Calibrate Your CPU Temp Reporting

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CALIBRATING YOUR CPU THERMAL DIODE OUTPUT

To ensure accurate temperature reports, you need to check for and correct two big issues in your readings: Linearity and Offset.

Essentially, if your temp readings are linear they may be wrong, but at least they are always wrong or offset by the same amount. Offset is pretty easy to fix, but non-linearity is much trickier. So tricky, in fact, that non-linearity is probably not correctable.

What you need for testing:

1. A CPU/motherboard combination that allows for reading from the CPU diode. If you are running an Intel CPU made in the past 5 years, you've got one. If you are running an AMD system, its a bit more complicated: First of all, it has to be an Athlon XP or newer to have a diode, and secondly, your motherboard has to read from the diode. Do not assume that your brand new motherboard does.Quite a few of the Socket A motherboards available today still read from an in-socket thermistor rather than the CPU diode. The trouble with reading from the socket is that its separation from the core ensures temperature compression and non-linearity especially as temperature rises.

A simple way to tell if your socket A board is reading from the diode is to watch the CPU temp change when a load is suddenly added to the system. Set the update time on Motherboard Monitor (or other CPU temp monitoring utility) to 1 second, and then fire up CPUBurn or Folding@Home. If the temp doesn't jump several degrees in the first second or two, you're not reading from the diode.


An illustration of the difference in Diode/Socket temperature readings from an AMD XP CPU. Note the 5° jump in the reading from the diode in the first second of CPUBurn running, while the socket temp hasn't even begun to move

A second source of information on how your motherboard reports CPU temperature is to look up your model in Motherboard Monitor's Motherboard List. It may be that your board has the ability to read the diode, but only if you have MBM set to be reading from the correct sensor. If your motherboard does not support diode readings, you're still welcome to try this method but there are no guarantees that your invested time will amount to anything.

2. A mobo/CPU combination that allows under/overclocking. You don't need a huge swing in CPU speed to get meaningful results, but statistically, the bigger the variation in speeds you can get, the better. Under/overclocking via either FSB, multiplier, or both is fine, but do not adjust the Vcore at any point during the testing.

3. Monitoring software, such as Motherboard Monitor 5. MBM5, although no longer being updated, is still probably the best choice. The ability to apply the calibration adjustment automatically to its readings is a very convenient feature.

4. Wattage calculation software, such as CPUHeat or CPUPower. Accuracy is surprising un-important. CPU wattage varies linearly with Mhz, so the other variables in the software's calculations have no real effect on the outcomes of these tests (they get canceled out in the calculations). So as long as you use the same source for all your wattages, and don't change the Vcore, you're fine.

5. CPU stressing software. CPUBurn is default choice here: it's simple, small, and unlike Prime95 or Folding@Home, it produces consistent results.

6. Fixed fan speed on CPU heatsink, and a PSU that does not increase fan speed so much (under load) as to affect CPU temperature. Basically, the airflow/cooling conditions in the system must remain constant through all the tests. If you have a HSF that adjusts the RPM based on temperature, you will need to find a way to lock the fan at the same RPM for all the tests, otherwise non-linearity with changing temperature is assured. For motherboard controlled fans, a BIOS setting tweak may be required. For a PSUs with bottom-intake fans that ramp up fast at load, you may need to move it outside the case temporarily.

TESTING

The basis of the calculations is a series of temperature readings taken while putting the CPU under maximum load over a range of speeds. Run a series of max CPU temp tests across as wide a range of FSB speeds as you can do stably. The general method to use is:

A. Run CPUBurn until the reported CPU temp stops increasing, and then record that temperature, the ambient temperature, and the CPU speed.

B. Adjust the FSB to the next level, and repeat A. Noting the ambient temperature is important, since it is unlikely that it will remain constant throughout your testing. Assuming your CPU heatsink fan draws air into the heatsink, the best place to measure this is within 6" of the intake point.

A wide CPU speed range is more important than the number of tests, but the more tests you have the more reliable your results will be, and the easier it will be to see patterns. I made 15 different readings for my testbed, but 5 would probably be enough. For my tests I set up a little spreadsheet to keep the values organized and to do the math for me, but just recording the results on the back of an envelope would work just fine too.

C. For each of the tests calculate and/or record the following values: Temperature Rise from Ambient (CPU max temp minus the ambient temp), wattage, MHz, and °C/W (°C/W=Temp Rise/Wattage).

ANALYSIS

Part 1: Linearity

Linearity is simple to check now that you've done all that testing. Just scroll down the table looking at the °C/W results. Are they the same, or fairly close to one another? If yes, then your CPU temperature monitoring system is linear. If not, then it is not linear. If it is not linear you could, in theory, derive a mathematical equation (that's the tricky part I mentioned above) to equate your results to linear ones.

If the system is linear, go on to part 2.

Part 2: Offset

This is slightly more complicated and it involves more math.

First, the premise: Since wattage scales linearly with clock speed, so should the change in temperature (dT). Luckily we have very accurate data on what the clock speeds are, and we can use that to determine the accuracy of the temperatures. Whatever the actual wattage is, it is irrelevant. (thus sparing the TDP vs MDP issues altogether)

Here comes the math....

Pick two different sets of test results from your data table. (the wider the spread the more accurate the math) For convenience we will call them Low and High. From the data we need 4 numbers:

  • Low clock speed = LS
  • High clock speed = HS
  • Low speed dT = LT
  • High speed dT = HT

In a perfect world: HS / LS = HT / LT

In plain English, the above equation means the ratio of the high clock speed to the low clock speed is equal to the ratio of the high clockspeed's temperature change to the low clock speed's temperature change.

What you'll likely find is that the above equation doesn't actually come out equal for your numbers. Do not despair, we really didn't think it would. Since we know from Part 1 that the temperature results are linear, we know that the HT and LT are must both be offset by the same constant. Adding that to the equation we get:

HS / LS = (HT + c) / (LT + c) where c is the calibration offset. (Note that c can be positive or negative)

You can do the algebra to solve for c:

c = ((HS / LS) * Lt - HT) / (1 - (HS / LS))

I created an XLS file to solve the above formula which you can download here for your convenience: Calculate C.

You can also just start plugging in integers for it and see whether it makes the equation closer to being equal. Accuracy to 0.5° is really the very best you can hope to achieve; chances are, this is already beyond the resolution of the CPU diode and motherboard circuitry. Eventually you will find a single value for c that will make the equation true, or at least pretty close to true. That is your motherboard/CPU temperature offset. Try repeating the offset calculation with a couple of other data set pairs to confirm it. If your numbers are accurate, it should come out the same each time (or pretty close)

Perhaps an example would help. From my testbed calibrations:

LS = 1300Mhz
HS = 2300Mhz
LT = 13.5°
HT = 25°

HS / LS = HT / LT
2300 / 1300 = 25 / 13.5

1.77 = 1.85 ......Hmm....not quite right. Lets start working out "c"
1.77 = (25 + c) / (13.5 + c) ..............I'll try -2° as a first guess
1.77 = (25 - 2) / (13.5 - 2)
1.77 = 2 ....................drat, the difference is bigger now, I must have gone the wrong way. Lets try +2°
1.77 = (25 + 2) / (13.5 + 2)
1.77 = 1.74 .........................pretty close, but I went a bit past it, I'll try +1.5°
1.77 = (25 + 1.5) / (13.5 + 1.5)
1.77 = 1.77.................. We have a winner! My CPU diode is off by 1.5°.

For my testing I set up a spreadsheet matrix to solve for c across all 15 sets of test data, then provide an average for c for all the pairs that produced valid answers. The average came out to something like 1.53C°. I settled on 1.5°C and entered this value into MBM to have it adjust the CPU readings automatically to ensure correct readings for my heatsink tests.


My AMD XP CPU test rig proved to be off by 1.5°.

SPCR reviewers have tried this method on a couple of different heatsink testbeds, one Socket A, and the other P4, and it appears to work as described here. Anyone who takes the time to complete the process is welcome to post their results. Perhaps with enough results we can look for patterns in the inaccuracies among CPU and motherboard models and makes.

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Discuss this article in the SPCR Forum.



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