Running Fast, Keeping Cool

PC buffs who are always hungry for the fastest systems and latest gadgets are often called “power users.” But PC processors are literally power users — and their appetite for electric power not only challenges the creators of unplugged notebook computers, but affects desktop design as well.

CPUs’ core voltages, heat and power dissipation, and temperature ratings are increasingly important aspects of PC architecture. And while Intel and AMD are busy increasing their processors’ clock speeds and adding new features, they’re also tweaking the chips’ internal power and cooling requirements. You’ve heard statements such as, “The Pentium 4 ‘Northwood’ runs cooler than the older ‘Willamette’ core”? There’s a lot of science behind them.

CPU Core Voltages

The core voltage of a processor denotes the power (technically, the electric potential) it needs to run at its default clock speed. This figure, different for different manufacturers’ CPU families, is expressed in volts, such as a core voltage of 1.5V for Intel’s 2.4GHz Pentium 4.

There are many factors that contribute to a given processor’s core voltage rating, such as speed, die process, and even platform or motherboard specifications. The most common voltage shift is an upward one, as a CPU model climbs up the gigahertz ladder: When the Pentium 4 reached 2.6GHz last year, Intel nudged up the core voltage rating for that and faster models to 1.525V. AMD did the same when moving from the Athlon XP 1900+ (1.5V) to the Athlon XP 2000+ (1.6V) and then the 2200+ (1.65V).

While this sounds like power inflation, you should know that older CPUs were even worse energy offenders: The original Pentium III gobbled up a sizzling 2.8V, and even the relatively cool-running — for the time — Slot 1 Celeron and original Pentium III required 2.0V.

One reason is that transitioning a processor core to a smaller die size, packing transistors tighter together, can decrease the CPU’s power requirements. Intel’s 2.0GHz Pentium 4 Willamette, an 0.18-micron-process chip, took 1.75V, but the 0.13-micron-process Northwood needed only 1.5V to achieve the same clock speed. The company’s upcoming “Prescott” will use an 0.09-micron (90-nanometer) process, and is rumored to have a core voltage of only 1.2V.

The platform also has an impact on power requirements, and no better example exists than the mobile Pentium 4 (what Intel calls the Pentium 4 Processor-M). Like the desktop Northwood chips, these processors have 0.13-micron design and 512K of Level 2 cache, and run at speeds of up to 2.4GHz — but their core voltage is not 1.5V but 1.3V. It seems obvious that some Northwoods are capable of 2.4GHz speeds at lower-than-desktop core voltages, and judging by the premium price of mobile processors, these prime chips are finding their way to the Pentium 4-M bin.

(And sometimes, core voltage is dictated by platform inertia: AMD’s 1.0GHz Duron had the same 1.75V rating as the then-standard Athlon processors, even though it was far in excess of what the processor really required.)

Core Voltage and Overclocking

The hardcore PC hot-rodders known as overclockers have long relied on increasing the core voltage to wring some extra megahertz out of stock CPUs, with some according this procedure almost mythical status. The option of increasing core voltage — usually via a system BIOS menu — is a standard one with performance motherboards, and sometimes accompanied by CPU speed, DDR voltage, and AGP voltage settings as well. These are the core tools of the seasoned overclocker.

Even though a mechanic will tell you that running a car built for 87-octane regular on 93-octane premium gas won’t actually make it perform better, there are myriad theories about the benefits of boosting the core voltage of a CPU. These range from manufacturers’ “burning in” of processors (with higher voltages for extended periods) to increasing the electrical flow through the various CPU components. Depending on processor design and architecture, the exact effect of a higher core voltage is still a bit of a mystery.

One real-world effect of jacking the voltage is to smooth out the electrical-current crests and valleys inherent in any processor. Let’s say you run an Athlon XP at 1.65V; the true internal voltage may fluctuate based on many different factors, such as power-supply rail voltage (which sometimes differs from specifications) or resistance from traveling though the CPU innards. By raising the current to 1.7V or higher, these bumps in the road can be smoothed out and any lack of voltage made up, thereby increasing overall CPU efficiencies and potentially yielding higher core speeds.

Of course, this does more than raise your electric bill: It also increases thermal power requirements and puts additional strain on the CPU cooling system.

Categories: Technology