root/lm-sensors/trunk/doc/FAQ @ 738

Sensors FAQ for lm_sensors version 2
------------------------------------

[Internal comments/questions/uncertainties are in square brackets.]

Note: some questions were specific for version 1. These are put together
under chapter 6: version 1 questions.

1 What sensors are available on my PC?
1.1 What can a sensor chip like the "LM78" do? 
1.2 Where do I find out more about any of these LMxx chips?

2 How are these sensors read?
2.1 What is the SMBus? And the I2C bus?
2.2 What sensors does the Pentium 2 (and P6) have?
2.3 How often are the sensor values updated?
2.4 How are alarms triggered?

3 Installation and management
3.1 Why so many modules, and how do I cope with them?
3.2 How do I know which chips I own?
3.3 Which modules should I insert?
3.4 Do I need the configuration file?
3.5 What about the 'No such file or directory' warnings when I compile it?
3.6 I get all kind of weird compilation errors?
3.7 It still does not compile or patch

4 Problems
4.1 Why do my fans report exactly half/double their values by your code
compared to the BIOS? 
4.2 Why do my two LM75's report "-48 degrees"?
4.3 Why do I have two Vcore readings, I have only one processor!
4.4 How do those ALARMS work? The current value is within range but there
is still an ALARM warning!
4.5 My voltage readings seem to drift a bit. Is something wrong with my power
supply?
4.6 Some measurements are way out of range. What happened?
4.7 What are VID lines?
4.8 I try to read sensor values several times a second, but it seems to be
updated only each second or so. Why?
4.9 It sometimes seems to take a fraction of a seconds before I see the
sensor reading results. Why?
4.10 Can I be alerted when an ALARM occurs?
4.11 SMBus transactions on my PIIX4 simply don't work (timeouts happen).  Why?
4.12 My BIOS reports a much higher CPU temperature than your modules!
4.13 I try to read the raw /proc files, but the values are strange?!?
4.14 How do I set new limits?
4.15 Some sensors are doubly detected?
4.16 I ran sensors-detect, but now I get very strange readings?!?

5 How to ask for help: 
5.1 What to do if it won't insert? 
5.2 What to do if it inserts, but nothing happens?
5.3 What to do if I read only bogus information?
5.4 What to do if you have other problems?
5.5 What if it just works like a charm?

6 Version 1 specific questions
6.1 My manufacturer swears that my mainboard has an SMBus, but your code 
reports that it can't find it.  What's wrong? 
6.2 The modules won't load, saying 'SMBus not detected'.
6.3 I try to read /proc/sensors, and I get a "No sensor data yet (try again in 
a few moments)" message. Why?

-------------------------------------------------------------

1 What sensors are available on my PC?

        Most medium/high-end computers since late 1997 now come with a LM78- or 
LM79-like hardware health monitoring chip and an SMBus. 
        
        
1.1 What can a sensor chip like the "LM78" do? 

        The LM78 is a chip made by National Semiconductor which can monitor 7 
voltages (5 positive, 2 negative) from 0 to 4.08V.  The inputs are usually in 
series with voltage dividers which lower the 12+/-V and 5+/-V supplies to 
measurable range.  Therefore, the readings for such inputs need to be 
re-scaled appropriately by software. 
        The LM78 also has 3 fan speed monitoring inputs, an internal 
temperature sensor, a chassis intrusion sensor, and a couple maskable interrupt 
inputs.  The LM78 can also relay the processor's (P6 or Pent II) VID lines
which are hardwired and used to indicate to the power regulator (usually on 
the mainboard close to the processor socket/slot) what voltage to supply to 
the processor. 
        The LM78 can be interfaced to a system via the ISA bus and/or the 
SMBus. 
        Most other sensor chips have comparable functionality. Each supported
chip should be documented somewhere in the doc/chips directory.

        
1.2 Where do I find out more about any of these LMxx chips?

        National Semiconductor has an excellent documentation resource on their
web-site.  Complete specs on the LMxx chips talked about in this FAQ can be 
downloaded at: 
        http://www.national.com
The Winbond documentation for Wxxxxxxx chips can be found at:
        http://www.winbond.com.tw/produ/perso7.htm
The Genesys Logic documentation for GLxxxxx chips is regrettably not freely
downloadable.
        Please see the file doc/useful_addresses.html for hyperlinks to
the available documentation.


2 How are these sensors read?

        The LM78 and most other chips sensors are usually read through
the ISA bus.  Our code looks for the presence of the chips on the ISA bus
and attempts to read the sensors from it by reading their different internal
registers.
        Many chips have also, or even exclusively, an SMBus interface,
explained more in detail below. 
        

2.1 What is the SMBus? And the I2C bus?

        The SMBus is the "System Management Bus".  More specifically, it is a 
2-wire, low-speed serial communication bus used for basic health monitoring 
and hardware management. It is a specific implementation of the more
general I2C (pronunciation: I-squared-C) bus. In fact, both I2C devices
and SMBus devices may be connected to the same (I2C) bus.
        The SMBus (or I2C bus) starts at the host controller, used for
starting transactions on the SMBus.  From the host interface, the
devices communicated with are the 'slave' devices.  Each slave device has a
unique 7-bit address in which the host must refer to it with. 
        For each supported SMBus host, there is a separate kernel module
which implements the communication protocol with the host. Some SMBus hosts
really operate on the SMBus level; these hosts can not cope with pure I2C
devices. Other hosts are in fact I2C hosts: in this case, we implement
the SMBus protocol in terms of I2C operations. But these hosts can also
talk to pure I2C devices.
        

2.2 What sensors does the Pentium 2 (and P6) have?

        The Pentium 2 'boxed' processor usually has an LM75 very close to the 
base of the box. It can be read through the SMBus to report the approximate 
temperature of the processor.  The processor also contains an internal 
temperature sensor (of low accuracy) used as a fail-safe to disable the 
processor in case it gets much too hot (usually around 130 degrees C).  And, 
the Pentium 2 also has a hard-wired signal (VID lines) on it's SEC (single 
edge connector) which indicates what power supply is required to operate the 
processor.
        Apparently, the SEC connector has connectivity to the thermal sensor 
for devices like the Analog Devices ADM1021 which has circuitry for 
converting the sense information into temperature degrees.  The Winbond 
chip's external temperature lines may be compatible with the P2 temperature 
lines as well.
        The P6 (Pentium-Pro) may have an LM75 in or just under the socket.  
P6's also have VID lines.
        Pentiums and Pentium w/ MMX do not have VID lines, and sometimes have 
LM75's under the sockets (depends on the mainboard, and how 'modern' the 
mainboard is).
        The P2 Xeon will be the first Intel processor to include the SMBus 
interface on the P2 Xeon SEC.
        

2.3 How often are the sensor values updated?

        The LM78, and most other sensor chips like it, reads its sensors one
by one. A complete scanning sweep will take about 1.5 seconds. The LM78 stops
readings sensors if you try to access it, so if you access it very often
(by reading sensor values; writing new limits is safe) it will not find the
time to update its sensor values at all! Fortunately, the kernel module takes
care not to do this, and only reads new values each 1.5 seconds. If you
read the values again, you will get the 'old' values again.


2.4 How are alarms triggered?

        It is possible to monitor each sensor and let an alarm go off if
it crosses some pre-determined limits.  There are two sorts of interrupts
which can be generated by sensor chips if this happens (it depends a bit on
the actual chip if both are supported; the LM80, for example, has only
IRQ interrupts): IRQ interrupts and SMI interrupts.  IRQ stands for 
Interrupt Request and are the interrupt lines you can find in /proc/interrupts.
SMI stands for System Management Interrupt, and is a special interrupt which
puts the processor in a secure environment independent of any other things
running.  SMI is currently not supported by the Linux kernel.  IRQs are 
supported, of course.
        Even if no interrupt is generated, some bits in a status register
will be set until the register is read the next time. If the alarm condition
persists after that, the bits will be set on the next scanning sweep, etc.
        At this moment, interrupts are not supported.


3 Installation and management

3.1 Why so many modules, and how do I cope with them?

        We tried to make this package as modular as possible. This makes it
easy to add new drivers, and unused drivers will take no precious kernel
space. On the other hand, it can be a bit confusing at first.
        It is strongly suggested you read doc/modules, and follow the
installation described there. It will allow you to use simple modprobe
commands to load the modules, instead of the more low-level insmod calls.
This file also describes how you can load the modules automatically at
boot-time.


3.2 How do I know which chips I own?

        By now, chip detection is fairly good. That means that it is 
relatively harmless to insert more chip drivers than you need. This approach
can still lead to problems, though.
        In the past, you had to muddle your way through and just tried
several drivers. But starting with 2.3.0, we have an excellent program
that scans all your hardware. It is called 'sensors-detect' and is
installed when you call 'make install'. Just execute this script, and
it will tell you exactly what you want to know.


3.3 Which modules should I insert?

        You need one module for each adapter you own; if there are 
chips on the ISA bus, this includes i2c-isa.o. Also, you need one module 
for each type of chip you own. That's all. On my computer, I could use the 
following line:
    modprobe i2c-isa && modprobe i2c-piix4 && \
                        modprobe lm78 && modprobe lm75 && modprobe i2c-dev


3.4 Do I need the configuration file?

        Usually, you do. It tells how to translate the values the chip
measures to real-world values. This is especially important for voltage
inputs. The default configuration file should usually do the trick.
It is automatically installed as /etc/sensors.conf, but it will not 
overwrite any existing file with that name.


3.5 What about the 'No such file or directory' warnings when I compile it?

        Don't worry about them. The dependency files (which tell which
files should be recompiled when certain files change) are created
dynamically. They are not distributed with the package. The `make' program
notices they are not there, and warns about that - and the first thing
it will do is generate them. So all is well.

3.6 I get all kinds of weird compilation errors?

        Check that the correct i2c header files are used. Depending on
how you installed, they should be under either /usr/local/include or
/usr/src/linux*/include. Try to edit the Makefile for the other setting.


3.7 It still does not compile or patch!
        Have you installed a recent version of the i2c package? Remember,
compilation is not enough, you also need to install it for the header
files to be found!
        If you want to patch the kernel, you will have to apply the i2c
patches first!


4 Problems

4.1 Why do my fans report exactly half/double their values by your code
compared to the BIOS? 

        The problem with much of the sensor data is that it is impossible to 
properly interpret some of the readings without knowing what the hardware 
configuration is.  Some fans report one 'tick' each rotation, some report
two 'ticks' each rotation. It is easy to resolve this through the
configuration file:
    chip lm78-*               # Or whatever chip this relates to
    compute fan1 2*@,@/2      # Copy for each fan present


4.2 Why do my two LM75's report "-48 degrees"?

        For starters, those aren't LM75's.  Your mainboard actually has the 
Winbond W83781D which emulates two LM75's, but many systems which use the 
Winbond chip (such as the Asus P2B) don't have the thermo-resisters connected 
to the chip resulting in these strange -48 degree readings.
        If you have an Asus P2B and want more information on adding thermal
sensing capability, check out:
http://ultimatepc.fsn.net/techinfo/p2bthermistor/p2bthermistor.htm
        In upcoming versions, you will be able to disable non-interesting
readings.


4.3 Why do I have two Vcore readings, I have only one processor!

        The LM78 family has seven voltage sensors. The default way of
connecting them is used in the configuration file. This includes a VCore2,
even if you do not have one. You can easily edit the configuration file
to give it another name, or (in upcoming versions) to make this reading
disappear.
        Note that Vcore2 is usually the same as Vcore on motherboards which
only support one processor. It is possible, though, that is monitors something
else, so you should not be too surprised if the values are completely 
different.


4.4 How do those ALARMS work? The current value is within range but there
is still an ALARM warning!

        An ALARM will go off when a minimum or maximum limit is crossed. It 
will stay there until the next internal update - which will be the next time
you read these values, but not within (usually) 1.5 seconds since the last
update. After that, it will only stay if the current value is out of range,
though this too depends a bit on the kind of chip. See the specific chip
documentation in doc/chips for more information.


4.5 My voltage readings seem to drift a bit. Is something wrong with my power
supply?

        No, probably not. If your motherboard heats up a bit, the sensed
voltages will drift a bit. If your power supply is loaded (because a disk
gets going, for example), the voltages may get a bit lower. As long as they
stay within a sensible range (say 10% of the expected value), there is no
reason to worry.


4.6 Some measurements are way out of range. What happened?

        Each module tries to set limits to sensible values on initialization,
but a module does not know how a chip is actually connected. This is
described in the configuration file, which is not read by kernel modules.
So limits can be strange, if the chip is connected in a non-standard way.
        Readings can also be strange; there are several reasons for this.
Temperature sensors, for example, can simply not be present, even though
the chip supports them. Also, it can be that the input is used in a
non-standard way. You can use the configuration file to describe how this
measurement should be interpreted; see the comments the example file for
more information.


4.7 What are VID lines?

        These describe the voltage your processors use. This is only supported
for Pentium 2 and newer processors, and even then they are not always
correctly connected to the sensor chip, so the readings may be out of
range. A value of +3.5 V is especially suspect.


4.8 I try to read sensor values several times a second, but it seems to be
updated only each second or so. Why?

        If we would read the registers more often, it would not find the
time to update them. So we only update our readings once each 1.5 seconds
(the actual delay is chip-specific; for some chips, it may not be needed
at all).


4.9 It sometimes seems to take a fraction of a seconds before I see the
sensor reading results. Why?

        ISA bus access is fast, but SMBus access is really slow. If you have
a lot of sensors, it just takes a lot of time to access them. Fortunately,
this has almost no impact on the system as a whole, as another job can run
while we are waiting for the transaction to finish.


4.10 Can I be alerted when an ALARM occurs?

        No, you can't; and it may well be never supported.
        Almost no mainboard we have encountered have actually connected the
IRQ-out pin of sensor chips. That means that we could enable IRQ reporting, but
nothing would happen. Also, even if a motherboard has it connected, it is
unclear what interrupt number would be triggered. And IRQ lines are a scarce
facility, which means that almost nobody would be able to use it anyway.
        The SMI interrupt is only available on a few types of chips. It is
really a very obscure way to handle interrupts, and supporting it under Linux
might be quite hard to do.
        Your best bet would be to poll the alarm file with a user-land daemon
which alerts you if an alarm is raised. I am not aware of any program which
does the job, though you might want to examine one of the graphical monitor
programs under X, see doc/useful_addresses.html for addresses.


4.11 SMBus transactions on my PIIX4 simply don't work (timeouts happen).  Why?

        Some chips which mainboard makers connect to the SMBus are not SMBus 
devices.  An example is the 91xx clock generator chips.  When read, these 
devices can lock up the SMBus until the next hard reboot.  This is because 
they have a similar serial interface (like the I2C), but don't conform to 
Intel's SMBus standard.
        Why did they connect these devices to the SMBus if they aren't 
compatible?  Good question! :')  Actually, these devices may support being 
written to, but lock things up when they are read.


4.12 My BIOS reports a much higher CPU temperature than your modules!

        We display the actual temperature of the sensor. This may not be the 
temperature you are interested in, though.  If a sensor should measure
the CPU temperature, it must be in thermal contact with it.  In practice,
it is just somewhere near it. Your BIOS may correct for this (by adding,
for example, thirty degrees to the measured temperature).  The correction 
factor is regrettably different for each mainboard, so we can not do this
in the module itself. You can do it through the configuration file, though:
  
    chip lm75-*-49                     # Or whatever chip this relates to
    label temp "Processor"
    compute temp @*1.2+13,(@-13)/1.2   # Or whatever formula


4.13 I try to read the raw /proc files, but the values are strange?!?

        Remember, these values do not take the configuration file
'compute' lines in account. This is especially obvious for voltage readings
(usually called in? or vin?). Use a program linked to libsensors (like
the provided 'sensors' program) instead.


4.14 How do I set new limits?

        This is a bit intricate right now. In the near future, this can be
done through the 'sensors' program, which will take account of the
computations specified in the configuration file. At this moment, you must
also put the 'set' commands into the configuration file. This will get
easier in newer versions.


4.15 Some sensors are doubly detected?

        Yes, this is still a problem. It will partially solved in the future,
but it is really tough. Double detections can be caused by two things: 
sensors can be detected to both the ISA and the SMBus (and if you have
loaded the approprate adapter drivers, it will be detected on both), and
some chips simulate other chips (the Winbond W83781D simulates LM75 chips
on the SMBus, for example). Remove the offending adapter or chip driver, or
run sensors-detect and add the insmod parameters it suggests.


4.16 I ran sensors-detect, but now I get very strange readings?!?

        Your SMBus (PIIX4?) is probably crashed. There are some mainboards
which connect a clock chip to the SMBus. Unfortunately, this clock chip
hangs the PIIX4 if it is read (it is an I2C device, but not SMBus compatible).
We have found no way of solving this, except for rebooting your computer.
Next time when you run sensors-detect, you may want to exclude addresses
0x69 and/or 0x6a, by entering 's' when you are asked whether you want to
scan the PIIX4.

5 How to ask for help:

        We are always willing to answer questions if things don't work out.
Please mail sensors@stimpy.netroedge.com, and not the individual authors,
unless you have something private to say. Especially, do not mail
Alexander Larsson, as he has left the development team. You can be assured
that any mail sent to sensors@stimpy.netroedge.com will arrive at anyone
who answered your email, so please do not CC: him again.
        Instead of using email, you can also use the web-based support
area, at http://www.netroedge.com/~lm78/support.html. You will be helped
just as fast, and others may profit from the answer too. You will be
noticed automatically when your question has been answered.


5.1 What to do if it won't insert?

        Always inspect the output of 'dmesg' and send it to us. Check
/proc/pci for your SMBus adapter, and send the output too. Check whether
you actually have a sensor chip - sometimes, it is only an upgrade option.
If your mainboard manual happens to mention the chip type, send it too.
Tell us what mainboard you own.


5.2 What to do if it inserts, but nothing happens?

        Check /proc/pci to see whether you have a supported adapter. If it
is not there, we do not support it. Future version may do it. If you want,
you can send us an email telling you have a such-and-so mainboard, together
with the PCI output - that way, we can see what SMBus hosts are used most.


5.3 What to do if I read only bogus information?

        It may be that this was a mis-detection: the chip may not be
present. If you are convinced there is something wrong, please send us
the usual information (see 5.1).


5.4 What to do if you have other problems?

        Again, send the output of 'dmesg', /proc/sensors and /proc/pci.


5.5 What if it just works like a charm?

        Drop us a mail if you feel like it, mentioning the mainboard and
detected chip type. That way, we have some positive feedback, too!


6 Version 1 specific questions

6.1 My manufacturer swears that my mainboard has an SMBus, but your code 
reports that it can't find it.  What's wrong? 

        Currently, our code only assumes that an SMBus exists if it originates 
from the Intel PIIX4 (82371AB).  If your computer doesn't have one, or if your 
SMBus originates from a different SMBus 'host', then you are out of luck. :'( 
Our experience is, though, that most machines have a PIIX4, and that it is 
where the SMBus is hosted.

Regarding the VIA chip set(s):
        Right now, the SMBus code depends on the Intel PIIX4 chip to handle 
SMBus transactions.  The VIA chip set is NOT supported at this time because 
it forces much of the SMBus protocol to be implemented by software.
        To implement the SMBus correctly with the VIA chip, it needs to be 
written at a low level to be quick.  A more attractive alternative is to 
use the Bios SMBus interface (not always available nor standard).

        Version 2 supports the VIA chipset, and will support other chipsets.
Version 1 never will.


6.2 The modules won't load, saying 'SMBus not detected'.

        This should no longer be an issue in 1.4.10 and later; it will
continue loading, but it won't support SMBus-connected devices, of course.


6.3 I try to read /proc/sensors, and I get a "No sensor data yet (try again in 
a few moments)" message. Why?

        It takes about 1.5 seconds for the LM78 to update all its sensor 
values.  If we would try to read it before it finished that, you would get
old garbage instead. So you have to wait 1.5 seconds after the module is
inserted before you can access /proc/sensors.
        Module versions 1.3.7 and later let the process sleep if it tries
to access sensor data right after insertion time, and do not display this
message anymore.


6.4 On my Dell, a LM80 is detected, but all readings are 0!

        This is a bug we have only observed on Dell computers. There is
probably a problem with the way the SMBus is accessed; but it is not yet
clear whether the problem is in our code or with the Dells.
        There are very probably no sensor chips at all on your computer;
but until somebody contacts Dell about this and tells us the results, we
can not be sure.


-----------
Rev 2.2 (Frodo) Corrections for lm_sensors 2.4, 19990920
Rev 2.1 (Frodo) Corrections for lm_sensors 2.2, 19990112
Rev 2.0 (Frodo) Major revision for lm_sensors 2.1, 19981229
Rev 1.10 (Frodo) Modified 3.8, updated some other things, 19980924
Rev 1.9 (Frodo) Added 3.15, 19980906
Rev 1.8 (Frodo) Added 3.14, 19980905
Rev 1.7 (Phil) Added 3.13 and some other minor changes, 19980901
Rev 1.6 (Frodo) Added 4, 4.1, 4.2, 4.3, 4.4, 4.5, 19980901
Rev 1.5 (Frodo) Added 2.3, 2.4, 3.9, 3.10, 3.11, 19980826
Rev 1.4 (Frodo) Added some more Winbond information, and 3.5-3.8, 19980817
Rev 1.3 Added info on the Winbond chip, 19980816
Rev 1.2 Adapation by Frodo Looijaard, 19980810
Rev 1.1 Modifications by Philip Edelbrock, 19980809
Rev 1.0 Written by Philip Edelbrock, 19980803