Tjmax

[Abdallah121]

My laptop is an HP ZBook 15 G3 with a Core i7 6th Gen HQ processor. Sometimes, when I turn it on or wake it up from sleep mode, the fans start running at maximum speed, even though nothing is running in the background, and the system is idle. I noticed that the temperature readings show that Tjmax is high, as shown in the attached image. What could be the reason for this?

[n_scott_pearson]

Tjmax is an representation for the Maximum Junction Temperature, which is the highest sustainable temperature that the transistors can handle without degradation. The Tjmax temperature is somewhere in the vicinity of 100c. In Mobile processors, they are bined (based upon clock speed) so that Tjmax is right at 100c. In Desktop processors, the Tjmax can vary somewhat, but the average is still in the vicinity of 100c. Now, when the temperature reaches Tjmax, the processor begins actively throttling its performance to pull the temperature down.

The processors contains some number of Digital Thermal Sensors (DTS). There is one DTS per Core, accessible by driver-level software, that measures the temperature as the point in the Core that gets the hottest. There are other DTS measuring the temperatures as points in the UnCore and Graphics portions of the processor, but these DTS are not accessible by software. They are used, along with the Core DTS, to determine the processor's Package Temperature (no, the equation they use in each processor design is not published).

Now, the DTS circuitry exposes how far the temperature currently is from Tjmax. That is, when the DTS temperature is right at Tjmax, a reading of 0 (zero) will be seen. As the reading number rises, the temperature at the DTS is getting lower and lower. You might ask, what happens when the temperature is above Tjmax? Well, the DTS will continue to expose a reading of 0 (zero). A bit, called the PROCHOT bit, will be turned on to indicate the processor is at or above Tjmax.

At some number of degrees above Tjmax (also not publicly documented), the processor will shut itself off to protect itself from silicon meltdown. This action, and the temperature at this point, is called THERMTRIP. The fact that the processor hit THERMTRIP and powered itself off is stored across the power loss and the BIOS can thus determine that it occurred and put an entry is its event log and indicate to the user in some way (beep, show message on screen, etc.) that it had occurred.

Ok, all that said, you obviously don't want the temperatures getting up to Tjmax - or, if they do, that it stays there as little time as possible. Let's face it, staying at high temperature levels for long periods of time is going to (eventually) cause silicon degradation and shorten the lifetime of the processor. Intel designs to ensure that, provided temperatures stay at or below Tjmax, they will outlast their warranted lifetime. Of course, your processor is old enough that it is well beyond its warranted lifetime - but still alive and kicking because of this design.

Having said all that, let's talk about cooling. The motherboard is responsible for the management of the processor's cooling, and has a subsystem that is implementing fan/pump speed control for the processor's cooling subsystem and, in desktop systems, across the entire chassis. In Desktop systems, the chassis fans are responsible for pulling cooler air into the chassis and exhausting hotter air from the chassis. Now, if the air being used to cool the processor is hotter, the efficiency of the processor's cooling subsystem will be retarded. Read this as, when the temperature of the air in the chassis is higher, the processor cooling fan(s) (or pump) must have to work harder (faster) to provide the same level of cooling. There is thus a symbiotic relationship between the chassis fan control and the processor fan control; they must work together to provide the cooling necessary overall.

As I said, you don't want to have any of the processor Cores reaching Tjmax for any appreciable amount of time. You thus want the processor fans running full out if they get this hot. Intel used to spec a recommended temperature above which the fan(s) should be at full speed. This temperature, called the Control Temperature (Tcontrol for short), used to be provided in a register inside the processor. Typically, this temperature was 10c-15c degrees below Tjmax. Intel no longer provides this setpoint, but you can approximate it yourself. For modern processors, I typically set this value to ~10c degrees below Tjmax. This is done in the configuration of the fan speed control subsystem, which is provided in the BIOS Setup program. For Desktop systems with chassis fans, I usually set the maximum temperature to ~65c.

Finally, you asked why you were seeing temperature rising when you aren't doing anything. This is occurring because Windows, when it sees that the system is idle, will run maintenance tasks (Windows Update, Storage Sense, Disk Cleanup, etc.) in the background.

Clear as mud?

...S