Athlon 64 for Quiet Power

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How does AMD define TDP?

“Thermal Design Power (TDP) is measured under the conditions of TCASE Max, IDD Max, and VDD=VID_VDD, and include all power dissipated on-die from VDD, VDDIO, VLDT, VTT, and VDDA.”

This means that TDP, as defined by AMD, is measured at the maximum current the CPU can draw, at the default voltage, under the worst-case temperature conditions. This is the maximum power that the CPU can possibly dissipate. Intel, however, has a different definition.

How does Intel define TDP?

From the Intel Datasheet for Northwood CPUs:

“The numbers in this column reflect Intel’s recommended design point and are not indicative of the maximum power the processor can dissipate under worst case conditions.”

And from Intel’s datasheet for Prescott CPUs:

“Thermal Design Power (TDP) should be used for processor thermal solution design targets. The TDP is not the maximum power that the processor can dissipate.”

And the most telling quote of all, contained in both documents:

“Analysis indicates that real applications are unlikely to cause the processor to consume maximum power dissipation for sustained periods of time. Intel recommends that complete thermal solution designs target the Thermal Design Power (TDP) indicated in Table 26 instead of the maximum processor power consumption. The Thermal Monitor feature is intended to help protect the processor in the unlikely event that an application exceeds the TDP recommendation for a sustained period of time.”

What this means is that Intel’s TDP is actually lower than the maximum power dissipation of the processor (and as you’ll see later, it can be significantly lower). This is in stark contrast to AMD’s TDP numbers, which are higher than the respective processor’s maximum power dissipation.

So what is the actual maximum power consumption of these CPUs?

Unfortunately, that’s a hard thing to determine. Fortunately, there have been some recent attempts to do just that.

This is a list of estimated maximum power consumption for Intel CPUs (calculated from Intel datasheets): On average, these numbers are roughly 10~15% higher than Intel's TDP. Here's the power table again, this time with the Maximum Power from the CPU Heat website linked above added beside the TDP numbers for the P4s.

TDP & Max Power for P4
Northwood P4
Prescott P4
89 / 103
100 / 115

This is an article (in German) comparing AC power draw of identically configured systems with different CPUs (thanks to jojo4u for the link):

The components they used in all these comparisons were the same, except for the motherboard, which naturally could not be the same:

  • 400-Watt PSU
  • Asus GeForce FX 5900 Ultra VGA
  • IBM 40 GB Desktar 120GXP HDD
  • TwinX1024(RE)-3200LL memory
  • Pentium-4 Asus P4C800-E Deluxe motherboard
  • AMD Athlon 64 MSI K8T Neo-FIS2R motherboard
  • Athlon 64 FX Asus SK8V motherboard
  • Athlon XP Asus nForce 2 400 Ultra motherboard

Some of the most telling charts from this excellent article are shown below. The first is the AC consumption of the various systems at idle. Note the extremely low power of the A64 systems running Cool 'n' Quiet (discussed in detail later)..

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Here is the maximum AC power consumption while running [email protected] Because the load of this program is almost entirely on the CPU, it best shows the power consumption differences between the various CPUs.

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The data above paints a very interesting picture.

Intel is listing TDP numbers that are significantly lower than the actual maximum power draw of their CPUs. They are then relying on the fact that most applications barely use the CPU, assuming that it will remain idle most of the time. In the case that an application does max out the CPU for any period of time, Intel relies on their “Thermal Monitor” to automatically slow down the CPU when it becomes too hot to protect it from overheating.

AMD, on the other hand, lists TDP numbers that are significantly higher than the maximum power draw of their CPUs. They also have listed the SAME TDP for every desktop Athlon 64 so far, and I have little reason to believe that future Athlon 64s will have a higher listed TDP (at least for the near future). We still don't have definitive information about the exact power dissipation of each of the Athlon 64 processors, but it is clear that other than the fastest clock models, it is far below the 89W TDP cited by AMD.

It is impossible to deny that the Athlon 64 dissipates considerably less power than even a Northwood Pentium 4, while the Prescott Pentium 4 has a power consumption that challenges even noise-indifferent builders to cool them effectively.

My personal experiences have also shown that the Athlon 64 is a very easy CPU to cool. I’ve used two Athlon 64 3000+ CPUs, one from a very early batch (when the 3000+ was first released) and one from a newer batch (currently in my computer). The first CPU would undervolt to 1.35V (from a default of 1.5V) and my current CPU will under-volt to at least 1.3V (I haven’t tried to go lower). The CPUs were 100% stable at these voltages.

Currently I’m cooling my CPU with a Zalman 7000AlCu, modified with a 92mm Panaflo L1A. With the L1A running at 5V, my CPU at 1.3V, and a case temperature of 27C, my CPU is running at 38C under full load ([email protected]). That’s an ice cold CPU load temperature, and all I have is a 5V Panaflo L1A fan which is virtually inaudible inside my case.

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