Intel TAT / CoreTemp / IDCC all different temperatures....

[unclewebb]

I originally posted this at the [H]ard forum but I think it's relevant and this procedure can be used to test how accurately programs that people take for granted actually track the DTS.

I use CrystalCPUID to read the on chip model specific register ( MSR ) that contains the raw DTS information. The 7 bits of temperature data are located in bits [22..16] of MSR 0x19C in all Core based processors.

Here's my E6400 ( 3600 MHz 1.504 volts ) at idle in a 22C room with the Tuniq Tower fan at its lowest speed of 1150 rpm.



When you enter 0x19C in the MSR Number box and click on RDMSR ( Read MSR ) you will get the value of the DTS. In this example bits [22..16] corresponds to 36 hexadecimal which is ( 3 X 16 + 6 ) = 54 in decimal. This means at idle I am 54C degrees away from TjMax which is the Intel documented highest safe operating temperature for a C2D cpu.

If RMClock is using the DTS and calculates the absolute core temperature to be 33C then it must be assuming that my TjMax = 33C + 54C = 87C. That's certainly possible but now let's check it again at full Orthos load.



At 3600 MHz and 1.480 volts it is happily running Orthos within 18C of TjMax. RMClock reports a core temperature of 63C so now it must be assuming that TjMax = 63C + 18C = 81C.

The maximum junction temperature of your processor ( TjMax ) is a fixed value, set at the factory and doesn't change yet RMClock is using two different values depending on whether the processor is hot or cold. That's not right. I also performed this test with TAT and the calculated TjMax could change from day to day which is also impossible.

Now you can run CoreTemp 0.95 or SpeedFan 4.32 and you will see that both of these programs track the on chip digital thermal sensors exactly. CoreTemp may be guessing wrong at TjMax but it can be used to report the DTS directly. When it is set like that, it reports the temperature data in processor register 0x19C exactly.

This is the only Intel documented core temperature information for a Core 2 Duo desktop processor.

Edit: On the main page of CrystalCPUID you can set it to read any of your cores. TAT is using different TjMax values at idle, at full load and even CPU 0 and CPU 1 are not consistent.