Intel's Ultra-Portable Atom: Unveiled
Power and the Platform
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As a result of Intel’s delicate balancing between power and performance, Atom is the company’s most energy-efficient design to date. Its thermal design power falls between .65 and 2.4 watts (contingent largely on the operating frequency of the SKU in question) and Intel says average power ranges from 160 to 220 mW. At idle, you’ll see numbers closer to 100 mW.
The decisions to incorporate an in-order execution engine and simplified scheduler contribute greatly to Atom’s effective power management. However, the CPU also includes a handful of specific power-saving technologies that keep consumption to a minimum.
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One of the most effective is a new C6 power state, first introduced as a feature of the mobile Penryn family. You’re probably familiar with some of the other C-states, such as C1E, where the core clock is turned off, the L1 caches are flushed, voltage is lowered, and power draw is reduced considerably. C6 is significantly more aggressive, shutting off the core clock, PLLs, and caches. Voltage consequently drops to near-zero and consumption is kept to an absolute minimum. Information normally stored in the registers (what Intel calls the architectural state) is saved to a small on-chip buffer. Waking back up from C6 happens, in turn, very quickly. For all but one of the launch SKUs, idle power in the C6 state is 100 mW.
The C6 state is enabled, in part, by a split power plane. Under full utilization, all 203 of Atom’s I/O pins are active across both planes. In its C6 state, 21 pins continue receiving power from the 1.05V VRM over the one plane still powered on while the remaining shut off. Only the circuitry needed to wake Atom out of C6 continues getting power. Hence the low draw from an Atom processor as it idles.
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Clock gating is another important tool Intel uses to ensure only the logic gates that need power are getting it. The power-saving technique isn’t new in the synchronous circuit business - it’s been a part of Intel’s mobile processors and discrete notebook GPUs since the Pentium 4 days. Atom is simply more aggressively optimized for the feature.
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Intel’s message of energy efficiency carries over from the processor to its platform. Like the Core 2 Duo and Quad CPUs, Atom communicates over a front side bus. As a percentage of total power consumption, the I/O pins on Intel’s FSB seem excessively high because the processor is so efficient relative to past designs.
The chipset on the other end of Atom’s front side bus is actually a single component consisting of functionality normally found on north and south bridges. Internally referred to as Poulsbo, the chip includes a capable integrated graphics engine and the I/O expansion of a mainstream desktop.
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Built from scratch like the Atom processor, Intel started development of Poulsbo in 2005. The company has been working with its software partners since then to ensure the most popular video players and codecs are all compatible with the chipset’s built-in hardware video acceleration. Pankaj Kedia points out that when he hops on YouTube with an iPhone, only the videos encoded in H.264 are available through Apple’s widget. Centrino Atom aims to change the need for “workarounds” on mobile devices. When you get online with a MID, you won’t be constrained by the proprietary nature of many of today’s solutions.
Integrated graphics is another important part of the Poulsbo story. The Centrino Atom platform was designed to accommodate a number of modern operating systems. And while Intel has standardized on the open source software projects hosted at moblin.org, you can expect at least a handful of OEMs to tap Vista as their operating system of choice. As a result, support for DirectX 9 and OpenGL are equally valuable to the core logic. Don’t expect breathtaking 3D performance from the core - Intel rates the theoretical maximum fill rate at 400 Mpixels/s - but that should be sufficient for the types of apps expected to run on MIDs.
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Poulsbo’s I/O consists of two PCI Express x1 ports, eight USB 2.0 host ports (one of which can be configured as a client port), three SDIO/MMC ports, support for up to 1GB of DDR2 memory, and an parallel ATA IDE controller. The idea, of course, is to facilitate PC-like capabilities in a miniaturized form factor. When OEMs start adding WiMAX and 3G support, they’ll use the System Controller Hub’s external connectivity to interface with the platform.
