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The Basics Of Undervolting

Undervolting Your Phenom II And Core 2 Processors
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Long-time overclockers can skip this page; everyone else should be aware of some facts related to processor voltage in the context of undervolting.

Drooping

First of all, it is important to know that the processor voltage you set in the BIOS (either automatically or set by the user) will not exactly correspond to the Vcore at which the processor effectively runs. Rather, it determines the maximum processor voltage, while the effective voltage will actually be lower. It can even vary depending on the processor’s operating conditions (such as temperature), which change as the CPU goes from idle to peak load conditions or vice versa.

This behavior is intentional, because silicon conductivity increases as a CPU heats up under load. With unchanged voltage, this would result in increased current as well, causing current and temperature to wind each other up. Drooping is the mechanism that slightly reduces processor voltage at high load to ensure that the processor stays within its electrical specifications.

If you use a tool such as CPU-Z to read the effective processor voltage, and take the time to check the set voltage using CoreTemp, you’ll notice that the two values differ. The difference between set voltage and effective voltage in idle is called offset (Voffset), while the voltage difference between idle and peak load is referred to as droop (Vdroop).

Undervolt Checking

A processor reaches peak voltage when it is going from a peak load back to idle, as the voltage never goes straight from one voltage to another, but rather levels out. The overshoot is what makes the CPU reach peak voltage, which is the set voltage.

For the same reason, it is relatively easy to check whether or not an undervolted processor runs reliably at peak loads: it will apply Vdroop and reduce the operating voltage to be sure it stays below the set voltage. We used Prime95, which is great for stressing a processor. After 30 minutes of peak activity without system crashes, we can be relatively sure that the undervolted system runs reliably at peak. This typically means it will also be reliable when idle, as this mode applies slightly higher voltage. However, this does not apply to power-saving modes like Intel’s SpeedStep, which reduces clock speed (multiplier) and voltage even further. We did all undervolt testing with SpeedStep enabled, but this was not necessary for AMD’s Cool’n’Quiet, as it has a default voltage and clock speed.

As always, there are no absolutes when it comes to overclocked or undervolted settings; it’s a matter of you being willing to do extensive test runs, or to live with a certain risk that the system might not be entirely stable. We can't claim that your results will necessarily mirror ours—which we describe on the following pages—and would prefer to go with slightly more conservative settings ( meaning slightly more voltage) to be sure we’re on the safe side. Still, the power savings potential remains significant.