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23 <h1 class="settitle">Sensors FAQ for lm_sensors version 2.18</h1>
24 Copyright (C) 1998 - 2005<br>
25 Frodo Looijaard,<br>
26 Philip Edelbrock,<br>
27 Mark D. Studebaker<br>
28 and<br>
29 Jean Delvare<br>
30
31    <div class="shortcontents">
32 <h2>Short Contents</h2>
33 <ul>
34 <li><a href="#toc_Top">lm_sensors</a></li>
35 <li><a href="#toc_Overview">1 PC and Sensor Overview</a></li>
36 <li><a href="#toc_Basics">2 Sensor and Bus Basics</a></li>
37 <li><a href="#toc_Installation">3 Installation and Management</a></li>
38 <li><a href="#toc_Problems">4 Problems</a></li>
39 <li><a href="#toc_Help">5 How to Ask for Help</a></li>
40 <li><a href="#toc_Contribute">6 How to Contribute</a></li>
41 <li><a href="#toc_Document-Revisions">Appendix A Revision History of This Document</a></li>
42 </ul>
43 </div>
44
45    <div class="contents">
46 <h2>Table of Contents</h2>
47 <ul>
48 <li><a name="toc_Top" href="#Top">lm_sensors</a>
49 <li><a name="toc_Overview" href="#Overview">1 PC and Sensor Overview</a>
50 <ul>
51 <li><a href="#Section-1_002e1">1.1 What sensors are available on my PC?</a>
52 <li><a href="#Section-1_002e2">1.2 What can a sensor chip like the "LM78" do?</a>
53 <li><a href="#Section-1_002e3">1.3 Where do I find out more about any of these chips?</a>
54 </li></ul>
55 <li><a name="toc_Basics" href="#Basics">2 Sensor and Bus Basics</a>
56 <ul>
57 <li><a href="#Section-2_002e1">2.1 How are these sensors read?</a>
58 <li><a href="#Section-2_002e2">2.2 What is the SMBus? And the I2C bus?</a>
59 <li><a href="#Section-2_002e3">2.3 I don't have an ISA bus!</a>
60 <li><a href="#Section-2_002e4">2.4 What sensors do processors have?</a>
61 <li><a href="#Section-2_002e5">2.5 How often are the sensor values updated?</a>
62 <li><a href="#Section-2_002e6">2.6 How are alarms triggered?</a>
63 </li></ul>
64 <li><a name="toc_Installation" href="#Installation">3 Installation and Management</a>
65 <ul>
66 <li><a href="#Section-3_002e1">3.1 Why so many modules, and how do I cope with them?</a>
67 <li><a href="#Section-3_002e2">3.2 How do I know which chips I own?</a>
68 <ul>
69 <li><a href="#Section-3_002e2">3.2.1 What chips are on motherboard XYZ?</a>
70 <li><a href="#Section-3_002e2">3.2.2 Do you support motherboard XYZ?</a>
71 <li><a href="#Section-3_002e2">3.2.3 Do you support chip XYZ?</a>
72 <li><a href="#Section-3_002e2">3.2.4 Anybody working on a driver for chip XYZ?</a>
73 </li></ul>
74 <li><a href="#Section-3_002e3">3.3 Which modules should I insert?</a>
75 <li><a href="#Section-3_002e4">3.4 Do I need the configuration file <samp><span class="file">/etc/sensors.conf</span></samp>?</a>
76 <ul>
77 <li><a href="#Section-3_002e4">3.4.1 The labels for the voltage and temperature readings in <samp><span class="command">sensors</span></samp> are incorrect!</a>
78 <li><a href="#Section-3_002e4">3.4.2 The min and max for the readings in <samp><span class="command">sensors</span></samp> are incorrect!</a>
79 <li><a href="#Section-3_002e4">3.4.3 The min and max settings in <samp><span class="file">/etc/sensors.conf</span></samp> didn't take effect!</a>
80 <li><a href="#Section-3_002e4">3.4.4 One sensor isn't hooked up on my board!</a>
81 <li><a href="#Section-3_002e4">3.4.5 I need help with <samp><span class="file">sensors.conf</span></samp>!</a>
82 <li><a href="#Section-3_002e4">3.4.6 Do you have a database of <samp><span class="file">sensors.conf</span></samp> entries for specific boards?</a>
83 </li></ul>
84 <li><a href="#Section-3_002e5">3.5 What about the `<samp><span class="samp">No such file or directory</span></samp>' warnings when I compile?</a>
85 <li><a href="#Section-3_002e6">3.6 I get all kinds of weird compilation errors?</a>
86 <ul>
87 <li><a href="#Section-3_002e6">3.6.1 `<samp><span class="samp">No rule to make target xxxx needed by xxxx</span></samp>' - how to fix?</a>
88 </li></ul>
89 <li><a href="#Section-3_002e7">3.7 It still does not compile or patch!</a>
90 <li><a href="#Section-3_002e8">3.8 <samp><span class="command">make install</span></samp> fails on Mandrake kernels</a>
91 <li><a href="#Section-3_002e9">3.9 I get unresolved symbols when I <samp><span class="command">modprobe</span></samp> modules (Red Hat especially)</a>
92 <li><a href="#Section-3_002e10">3.10 I2C_DRIVERID_ADM1024 undefined (Red Hat especially)</a>
93 </li></ul>
94 <li><a name="toc_Problems" href="#Problems">4 Problems</a>
95 <ul>
96 <li><a href="#Section-4_002e1">4.1 My fans report exactly half/double their values compared to the BIOS?</a>
97 <ul>
98 <li><a href="#Section-4_002e1">4.1.1 Fans sometimes/always read 0!!</a>
99 <li><a href="#Section-4_002e1">4.1.2 I doubled the fan divisor and the fan still reads 7000!</a>
100 </li></ul>
101 <li><a href="#Section-4_002e2">4.2 Why do my two LM75's report "-48 degrees"?</a>
102 <li><a href="#Section-4_002e3">4.3 Why do I have two Vcore readings, I have only one processor!</a>
103 <li><a href="#Section-4_002e4">4.4 How do those ALARMS work? The current value is within range but there is still an ALARM warning!</a>
104 <li><a href="#Section-4_002e5">4.5 My voltage readings seem to drift a bit. Is something wrong?</a>
105 <li><a href="#Section-4_002e6">4.6 Some measurements are way out of range. What happened?</a>
106 <ul>
107 <li><a href="#Section-4_002e6">4.6.1 -5V and -12V readings are way out of range!</a>
108 </li></ul>
109 <li><a href="#Section-4_002e7">4.7 What are VID lines? Why is the VID reading wrong?</a>
110 <li><a href="#Section-4_002e8">4.8 I read sensor values several times a second, but they are only updated only each second or so. Why?</a>
111 <li><a href="#Section-4_002e9">4.9 It sometimes seems to take almost a second before I see the sensor reading results. Why?</a>
112 <li><a href="#Section-4_002e10">4.10 Can I be alerted when an ALARM occurs?</a>
113 <li><a href="#Section-4_002e11">4.11 SMBus transactions on my PIIX4 simply don't work (timeouts happen).  Why?</a>
114 <li><a href="#Section-4_002e12">4.12 My BIOS reports a much higher CPU temperature than your modules!</a>
115 <li><a href="#Section-4_002e13">4.13 I try to read the raw <samp><span class="file">/proc</span></samp> files, but the values are strange?!?</a>
116 <li><a href="#Section-4_002e14">4.14 How do I set new limits?</a>
117 <ul>
118 <li><a href="#Section-4_002e14">4.14.1 I set new limits and it didn't work?</a>
119 </li></ul>
120 <li><a href="#Section-4_002e15">4.15 Some sensors are doubly detected?</a>
121 <li><a href="#Section-4_002e16">4.16 I ran sensors-detect, but now I get very strange readings?!?</a>
122 <li><a href="#Section-4_002e17">4.17 Bad readings from particular chips</a>
123 <ul>
124 <li><a href="#Section-4_002e17">4.17.1 Bad readings from the AS99127F!</a>
125 <li><a href="#Section-4_002e17">4.17.2 Bad readings from the VIA 686A!</a>
126 <li><a href="#Section-4_002e17">4.17.3 Bad readings from the MTP008!</a>
127 <li><a href="#Section-4_002e17">4.17.4 Bad temperature readings from the SIS5595!</a>
128 <li><a href="#Section-4_002e17">4.17.5 Bad readings from a w8378[12]d!</a>
129 <li><a href="#Section-4_002e17">4.17.6 Bus hangs on Ali 1543 on Asus P5A boards!</a>
130 <li><a href="#Section-4_002e17">4.17.7 Bad readings from LM75!</a>
131 <li><a href="#Section-4_002e17">4.17.8 Bad readings from LM78!</a>
132 <li><a href="#Section-4_002e17">4.17.9 Bad readings from LM80!</a>
133 </li></ul>
134 <li><a href="#Section-4_002e18">4.18 How do I configure two chips (LM87) differently?</a>
135 <li><a href="#Section-4_002e19">4.19 Dmesg says `<samp><span class="samp">Upgrade BIOS</span></samp>'! I don't want to!</a>
136 <ul>
137 <li><a href="#Section-4_002e19">4.19.1 Dmesg says `<samp><span class="samp">use force_addr=0xaddr</span></samp>'! What address do I use?</a>
138 </li></ul>
139 <li><a href="#Section-4_002e20">4.20 Sensors says `<samp><span class="samp">Can't access procfs/sysfs file</span></samp>'</a>
140 <li><a href="#Section-4_002e21">4.21 Sensors says `<samp><span class="samp">No sensors found!</span></samp>'</a>
141 <li><a href="#Section-4_002e22">4.22 Sensors output is not correct!</a>
142 <li><a href="#Section-4_002e23">4.23 What is at I2C address XXX?</a>
143 <ul>
144 <li><a href="#Section-4_002e23">4.23.1 What is at I2C address 0x69?</a>
145 <li><a href="#Section-4_002e23">4.23.2 What is at I2C addresses 0x50 - 0x57?</a>
146 <li><a href="#Section-4_002e23">4.23.3 What is at I2C addresses 0x30 - 0x37?</a>
147 </li></ul>
148 <li><a href="#Section-4_002e24">4.24 Sensors-detect doesn't work at all</a>
149 <ul>
150 <li><a href="#Section-4_002e24">4.24.1 Sensors-detect says "Couldn't open /proc/bus/i2c?!?"</a>
151 <li><a href="#Section-4_002e24">4.24.2 Sensors-detect says "Can't open /dev/i2c[-/]0"</a>
152 <li><a href="#Section-4_002e24">4.24.3 Sensors-detect doesn't find any sensors!</a>
153 </li></ul>
154 <li><a href="#Section-4_002e25">4.25 Sensors says `<samp><span class="samp">Error: Line xxx: zzzzzzz</span></samp>'</a>
155 <li><a href="#Section-4_002e26">4.26 Sensors only gives the name and adapter for my chip</a>
156 <li><a href="#Section-4_002e27">4.27 Sensors says `<samp><span class="samp">ERROR: Can't get xxxxx data!</span></samp>'</a>
157 <li><a href="#Section-4_002e28">4.28 Sensors doesn't find any sensors, just eeproms.</a>
158 <li><a href="#Section-4_002e29">4.29 Inserting modules hangs my board</a>
159 <li><a href="#Section-4_002e30">4.30 Inserting modules slows down my board</a>
160 <li><a href="#Section-4_002e31">4.31 Problems on particular motherboards</a>
161 <ul>
162 <li><a href="#Section-4_002e31">4.31.1 Asus P4B</a>
163 <li><a href="#Section-4_002e31">4.31.2 Tyan 2460, 2462</a>
164 <li><a href="#Section-4_002e31">4.31.3 Tyan 2466</a>
165 <li><a href="#Section-4_002e31">4.31.4 Tyan 2688</a>
166 </li></ul>
167 <li><a href="#Section-4_002e32">4.32 Problems on 2.6 kernels</a>
168 <ul>
169 <li><a href="#Section-4_002e32">4.32.1 i2c-viapro and via686a</a>
170 <li><a href="#Section-4_002e32">4.32.2 Where are my EEPROMs?</a>
171 </li></ul>
172 </li></ul>
173 <li><a name="toc_Help" href="#Help">5 How to Ask for Help</a>
174 <ul>
175 <li><a href="#Section-5_002e1">5.1 What to send us when asking for help</a>
176 <li><a href="#Section-5_002e2">5.2 What to do if a module won't insert?</a>
177 <li><a href="#Section-5_002e3">5.3 What to do if it inserts, but nothing happens?</a>
178 <li><a href="#Section-5_002e4">5.4 What to do if I read only bogus information?</a>
179 <li><a href="#Section-5_002e5">5.5 What to do if you have other problems?</a>
180 <li><a href="#Section-5_002e6">5.6 What if it just works like a charm?</a>
181 <li><a href="#Section-5_002e7">5.7 How do I update a ticket?</a>
182 <li><a href="#Section-5_002e8">5.8 How do I follow up on a ticket?</a>
183 <li><a href="#Section-5_002e9">5.9 Why did you decide not to support undocumented chips?</a>
184 </li></ul>
185 <li><a name="toc_Contribute" href="#Contribute">6 How to Contribute</a>
186 <ul>
187 <li><a href="#Section-6_002e1">6.1 How to write a driver</a>
188 <li><a href="#Section-6_002e2">6.2 How to get SVN access</a>
189 <li><a href="#Section-6_002e3">6.3 How to donate hardware to the project</a>
190 <li><a href="#Section-6_002e4">6.4 How to join the project mailing lists</a>
191 <li><a href="#Section-6_002e5">6.5 How to access mailing list archives</a>
192 <li><a href="#Section-6_002e6">6.6 How to submit a patch</a>
193 <li><a href="#Section-6_002e7">6.7 How to REALLY help</a>
194 <li><a href="#Section-6_002e8">6.8 How to get release announcements</a>
195 </li></ul>
196 <li><a name="toc_Document-Revisions" href="#Document-Revisions">Appendix A Revision History of This Document</a>
197 </li></ul>
198 </div>
199
200 <div class="node">
201 <p><hr>
202 <a name="Top"></a>
203 Next:&nbsp;<a rel="next" accesskey="n" href="#Overview">Overview</a>,
204 Up:&nbsp;<a rel="up" accesskey="u" href="#dir">(dir)</a>
205
206 </div>
207
208 <h2 class="unnumbered">lm_sensors</h2>
209
210 <p>The lm_sensors package includes a collection of modules for general SMBus
211 access and hardware monitoring.  NOTE: this requires special support which
212 is not in standard 2.2-vintage kernels.
213
214 <ul class="menu">
215 <li><a accesskey="1" href="#Overview">Overview</a>:                 PC and Sensor Overview
216 <li><a accesskey="2" href="#Basics">Basics</a>:                   Sensor and Bus Basics
217 <li><a accesskey="3" href="#Installation">Installation</a>:             Installation and Management
218 <li><a accesskey="4" href="#Problems">Problems</a>:                 Problems
219 <li><a accesskey="5" href="#Help">Help</a>:                     How to Ask for Help
220 <li><a accesskey="6" href="#Contribute">Contribute</a>:               How to Contribute
221 <li><a accesskey="7" href="#Document-Revisions">Document Revisions</a>:       Revision History of This Document
222 </ul>
223
224 <div class="node">
225 <p><hr>
226 <a name="Overview"></a>
227 Next:&nbsp;<a rel="next" accesskey="n" href="#Basics">Basics</a>,
228 Previous:&nbsp;<a rel="previous" accesskey="p" href="#Top">Top</a>,
229 Up:&nbsp;<a rel="up" accesskey="u" href="#Top">Top</a>
230
231 </div>
232
233 <h2 class="chapter">1 PC and Sensor Overview</h2>
234
235 <ul class="menu">
236 <li><a accesskey="1" href="#Section-1_002e1">Section 1.1</a>:              What sensors are available on my PC?
237 <li><a accesskey="2" href="#Section-1_002e2">Section 1.2</a>:              What can a sensor chip like the "LM78" do?
238 <li><a accesskey="3" href="#Section-1_002e3">Section 1.3</a>:              Where do I find out more about any of these chips?
239 </ul>
240
241 <div class="node">
242 <p><hr>
243 <a name="Section-1.1"></a>
244 <a name="Section-1_002e1"></a>
245 Next:&nbsp;<a rel="next" accesskey="n" href="#Section-1_002e2">Section 1.2</a>,
246 Up:&nbsp;<a rel="up" accesskey="u" href="#Overview">Overview</a>
247
248 </div>
249
250 <h3 class="section">1.1 What sensors are available on my PC?</h3>
251
252 <p>Most PC's built since late 1997 now come with a
253 hardware health monitoring chip. This chip may be accessed via the
254 ISA bus or the SMBus, depending on the motherboard.
255
256    <p>Some motherboard chipsets, notably the Via 686 and the SiS 5595,
257 contain hardware monitor functions.
258
259    <p>This FAQ frequently refers to the "LM78". This chip has been
260 obsoleted by National Semiconductor. Most motherboards today contain
261 a chip with similar functions.
262
263 <div class="node">
264 <p><hr>
265 <a name="Section-1.2"></a>
266 <a name="Section-1_002e2"></a>
267 Next:&nbsp;<a rel="next" accesskey="n" href="#Section-1_002e3">Section 1.3</a>,
268 Previous:&nbsp;<a rel="previous" accesskey="p" href="#Section-1_002e1">Section 1.1</a>,
269 Up:&nbsp;<a rel="up" accesskey="u" href="#Overview">Overview</a>
270
271 </div>
272
273 <h3 class="section">1.2 What can a sensor chip like the "LM78" do?</h3>
274
275 <p>The LM78 is a chip made by National Semiconductor which can monitor 7
276 voltages (5 positive, 2 negative) from 0 to 4.08V.  The inputs are usually in
277 series with voltage dividers which lower the +/- 12V and +/- 5V supplies to
278 measurable range.  Therefore, the readings for such inputs need to be
279 re-scaled appropriately by software.
280
281    <p>The LM78 also has 3 fan speed monitoring inputs, an internal
282 temperature sensor, a chassis intrusion sensor, and a couple maskable interrupt
283 inputs.  The LM78 can also relay the processor's (P6 or Pent II) VID lines
284 which are hardwired and used to indicate to the power regulator (usually on
285 the mainboard close to the processor socket/slot) what voltage to supply to
286 the processor.
287
288    <p>The LM78 can be interfaced to a system via the ISA bus and/or the
289 SMBus.
290
291    <p>Most other sensor chips have comparable functionality. Each supported
292 chip is documented in the <samp><span class="file">doc/chips</span></samp> directory.
293
294 <div class="node">
295 <p><hr>
296 <a name="Section-1.3"></a>
297 <a name="Section-1_002e3"></a>
298 Previous:&nbsp;<a rel="previous" accesskey="p" href="#Section-1_002e2">Section 1.2</a>,
299 Up:&nbsp;<a rel="up" accesskey="u" href="#Overview">Overview</a>
300
301 </div>
302
303 <h3 class="section">1.3 Where do I find out more about any of these chips?</h3>
304
305 <p>Most semiconductor companies have comprehensive documentation,
306 including complete datasheets, on their websites. Analog Devices,
307 Dallas Semiconductor, Maxim, and National Semiconductor have the widest selection
308 of sensor chips. Their websites are:
309
310      <ul>
311   <li><a href="http://www.analog.com">http://www.analog.com</a>
312   <li><a href="http://www.dalsemi.com">http://www.dalsemi.com</a>
313   <li><a href="http://www.maxim-ic.com">http://www.maxim-ic.com</a>
314   <li><a href="http://www.national.com">http://www.national.com</a>
315 </ul>
316
317    <p>Please see the file <a href="http://www.lm-sensors.org/wiki/UsefulLinks">http://www.lm-sensors.org/wiki/UsefulLinks</a>
318 for links to other companies' websites.
319
320 <div class="node">
321 <p><hr>
322 <a name="Basics"></a>
323 Next:&nbsp;<a rel="next" accesskey="n" href="#Installation">Installation</a>,
324 Previous:&nbsp;<a rel="previous" accesskey="p" href="#Overview">Overview</a>,
325 Up:&nbsp;<a rel="up" accesskey="u" href="#Top">Top</a>
326
327 </div>
328
329 <h2 class="chapter">2 Sensor and Bus Basics</h2>
330
331 <ul class="menu">
332 <li><a accesskey="1" href="#Section-2_002e1">Section 2.1</a>:              What sensors are available on my PC?
333 <li><a accesskey="2" href="#Section-2_002e2">Section 2.2</a>:              What can a sensor chip like the "LM78" do?
334 <li><a accesskey="3" href="#Section-2_002e3">Section 2.3</a>:              Where do I find out more about any of these chips?
335 <li><a accesskey="4" href="#Section-2_002e4">Section 2.4</a>:              What sensors are available on my PC?
336 <li><a accesskey="5" href="#Section-2_002e5">Section 2.5</a>:              What can a sensor chip like the "LM78" do?
337 <li><a accesskey="6" href="#Section-2_002e6">Section 2.6</a>:              Where do I find out more about any of these chips?
338 </ul>
339
340 <div class="node">
341 <p><hr>
342 <a name="Section-2.1"></a>
343 <a name="Section-2_002e1"></a>
344 Next:&nbsp;<a rel="next" accesskey="n" href="#Section-2_002e2">Section 2.2</a>,
345 Up:&nbsp;<a rel="up" accesskey="u" href="#Basics">Basics</a>
346
347 </div>
348
349 <h3 class="section">2.1 How are these sensors read?</h3>
350
351 <p>Sensor chips reside on either the ISA bus, the SMBus, or both.
352 See the file <samp><span class="file">doc/chips/SUMMARY</span></samp> in our package for a list.
353
354    <p>To communicate with chips on the ISA bus, the software uses
355 simple I/O reads and writes.
356
357    <p>To communicate with chips on the SMBus, the software must
358 use an SMBus interface device, explained below.
359
360 <div class="node">
361 <p><hr>
362 <a name="Section-2.2"></a>
363 <a name="Section-2_002e2"></a>
364 Next:&nbsp;<a rel="next" accesskey="n" href="#Section-2_002e3">Section 2.3</a>,
365 Previous:&nbsp;<a rel="previous" accesskey="p" href="#Section-2_002e1">Section 2.1</a>,
366 Up:&nbsp;<a rel="up" accesskey="u" href="#Basics">Basics</a>
367
368 </div>
369
370 <h3 class="section">2.2 What is the SMBus? And the I2C bus?</h3>
371
372 <p>The SMBus is the "System Management Bus".  More specifically, it is a
373 2-wire, low-speed serial communication bus used for basic health monitoring
374 and hardware management. It is a specific implementation of the more
375 general I2C (pronunciation: I-squared-C) bus. In fact, both I2C devices
376 and SMBus devices may be connected to the same (I2C) bus.
377
378    <p>The SMBus (or I2C bus) starts at the host controller, used for
379 starting transactions on the SMBus.  From the host interface, the
380 devices communicated with are the <dfn>slave</dfn> devices.  Each slave device has a
381 unique 7-bit address which the host uses to refer to that device.
382
383    <p>For each supported SMBus host, there is a separate kernel module
384 which implements the communication protocol with the host. Some SMBus hosts
385 really operate on the SMBus level; these hosts can not cope with pure I2C
386 devices. Other hosts are in fact I2C hosts: in this case, we implement
387 the SMBus protocol in terms of I2C operations. But these hosts can also
388 talk to pure I2C devices.
389
390 <div class="node">
391 <p><hr>
392 <a name="Section-2.3"></a>
393 <a name="Section-2_002e3"></a>
394 Next:&nbsp;<a rel="next" accesskey="n" href="#Section-2_002e4">Section 2.4</a>,
395 Previous:&nbsp;<a rel="previous" accesskey="p" href="#Section-2_002e2">Section 2.2</a>,
396 Up:&nbsp;<a rel="up" accesskey="u" href="#Basics">Basics</a>
397
398 </div>
399
400 <h3 class="section">2.3 I don't have an ISA bus!</h3>
401
402 <p>We promise, you do, even if you don't have any old ISA slots.
403 The "ISA Bus" exists in your computer even if you don't have ISA slots;
404 it is simply a memory-mapped area, 64KB in size (0x0000 - 0xFFFF)
405 where many "legacy" functions, such as keyboard and interrupt controllers,
406 are found. It isn't necessarily a separate physical bus.
407 See the file <samp><span class="file">/proc/ioports</span></samp> for a list of devices living on
408 the "ISA Bus" in your system. If you don't like the term "ISA Bus"
409 think "I/O Space".
410
411 <div class="node">
412 <p><hr>
413 <a name="Section-2.4"></a>
414 <a name="Section-2_002e4"></a>
415 Next:&nbsp;<a rel="next" accesskey="n" href="#Section-2_002e5">Section 2.5</a>,
416 Previous:&nbsp;<a rel="previous" accesskey="p" href="#Section-2_002e3">Section 2.3</a>,
417 Up:&nbsp;<a rel="up" accesskey="u" href="#Basics">Basics</a>
418
419 </div>
420
421 <h3 class="section">2.4 What sensors do processors have?</h3>
422
423 <p>Most new processors contain a thermal diode on the die itself.
424 The electical properties of all diodes and transistors vary
425 slightly with temperature. The thermal diode is exceptionally accurate
426 because it is directly on the die. Newer temperature sensor chips,
427 like the Analog Devices ADM1021 and clones, and the Winbond chips,
428 have circuitry for measuring the the electrical properties of
429 an external diode and converting this data to a temperature.
430 Any sensor chip listed in <samp><span class="file">doc/chips/SUMMARY</span></samp> in our package which
431 has support for more than one temperature supports external temperature sensing.
432
433    <p>Older motherboards and processors without this feature generally use
434 an LM75 placed close to the processor. This is much less accurate.
435
436    <p>The Pentium 2 'boxed' processor usually has an LM75 very close to the
437 base of the box. It can be read through the SMBus to report the approximate
438 temperature of the processor.  The processor also contains an internal
439 temperature sensor (of low accuracy) used as a fail-safe to disable the
440 processor in case it gets much too hot (usually around 130 degrees C).  And,
441 the Pentium 2 also has a hard-wired signal (VID lines) on it's SEC (single
442 edge connector) which indicates what power supply is required to operate the
443 processor.
444
445    <p>The P6 (Pentium-Pro) may have an LM75 in or just under the socket.
446 P6's also have VID lines.
447
448    <p>Pentiums and Pentium w/ MMX do not have VID lines, and sometimes have
449 LM75's under the sockets (depends on the mainboard, and how 'modern' the
450 mainboard is).
451
452    <p>The P2 Xeon was the first Intel processor to include the SMBus
453 interface on the P2 Xeon SEC.
454
455 <div class="node">
456 <p><hr>
457 <a name="Section-2.5"></a>
458 <a name="Section-2_002e5"></a>
459 Next:&nbsp;<a rel="next" accesskey="n" href="#Section-2_002e6">Section 2.6</a>,
460 Previous:&nbsp;<a rel="previous" accesskey="p" href="#Section-2_002e4">Section 2.4</a>,
461 Up:&nbsp;<a rel="up" accesskey="u" href="#Basics">Basics</a>
462
463 </div>
464
465 <h3 class="section">2.5 How often are the sensor values updated?</h3>
466
467 <p>The LM78, and most other sensor chips like it, reads its sensors one
468 by one. A complete scanning sweep will take about 1.5 seconds. The LM78 stops
469 readings sensors if you try to access it, so if you access it very often
470 (by reading sensor values; writing new limits is safe) it will not find the
471 time to update its sensor values at all! Fortunately, the kernel module takes
472 care not to do this, and only reads new values each 1.5 seconds. If you
473 read the values again, you will get the 'old' values again.
474
475 <div class="node">
476 <p><hr>
477 <a name="Section-2.6"></a>
478 <a name="Section-2_002e6"></a>
479 Previous:&nbsp;<a rel="previous" accesskey="p" href="#Section-2_002e5">Section 2.5</a>,
480 Up:&nbsp;<a rel="up" accesskey="u" href="#Basics">Basics</a>
481
482 </div>
483
484 <h3 class="section">2.6 How are alarms triggered?</h3>
485
486 <p>It is possible to monitor each sensor and have an alarm go off if
487 it crosses some pre-determined limits.  There are two sorts of interrupts
488 which can be generated by sensor chips if this happens (it depends a bit on
489 the actual chip if both are supported; the LM80, for example, has only
490 IRQ interrupts): IRQ interrupts and SMI interrupts.  IRQ stands for
491 Interrupt Request and are the interrupt lines you can find in <samp><span class="file">/proc/interrupts</span></samp>.
492 SMI stands for System Management Interrupt, and is a special interrupt which
493 puts the processor in a secure environment independent of any other things
494 running.  SMI is currently not supported by the Linux kernel.  IRQs are
495 supported, of course.
496
497    <p>Even if no interrupt is generated, some bits in a status register
498 will be set until the register is read the next time. If the alarm condition
499 persists after that, the bits will be set on the next scanning sweep, etc.
500
501    <p>Most drivers in our package do not support interrupts at this time.
502
503 <div class="node">
504 <p><hr>
505 <a name="Installation"></a>
506 Next:&nbsp;<a rel=&qu