[lm-sensors] About the "CPU temp" output of ADM1025A chip

[Rudolf Marek]

Hi,

> I have an Intel D850MV motherboard. There is an ADM1025A sensor chip on
> the motherboard. Also there is an LPC47M142 I/O chip on the motherboard.
> My CPU is a Pentium 4 CPU, 1.7GHz.

And here is your motherboard
ftp://download.intel.com/design/motherbd/mv/A6514501.pdf

technical guide ;)


> The following is a sample output for my lm-sensors software.
> ---------------------------------
> $ sensors
> adm1025-i2c-0-2d
> Adapter: SMBus I801 adapter at efa0
>  
> +2.5V:     +2.49 V  (min =  +2.25 V, max =  +2.75 V)
> VCore:     +1.68 V  (min =  +1.66 V, max =  +1.84 V)
> +3.3V:     +3.30 V  (min =  +2.97 V, max =  +3.63 V)
> +5V:       +5.13 V  (min =  +4.50 V, max =  +5.50 V)
> VCC:       +3.30 V  (min =  +2.97 V, max =  +3.63 V)
> CPU Temp:  +35.0°C  (low  =   +10°C, high =   +60°C)
> M/B Temp:  +34.0°C  (low  =   +10°C, high =   +45°C)
> vid:      +1.750 V  (VRM Version 9.0)
>  
> smsc47m1-isa-0800
> Adapter: ISA adapter
> fan1:     3233 RPM  (min =  640 RPM, div = 8)
> fan2:        0 RPM  (min =  640 RPM, div = 8)
> --------------------------------------------------------------
>  
> I am interested in the temperature readings from the sensor chip. There
> are two temperature readings: CPU temp and M/B temp. According the
> ADM1025A datasheet, I think the M/B temp is the on-chip sensor in the
> ADM1025A chip, and the CPU temp is the External Temperature Measurement
> with Remote Diode.
>  
> My question is: Where is this remote diode located? Is this remote diode
> the internal thermal diode inside the Pentium 4 CPU chip, or a
> thermal sensor provided by the motherboard which is attached to the CPU
> chip very closely?

According to technical docs from intel, it is Remote thermal diode sensing for
direct monitoring of processor temperature

inside the CPU.

> I downloaded the manual of Intel D850MV motherboard from Intel's
> website, but could not get the answer to my question.

I did ;)

> I opened the computer case, removed the CPU fan, and saw the CPU and the
> ADM1025A chip sit together on the motherboard, but I could not see from
> the circuit board whether the PIN 9 and 10 (the two pins connected to
> the remote diode) of ADM1025A chip are connected to the internal thermal
> diode inside the Pentium 4 CPU chip.

Well there is a lot of layers PCB so it is impossible to track it down.

The manual also suggest that the fan speeds are somehow controlled. This is
quite interesting. Page 43 Quote:

The temperature sensors that control the fans are initialized by the BIOS at
power-up to turn on only when the sensor reaches 36 C (96.8 F). The fans switch
off when their respective sensors
cool down to 31 C (87.8 F). This prevents the fans from turning off and on when
the ambient air around the sensor fluctuates around 35­36 C. When the fans
switch on, they may appear to rotate
slowly because the fan's duty cycle starts at 70 percent and rises to 100
percent when the sensor reaches 46 C (114.8 F).

Datasheet is here http://www.datasheets.org.uk/datasheet.php?article=2040255

It seems it somehow changes the PWM via software (system management interrupt)

I hope it helps,

Regards
Rudolf