Real Temp - New temp program for Intel Core processors

[unclewebb]

The first Core 2 I ever owned was an E6400 B2 stepping. I definitely did some hands on testing with my temperature gun on that one.
Happy memories. That one could run Prime stable at 3640 MHz for enough hours to get me into the [H]ard OCP database at a very respectable level for an air cooled processor. Of course, I had to drag my computer out to the garage in the middle of winter for that run but that's one of the perks of living in Canada.

Here's the history of TJMax. When Core 2 CPUs were first introduced, there was a bit in the CPU that software could read to determine what TJMax the CPU was, supposedly. If this magic bit was a 1 then the CPU was TJMax = 100C and if it was a 0 then it had to be TJMax = 85C or maybe that was reversed, I can't remember. It had to be one or the other though, based on this bit. This information was never in any Core 2 documentation. It instead came from some old Pentium 4 documentation that had absolutely nothing to do with the new Core 2 CPUs. One programmer made an assumption and then another one followed and another and etc.

Intel's original IDF presentation brought up the fact that this bit was not defined for Core 2 Desktop CPUs but I guess the other programmers decided to ignore this and they still continue to use TJMax = 85C to this day for the original Core 2 CPUs.



The question becomes, why didn't Intel come forward and set the record straight about TJMax when these CPUs were first released. If TJMax is closest to 90C, like I claim, and software assumes TJMax = 85C then all software will report that these CPUs are running 5C cooler than they actually are. That's a convenient way to create a cool running CPU. If the truth is that the average CPU has a TJMax closer to 90C and not 85C then if software was updated, everyone's CPU would be running 5C hotter than originally thought. Customers wouldn't be happy with that and RMA returns would likely go up.

I can certainly understand the motivation to remain as quiet as possible about this issue. Intel has nothing to gain so why bother.

I will continue to use TJMax = 90C on these early CPUs and it makes me happy to know that no one else has the guts to tell it like it is.

RealTemp originally followed the herd and used TJMax = 85C just like everyone else. By using this number, when the surface temperature of the IHS was 80C RealTemp would also report 80C. Everything looked good.

rge and his testing and papers about heat transfer in CPUs showed me the light. If the surface temperature of the IHS is 80C then the hottest spot on the core is not the same, it has to be higher. Heat dissipates very rapidly over a small distance in CPUs. The docs and rge's testing with a calibrated sensor showed me that the hottest spot on the core is approximately 5C hotter than the IHS surface temperature that I was measuring.

I hated to admit that I was wrong but I was wrong. This proved that TJMax couldn't be 85C. It had to be higher and for this E6400 B2, it was likely very close to 90C. That's why I use that number and why I have decided to go against Intel's PR news release about TJMax where they introduced the term TJ Target which has only confused users even more.

I've tested more CPUs since then and nothing has changed my mind. Intel says they raised TJMax 10C when they introduced the new G0 CPUs to save some money on heatsink costs. They also said that TJMax = 100C for the 45nm E8000 series of CPUs and everyone is happy with that number. When I use TJMax = 100C for a 45nm CPU, the core temperature reports 5C higher than the IHS surface temperature like it should. To get that same relationship with an early B2 CPU, I need to use TJMax = 90C. Using the 80C TJ Target number they released as TJMax would mean that the IHS surface temperature is hotter than the hottest spot on the core which goes against the laws of physics. The source of the heat, the core, can't possibly be cooler than the IHS surface temperature. When my testing was brought to Intel's attention, they chose to quietly ignore it. No surprise.

Kind of a long winded answer to your question. Thanks for bringing this up. Lots of users assume that TJ Target and actual TJMax are the same but they are not.

KURTZ: I just noticed in your screen shot that SpeedFan reports 50C for the CPU temperature. It's impossible for the core temperature to be less than this number. Only RealTemp reports the cores higher than the approximated Tcase temperature which is how it should be.