[lm-sensors] pc87360 voltage reference constants

Jim Cromie jcromie at divsol.com
Sat Aug 13 05:54:46 CEST 2005

Jean Delvare wrote:

>Hi Jim,
>>Im looking to understand why my temps are so high,
>>esp in comparison to values from my soekris as obtained from
>>    VREF = 1.214  V245 = 2.450
>>    Temp 2 (status=0x81)   54 C
>>in your code,
>>        data->in_vref = (i&0x02) ? 3025 : 2966;
>>        dev_dbg(&new_client->dev, "Using %s reference voltage\n",
>>            (i&0x02) ? "external" : "internal");
>>youve got 2 magical constants,  where are they from ?
>>I cant find mention of them in the pdf,
>3.025 is 1.235 * 2.45. Page 227 of the datasheet mentions internal Vref
>= 1.235V, and at page 180, section 11.3.2 has: "Analog input voltage is
>measured relative to 2.45 * Vref".
>In section 14.4.1, external Vref is given at 1.211V, and full scale is
>given at 2.097, which happens to be 1.211 * 2.45.
>This is where the numbers are coming from. 
thanks for the (patient) orientation.  Theres a lot to miss/get in a 230 
page spec.
(Ill admit, I did a literal search.)

>Now I always found it strange
>that you could pick an external voltage with a *lower* value than the
>internale voltage, and not much lower at that. Remember that I do not
>have a PC87366 chip myself, so had no chance to measure the physical
>values at a chip's pins to confirm that the driver does the right thing.
>If you are using an external reference, and it is NOT 1.211 volt, then I
>am not surprised if your voltages are not correct.
Ahh.  I believe Ive found a bug.
the ? : assignment above chooses the larger number for the external vref,
which disagrees with your explanation, and the doc refs.

So attached patch fixes that,
and adds 3 module_params
    vrefext   allows changes to accommodate different boards
    vrefscale   allows +/-50mv around nominal 2450 mv
    vrefint    - not entirely necessary, for real tweakers.

BTW, board uses external ref, and is 1.211v (per PHK's measures)
I havent checked vrefext for myself.  Im getting numbers in close 
with PHKs.

I'll trim it back (the params) to suit.

FWIW, this module-side tweak could be avoided by hacking sensors.conf
but its ugly to work vref scaling into ALL the voltage formulas,
and thermistors too.

I gather one can use inputs as factors in computations too, but the man-page
shows no way of adding new symbolic constants. (I added this as a 
support ticket
so it doent get lost b4 the libsensors rewrite)

other thought was to add a new sysfs node - a vref-set-point.
This makes it easy to tweak the scaling/calcs done by the drivers,
so is transparent to sensors.conf, keeping it less cluttered than constants.
But perhaps this is too available for disciplined use;
the mod-param can be set once in /etc/modprobe.d/sensors, and forgotten.

>Note that the reference voltage is used to compute the voltages and the
>thermistor-based temperatures, NOT the diode-based temperatures. The TMS
>logical device is influenced by the reference voltage, but the value
>isn't used in computations AFAIR.
yes - thats what I meant by saying:
    I think my temp answers lie elsewhere (perhaps a cast to s8 for the 
register value),

BTW, you recall correctly;
for thermistors:
#define IN_FROM_REG(val,ref)            (((val) * (ref) + 128) / 256)
for diodes:
#define TEMP_FROM_REG(val)              ((val) * 1000)

but that still leave me with an impossibly high value for a low-power, 
fanless cpu

>>BTW, (since Im writing),  my patchset for pc87366 sensors-dev-attrs
>>(done against rc4-mm1), also applies (clean, iirc) to rc5-mm1, and
>>works  there.
>I'm sorry, I didn't have the time to look at it yet. I'm currently busy
>with i2c core changes and other drivers (i2c-viapro and it87.) Please be
I can see youre busy, I hope my update wasnt an annoyance, however 
In any case, I gather that 13-rcX is closed for all new features, so I 
regard it was info only.
Ill revalidate when 13-final is out, and update if needed.
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