[Cathar]

Originally posted by joemac
Maybe the reasons the temperatures are so far off on an Intel CPU is due to Intel wanting the user to know the case temperature and not the die temperature. The case being the I.H.S. I am not sure but maybe the sensor used by motherboards to display temperatures is not in the die but on the outside measuring the air temp in the I.H.S.

The P4 diode that is used to measure the P4's temperature is located on-die, but it is off to one corner, away from the major heat producing regions of the CPU. I can't remember where I saw the picture now, but the diode is basically sitting by itself in a bit of a dead-zone with not much around it. The P4 die does have some small "gaps" due to the block layout nature of the functional units not all lining up happily all the time, and the diode sits in the middle of one of these areas, with basically no real heat-producing bits near it. It seems to be more of a measure, in the way in which it is located and implemented, of the IHS temperature in that area, and the temperature as a result of heat from hotter sections of the CPU conducting laterally through the CPU die. By itself, it is a rather poor measure of how hot the die can actually get (as indicated by the XBitLabs tests).

Found a link to the XbitLabs article here:

http://www.xbitlabs.com/articles/cpu/print/p4-temp.html

The three most relevent quotes from it are:

In order to save the CPU from damages in case of such breakdowns, the second thermal diode built into the CPU keeps track of another CPU temperature value. When this value is notched, it doesn't "slow down" the CPU anymore, but sends THERMTRIP# signal for the system to shut down.

For your information: the temperature when THERMTRIP# signal is sent equals about 135oC, according to Intel.

So all we could do, was to turn off the CPU cooler. After that, the CPU temperature rose to 94oC in less than one minute and the system shut down

So basically the second hidden diode is monitoring the peak CPU temperature, and shuts the system down when it hits 135C, yet the motherboard visible temperature diode is only reporting 94C when the system shuts down.