Calibrate Your CPU Temp Reporting

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October 6, 2004 by Russ Kinder

In the quest for a quieter PC, most people quickly discover that heat is the fundamental cause of noise problems. Every source of noise in the computer, with the exception of the hard drives, is a fan that is there to deal with heat. Because of this, the quest for PC silence hinges on understanding thermal issues.

The single greatest source of heat is the CPU, which is also one of the most heat-sensitive components in the machine and often the most expensive. Unfortunatly, our understanding of the CPU heat is limited by how the temperature is reported. The CPU temperature reporting mechanisms incorporated into current processors and motherboards are incredibly inaccurate, with results that can vary by 10°C or more. The causes of the inaccuracies are complex, but correcting them to a more reasonable margin of error is not terribly difficult.

The single greatest source of heat in a PC is the CPU.

Before we get into correcting the CPU temperature reporting inaccuracies, a few observations:

1. Most PC users do not need to worry about precise CPU temps. It's an odd disclaimer for an article about how to correct CPU temps, but the oft-quoted axiom of "If your PC is stable, your temps are fine" is true. As long as your system is stable, you really don't need to worry about whether your reported CPU temperatures are correct; running at 50°C or 60°C makes no difference to any measurable performance benchmark. Computer tinkerers and power enthusiasts have become temperature obsessed, but in reality, rounding your CPU temps off to the nearest 10°C is all the accuracy 99% of users need.

So why write this? Well, the temperature obsession isn't likely to go away anytime soon, so at the very least we can help clear some of the fog of mis-information surrounding CPU temps. Secondly, there will always be a small group of people who want to know precisely what their machine is doing, so that they can tread more closely to that fine line at the limit of the performance envelope. A third even smaller group is people for whom having accurate temperature data is an integral part of their work. If you want to compare the efficiency of different cooling systems, or benchmark your current system, you need to have a higher level of accuracy. This group obviously includes technical reviewers such as the SPCR staff.

2. What this article describes as "accurate" is a matter of debate. This methodology attempts to remove the motherboard-induced variations to the CPU on-die thermal diode readings. However, the accuracy of the temperature output from the CPU thermal diode is a completely different issue. The P4 is known to report temperatures well below the hottest portions of the core, and to a lesser extant this is true of the AMD processors as well. The nice aspect of reading directly from the CPU diode is that at least you know that everyone else's readings are equally inaccurate. (On a philosophical level, if everyone is equally wrong, does that make everyone right?)

Simulated temperature plot of a P4 processor. The arrow on the upper left marks the location of the P4's thermal diode, while the arrow on the lower right is the hottest portion of the die. The image, many times larger than the actual size of a P4 core, is from Differentiating PCs in a 'Toaster World' by Robin Getz of Analog Devices, published in the April 2002 edition of the Intel Developer Update Magazine.

3. The calibration described here only works for a specific CPU in a specific motherboard. Swap the CPU after you calibrate the thermal reporting system, and it won't be accurate anymore. This is because there is enough variance between different samples of the same model processor that their heat output will not be the same. Change the motherboard, and it won't be accurate, again because of sample variances. Changing to a different model virtually guarantees a different temp result. Even removing and replacing the CPU from the socket may effect the results, due to the potential for altered resistance between the thermal diode output pins and the motherboard socket. Changing heatsinks will have little effect, as long as the heatsink is installed the same way with the same thermal interface material. (There is some debate about variability of TIM applications and the effects on CPU temp, but it's generally true that over time, any differences become nullified.)

4. This article contains very little original thought. Nearly everything in here is based on the work of someone else, or upon commonly understood principles that have been explained elsewhere, often in much greater detail. This methodology is a distillation of these ideas into a single convenient place. If the subject interests you, I encourage you to do more reading elsewhere. Some excellent places to start are be the articles by Derek Peak (aka pHaestus) at Reading from AMD's Thermal Diode, and Fun with AMD Diodes: CPU Mutilation, calibration and testing. Another important source is the useful, if obscure Calibrating the Internal Thermal Diode in an Intel PIII CPU at Arctic Silver.

Ok, for those of you brave enough to proceed...

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