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AMD FX CPU Offset setting ???

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AMD FX CPU Offset setting ???

Postby CombatDoc » Sun Jul 22, 2012 2:22 am

Is there a resource that shows what the offset should be for my CPU ?
CoreTemp seems to be showing a lower temperature reading for my CPU than what I was expecting, I just built this system and am still learning...

AMD FX-6200 @ 3.8 GHz (4.1 GHz Turbo) - Stock heatsink for now.
ASUS Sabertooth 990FX
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Gigabyte GeForce GTX 560
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Re: AMD FX CPU Offset setting ???

Postby The Coolest » Sun Aug 05, 2012 11:19 am

From my tests, the offset should be around 15c.
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Re: AMD FX CPU Offset setting ???

Postby CombatDoc » Sun Aug 05, 2012 11:40 am

OK, thanx, but my temps are still running between 75- 105 degrees Fahrenheit (not sure of the Celsius temps as I have it set to F)
But that just seems a bit low on the stock heatsink... But then again, I'm not overclocking it either...
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Re: AMD FX CPU Offset setting ???

Postby The Coolest » Sun Aug 05, 2012 11:45 am

I'm not sure how to calculate the correct offset in F since it is not in a linear ratio to Celsius.
The processor reports temps in Celsius, and the offset should be 15 Celsius.
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Re: AMD FX CPU Offset setting ???

Postby Jorge » Mon Jan 07, 2013 12:06 am

It doesn't appear to me that you can have a fixed offset for the FX series processors based on the information that AMD has provided in doc #42301 Rev. 3.12 which explains that the value generated (on it's own scale), is not the actual temp as it is based on the difference from the maximum Tctl temp which can vary based on FX CPU model in addition to the value being generated being on an unknown scale which is not necessarily linear to the Celsius scale.

Here's the excerpt from the AMD document:

"Page 219, section 2.11.1

Tctl is a processor temperature control value used for processor thermal management. Tctl is accessible
through D18F3xA4[CurTmp]. Tctl is a temperature on its own scale aligned to the processors cooling requirements.
Therefore Tctl does not represent a temperature which could be measured on the die or the case of the
processor. Instead, it specifies the processor temperature relative to the maximum operating temperature,
42301 Rev 3.12 - October 11, 2012 BKDG for AMD Family 15h Models 00h-0Fh Processors

...continued on Page 220

Tctl,max. Tctl,max is specified in the power and thermal data sheet. Tctl is defined as follows for all parts:

A: For Tctl = Tctl_max to 255.875: the temperature of the part is [Tctl - Tctl_max] over the maximum operating
temperature. The processor may take corrective actions that affects performance, such as HTC, to support
the return to Tctl range A.

B: For Tctl = 0 to Tctl_max - 0.125: the temperature of the part is [Tctl_max - Tctl] under the maximum operating
temperature."

It seems like quite a dilemma to get an accurate temp readout on the FX series CPUs. :(
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Re: AMD FX CPU Offset setting ???

Postby Jorge » Mon Jan 14, 2013 1:15 am

I thought "The Coolest" may have some information to offer on the FX processor core thermal readout but I don't see any responses from him in the forum for a week? Maybe he's busy with something else???
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Re: AMD FX CPU Offset setting ???

Postby The Coolest » Fri Jan 18, 2013 12:32 am

Hi Jorge,

Yes, I'm aware of this document. As there is no other DTS in recent AMD chips (might have changed with their latest APU, still requires more testing) there is no other way of getting core temperature from the CPU.
The scale is not absolute, that is true, but it is usually around 15C under-reporting for most processors.
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Re: AMD FX CPU Offset setting ???

Postby Jorge » Sat Jan 19, 2013 3:55 am

Based on the information that AMD has provided to me on the FX processors, I don't believe that you can use an offset and have accurate temps at the important temp range where the CPU is under heavy load.

AMD engineers have publicly stated that the FX series CPU temps are NOT accurate in the low range but that they are accurate from about 40C upwards. If that is true, then using an offset to make the low temps "look normal", which is totally unimportant, would inflate the true temps from 40C upwards. Obviously this would not be good as enthusiasts would think their CPU is running too hot, when in fact it is not. They could also conclude their HSF is insufficient, when it is perfectly fine. Many enthusiasts do not understand that the only CPU temps that really count are the full load/max operating temp, so the 15 degree offset could be a real issue?

In addition AMD is using different max 24/7 temps for the 4xxx, 6xxx and 8xxx series processors and they could also be using higher/different TjMAX limits so it might become necessary to list TjMAX by the CPU I.D. in Core Temp, if this is the case.

If AMD provides the additional documentation that I have requested I may be able to shed more light on this subject. For now there are a lot of unknowns including the true CPU core temps on the FX series processors. :(
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Re: AMD FX CPU Offset setting ???

Postby The Coolest » Sat Jan 19, 2013 12:14 pm

From my own testing, the sensor is still inaccurate at high loads as well and requires an offset then.
The TjMax is listed for any CPU that supports it.
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Re: AMD FX CPU Offset setting ???

Postby Jorge » Sat Jan 19, 2013 5:05 pm

How are you determining that the FX processor core temp readings are inaccurate at "higher temps" when you have no physical means to measure the core temps?

As I suggested before, the fact that AMD's internal temp sensor does not use the same scale as the celsius scale and it probably does not use the same resolution, since they use a monitor readout range from 0-255, then without knowing the resolution and appropriate formula to convert from the AMD readout to Celsius, - if this is even possible, none of the temp data is going to be accurate.

I'd be far more inclined to believe AMD engineers that the temps are accurate from 40C up as this is the only range that matters and what technically astute enthusiasts would want to monitor. It's quite possible that AMD has decided to forgo a wide temp range readout for a more precise and accurate temp reading in the critical max load temp range?

Does Core Temp determine the TjMAX based on the CPU I.D.? If not then the listed TjMAX temp in Core Temp may be incorrect? As an example the latest version of Core Temp lists the TjMAX for the FX-8350 as 70C, the same as it lists for most FX processors that I have seen listed online, even though the temps AMD list are not the same for the various FX processors. AMD states that the max temp for the FX-8350 is 60C. Adding 15C as an offset would even exceed what Core Temp list as the 70C max temp. so obviously the numbers don't work our at all.

Please understand that my goal is to obtain accurate CPU core temps and not to find fault with the Core Temp applet, which I have used and recommended for years. I think however with the changes in the FX processor temp readout, the previous methodology to determine core temps may not be valid and can lead to very inaccurate and misleading data that confuses and misleads PC enthusiasts. Hopefully with more technical documentation from AMD we can determine exactly what the status is.
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Re: AMD FX CPU Offset setting ???

Postby The Coolest » Sun Jan 20, 2013 11:34 am

You are right in saying that it's impossible to get an accurate absolute temperature from AMD's DTS, no matter how you spin it.
The TjMax is recorded in a field in the NB, and Core Temp reports the value that is stored in said field. This value may vary between processors.
It's similar to the system Intel uses, the same processor model may have different TjMax values.
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Re: AMD FX CPU Offset setting ???

Postby Jorge » Sun Jan 20, 2013 4:56 pm

I am aware that Intel does use different TjMAX values for the same model processors, which is probably due to higher and inconsistent electrical leakage. I do not believe AMD is using different TjMAX for the same processors. They may however use a different TjMAX for different model FX processors, which is why I asked if Core Temp listed the TjMAX based on the CPU I.D.
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Re: AMD FX CPU Offset setting ???

Postby The Coolest » Sun Jan 20, 2013 8:16 pm

You don't have to believe it, it's mentioned in one of AMDs own technical documents.
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Re: AMD FX CPU Offset setting ???

Postby Jorge » Mon Jan 21, 2013 7:03 pm

The Coolest wrote:You don't have to believe it, it's mentioned in one of AMDs own technical documents.


It's not a matter of "not believing", I was not aware of this and have never seen AMD indicate that they use different TjMAX for the same part number CPU. I know they use different Vcore's, but I was not aware of them using different TjMAX for the same model CPU. If you have the AMD doc number, please advise as I'd like to read up on this issue.
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Re: AMD FX CPU Offset setting ???

Postby The Coolest » Mon Jan 21, 2013 7:25 pm

Here it is:
Image

BIOS and Kernel Developer’s Guide (BKDG) For AMD Family 10h Processors - Doc #31116
So this is not exactly a TjMax per-se, but it's close enough for our uses I would say.
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Re: AMD FX CPU Offset setting ???

Postby Jorge » Mon Jan 21, 2013 8:11 pm

Thanks for the document. Unfortunately it is not formatted so that I can read it all, but I'll try to find the original to see what it states. If you are refering to section 22:16, obviously TjMAX and HTC cut-in can be two different temps, if that is what you are referring to and of course as I already indicated AMD is using different max CPU temps for the different model FX processors, which is why I asked if Core Temp generated the indicated TjMAX based on the CPU I.D. I'll take a look at the full document.

The above information however still doesn't answer the original question, which is how to obtain accurate FX core temps... and if an offset is appropriate or not on the FX processors.
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Re: AMD FX CPU Offset setting ???

Postby The Coolest » Mon Jan 21, 2013 8:49 pm

"Core Temp" on AMD is in register F3xA4 or F3xE4, in the same document.
http://support.amd.com/us/Processor_TechDocs/31116.pdf
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Re: AMD FX CPU Offset setting ???

Postby Jorge » Mon Jan 21, 2013 9:24 pm

The Coolest wrote:"Core Temp" on AMD is in register F3xA4 or F3xE4, in the same document.
http://support.amd.com/us/Processor_TechDocs/31116.pdf


Actually on the FX processors the reading on register F3xA4 is not the core temp in Celsius, it's a "number" generated on whatever scale resolution AMD is using between 0-255.875, which may or may not be linear. This "number" which is defined as "Tctl" is not in Celsius and as doc #42301, Rev. 3.12 states, you need to subtract this number from the Tctl_max figure for the specific model CPU, (up to the Tctl_MAX temp), to determine the core temp, presumably in degrees Celsius. This was all discussed in my original post above.
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