Undervolting Made LM75 Sensor Extremely Hot!

elasticdog · Post by **elasticdog** » Mon Feb 28, 2005 1:39 pm

See "System 1" below...running at stock speeds, it's very quiet and theoretically shouldn't draw too much power considering the processor, but I wanted to fool around with undervolting to see what would happen and to lower its power consumption even more.

I set it down to 6 x 200 FSB, and got it to boot just fine with 1.1V, however what concerned me was when I loaded up Speedfan, it showed an extremely high temperature for a sensor that was simply labeled "Temp". Looking it up under the configuration page, it said that it was the LM75 sensor, although I have no idea what that would be for my board (DFI LANParty NFII Ultra B). It's not a sensor that is monitored by any of my other programs (at least not by default). All the other temperatures were where I'd expect them to be...low.

Any ideas what that sensor would be, or why that would happen with undervolting?

ilh · Post by **ilh** » Mon Feb 28, 2005 1:55 pm

Is there any chance your CPU fan is thermally controlled, and because you were underclocking/undervolting the CPU fan ran slowly if at all, causing some other nearby components to not get much/any cooling air?

Tibors · Post by **Tibors** » Mon Feb 28, 2005 2:51 pm

Not sure that what I am going to describe next is happening to you, but it is a possibility.

One of my boards (Asus P4P800-VM) has a hardware monitor chip that is capable of reading temps of three different probes. Asus only connected two probes: the CPU temp and case temp. The third (not implemented) probe registers a complete random temp at startup. This can range anywhere from -20°C to 50°C. After startup it either follows the changes of the CPU temp or it changes in the opposite direction. This third temp is only visible in SpeedFan, not in the BIOS. After I had observed its strange behaviour for a few weeks, I just disable it.

elasticdog · Post by **elasticdog** » Mon Feb 28, 2005 4:15 pm

I'm hoping that something like your random temperature reading is happening with mine as well. I'm at work right now, but will look into the specifics of it more thoroughly once I get home. There are actually two temp sensors showing on SpeedFan that I'm not sure what they're for, but this is the only one that seemed out of whack when I started messing with the undervolting.

Does anyone know of a place you can look up and see what sensors are on specific boards?

Unfortunately, I suspect the temperatures that are reported are a bit off...for instance my Diode sensor has a weird range, and I've even seen it as low as -1°C before, which makes no sense. I can monitor System, CPU, Diode, Hard Drive, Temp, and Temp 1 (the two mystery sensors) with SpeedFan, but I've pretty much ignored everthing except for the CPU temp. I'll have to double check what shows up in BIOS, but I think it's only System and CPU. On Mother Board Monitor, I think I only see System, CPU, Diode, and Hard Drive. Same for Everest. I'll do some more fiddling around and check back...

elasticdog · Post by **elasticdog** » Tue Mar 01, 2005 11:36 am

From what I can tell, this sensor is most likely not hooked up to anything. I have not been able to find an official source of information about what sensors are on specific boards and where, but I now think the only two on my board actually hooked up to anything are the system temp (although I'm not exactly sure where it's reading that temp from), and the cpu temp.

Those are the only two shown in bios, the only two shown on the official Hardware Doctor Winbond monitoring program that came with the motherboard, and the only two that show up in Everest. MBM did select the diode temperature as well by default, but I think it might have been a sensor that was only used on the first revision of my board, and not the version I have.

I was able to find a bit of information about the sensors, specifically the LM75 sensor on the SpeedFan site under their support section, but it didn't really tell me anything pertinent to my situation. The system and cpu temps are really the only ones that are consistent and make sense, so I think it's safe for me to ignore everything else.

I may write to DFI directly to see if they have that technical information, but with what I've read, it's very common to have sensors that are not hooked up to anything and that give erroneous readouts in monitoring software. I hope that information can help some other people out...

StarfishChris · Post by **StarfishChris** » Tue Mar 01, 2005 12:52 pm

Tibors wrote:One of my boards (Asus P4P800-VM) has a hardware monitor chip that is capable of reading temps of three different probes. Asus only connected two probes: the CPU temp and case temp. The third (not implemented) probe registers a complete random temp at startup.

Ditto with my board. Speedfan says this is an LM75 too. Interestingly I had the random temperatures with version 4.17, now they are simply 0C.

elasticdog wrote:...with what I've read, it's very common to have sensors that are not hooked up to anything and that give erroneous readouts in monitoring software.

I'm quite sure it's because it's more cost-effective to do this than creating a new board design (and production line?) by removing the chip / replacing it with a different one.