-bit superscalar “Isaiah” architecture, which allows flawless playback of high bit rate 1080p HD video. At speeds from 800 MHz to 1.8 GHz, Nano 3000 Series processors deliver up to 20 percent higher performance using up to 20 percent less power than earlier Nano processors. The Nano 3000E also supports CPU virtualization technology, SSE4 and security capabilities integrated in the Via PadLock Security Engine.

The CPU TDP is usually the most important component affecting system cooling. To help achieve low thermal design power and/or long battery life, the more features that can be moved from software to hardware such as AES hardware encryption (FIPS 197 certified) or H.264 decoding using the latest integrated chipsets, helps lessen the CPU utilization and can help lower the overall board or system level TDP. Standby or idle power is also important; obviously, designs include an off switch that takes power consumption to zero. But many designs retain data while idling in standby with a minimum complement of functions powered. This is becoming an increasingly important figure for battery-powered devices as they spend most of their time in standby. Standby usage also has the benefit of allowing a rapid power-up to full operation for a better user experience.

For even more processing power in the future, it is important to take advantage of the continuous ongoing improvements to transistor and other submicron technologies to deliver the additional performance customers are asking for. The fabs’ incremental improvements to their process technology will allow new CPUs to offer faster or multicore processors while remaining within similar thermal envelopes. 

For the small form factor board-level market there have been a number of innovative open reference design motherboards, the most notable and widely adopted being Mini-ITX back in 2002. Mini-ITX boards can often be passively cooled due to their low power consumption architecture, which makes them useful for extremely quiet PC systems, where fan noise can detract from the user experience or the fan can be seen as an additional potential point of failure.

Following on the heels of the Mini-ITX, the industry is going to even smaller platforms, sometimes halving the overall size for even smaller form factor embedded systems. To passively cool each of the motherboards usually involved developing a heat pipe from the CPU and chipset to the chassis, which would provide additional heat dissipation (Figures 1-3).

Creative Solutions

Designing different heat pipes is difficult when the size and location of the CPUs and chipset are continually changing, along with fact that the other components need to be designed around to ensure good contact for heat dissipation. For example, the Embedded ITX form factor Em-ITX line (17 x 12 cm) places the CPU and chipset on the bottom side of the motherboard. This makes it a lot easier for system designers to passively cool the main hot components on the motherboard as there are no components in the way when designing into a cooling solution. The Em-TX form factor was designed for ultra-thin embedded applications and can withstand temperatures ranging from -10°C up to 70°C. 

Longer dual I/O coastlines provide space for an array of I/O options that OEMs demand such as LVDS, VGA, Gigabit Ethernet, ample USB 2.0 and COM ports. Having the I/O built into the coastline rather than using headers, allows for a cableless system assembly and better internal airflow as there are no cables to restrict airflow. 

The growing digital signage market, including POS, Kiosks, Digital Signage and Panel PCs, presents other challenges. Faced with limited airflow, they must deliver sophisticated features such as onboard touchscreen control and 1080P and H.264, and offer the required 7-year embedded lifecycle. OEMs must look for platforms that offer built-in hardware acceleration, a variety of video I/O and resistive touchscreen control, which allow digital displays to achieve their purpose yet work in tight spaces such as gaming systems, back-of-seat transportation entertainment, or hospital patient entertainment systems.

Reducing size and TDP has typically been an aim for many new computing platforms. More powerful CPUs at similar TDP levels coupled with new integrated chipsets will broaden the target applications for embedded computing. However, many embedded developers do not need fanless as their systems are in controlled environments. Developers are not just looking for smaller and lower TDP fanless offerings. New features like High Definition and 3D are requiring board designers to embed GPUs on board for additional display capabilities. 

The addition of onboard GPUs gives powerful and feature-rich mid-level top-to-bottom solutions, especially for graphics-intensive applications such as digital signage where manufacturers want to integrate the processing solution into the back of the display and need high-quality and high-performance graphics in a small space. Companies can design systems that disable the GPU when all the features are not required, and can switch back to the integrated chipset and improve battery life or lower the chassis thermals to increase MTBF. In the future, more and more features now seen only in systems utilizing GPUs will be included in more powerful integrated chipsets for improved thermals and cost

Embedded system designers focusing on their next-generation products will have more options on Silicon, board and chassis design, and will continue to see benefits commonly associated with industry-standard x86 processors and boards including streamlined fanless design capability, a large software ecosystem and fast time-to-market.  

VIA Technologies
Fremont, CA.
(510) 683-3300.
[www.viaembedded.com].