lm_sensors-FAQ.html

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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, adapter, and algorithm 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 particular systems</a>
168	<li><a href="#Section-4_002e33">4.33 Problems on 2.6 kernels</a>
169	<ul>
170	<li><a href="#Section-4_002e33">4.33.1 i2c-viapro and via686a</a>
171	<li><a href="#Section-4_002e33">4.33.2 Where are my EEPROMs?</a>
172	</li></ul>
173	</li></ul>
174	<li><a name="toc_Help" href="#Help">5 How to Ask for Help</a>
175	<ul>
176	<li><a href="#Section-5_002e1">5.1 What to send us when asking for help</a>
177	<li><a href="#Section-5_002e2">5.2 What to do if a module won't insert?</a>
178	<li><a href="#Section-5_002e3">5.3 What to do if it inserts, but nothing happens?</a>
179	<li><a href="#Section-5_002e4">5.4 What to do if I read only bogus information?</a>
180	<li><a href="#Section-5_002e5">5.5 What to do if you have other problems?</a>
181	<li><a href="#Section-5_002e6">5.6 What if it just works like a charm?</a>
182	<li><a href="#Section-5_002e7">5.7 How do I update a ticket?</a>
183	<li><a href="#Section-5_002e8">5.8 How do I follow up on a ticket?</a>
184	<li><a href="#Section-5_002e9">5.9 Why did you decide not to support undocumented chips?</a>
185	</li></ul>
186	<li><a name="toc_Contribute" href="#Contribute">6 How to Contribute</a>
187	<ul>
188	<li><a href="#Section-6_002e1">6.1 How to write a driver</a>
189	<li><a href="#Section-6_002e2">6.2 How to get SVN access</a>
190	<li><a href="#Section-6_002e3">6.3 How to donate hardware to the project</a>
191	<li><a href="#Section-6_002e4">6.4 How to join the project mailing lists</a>
192	<li><a href="#Section-6_002e5">6.5 How to access mailing list archives</a>
193	<li><a href="#Section-6_002e6">6.6 How to submit a patch</a>
194	<li><a href="#Section-6_002e7">6.7 How to REALLY help</a>
195	<li><a href="#Section-6_002e8">6.8 How to get release announcements</a>
196	</li></ul>
197	<li><a name="toc_Document-Revisions" href="#Document-Revisions">Appendix A Revision History of This Document</a>
198	</li></ul>
199	</div>
200
201	<div class="node">
202	<p><hr>
203	<a name="Top"></a>
204	Next: <a rel="next" accesskey="n" href="#Overview">Overview</a>,
205	Up: <a rel="up" accesskey="u" href="#dir">(dir)</a>
206
207	</div>
208
209	<h2 class="unnumbered">lm_sensors</h2>
210
211	<p>The lm_sensors package includes a collection of modules for general SMBus
212	access and hardware monitoring. NOTE: this requires special support which
213	is not in standard 2.2-vintage kernels.
214
215	<ul class="menu">
216	<li><a accesskey="1" href="#Overview">Overview</a>: PC and Sensor Overview
217	<li><a accesskey="2" href="#Basics">Basics</a>: Sensor and Bus Basics
218	<li><a accesskey="3" href="#Installation">Installation</a>: Installation and Management
219	<li><a accesskey="4" href="#Problems">Problems</a>: Problems
220	<li><a accesskey="5" href="#Help">Help</a>: How to Ask for Help
221	<li><a accesskey="6" href="#Contribute">Contribute</a>: How to Contribute
222	<li><a accesskey="7" href="#Document-Revisions">Document Revisions</a>: Revision History of This Document
223	</ul>
224
225	<div class="node">
226	<p><hr>
227	<a name="Overview"></a>
228	Next: <a rel="next" accesskey="n" href="#Basics">Basics</a>,
229	Previous: <a rel="previous" accesskey="p" href="#Top">Top</a>,
230	Up: <a rel="up" accesskey="u" href="#Top">Top</a>
231
232	</div>
233
234	<h2 class="chapter">1 PC and Sensor Overview</h2>
235
236	<ul class="menu">
237	<li><a accesskey="1" href="#Section-1_002e1">Section 1.1</a>: What sensors are available on my PC?
238	<li><a accesskey="2" href="#Section-1_002e2">Section 1.2</a>: What can a sensor chip like the "LM78" do?
239	<li><a accesskey="3" href="#Section-1_002e3">Section 1.3</a>: Where do I find out more about any of these chips?
240	</ul>
241
242	<div class="node">
243	<p><hr>
244	<a name="Section-1.1"></a>
245	<a name="Section-1_002e1"></a>
246	Next: <a rel="next" accesskey="n" href="#Section-1_002e2">Section 1.2</a>,
247	Up: <a rel="up" accesskey="u" href="#Overview">Overview</a>
248
249	</div>
250
251	<h3 class="section">1.1 What sensors are available on my PC?</h3>
252
253	<p>Most PC's built since late 1997 now come with a
254	hardware health monitoring chip. This chip may be accessed via the
255	ISA bus or the SMBus, depending on the motherboard.
256
257	<p>Some motherboard chipsets, notably the Via 686 and the SiS 5595,
258	contain hardware monitor functions.
259
260	<p>This FAQ frequently refers to the "LM78". This chip has been
261	obsoleted by National Semiconductor. Most motherboards today contain
262	a chip with similar functions.
263
264	<div class="node">
265	<p><hr>
266	<a name="Section-1.2"></a>
267	<a name="Section-1_002e2"></a>
268	Next: <a rel="next" accesskey="n" href="#Section-1_002e3">Section 1.3</a>,
269	Previous: <a rel="previous" accesskey="p" href="#Section-1_002e1">Section 1.1</a>,
270	Up: <a rel="up" accesskey="u" href="#Overview">Overview</a>
271
272	</div>
273
274	<h3 class="section">1.2 What can a sensor chip like the "LM78" do?</h3>
275
276	<p>The LM78 is a chip made by National Semiconductor which can monitor 7
277	voltages (5 positive, 2 negative) from 0 to 4.08V. The inputs are usually in
278	series with voltage dividers which lower the +/- 12V and +/- 5V supplies to
279	measurable range. Therefore, the readings for such inputs need to be
280	re-scaled appropriately by software.
281
282	<p>The LM78 also has 3 fan speed monitoring inputs, an internal
283	temperature sensor, a chassis intrusion sensor, and a couple maskable interrupt
284	inputs. The LM78 can also relay the processor's (P6 or Pent II) VID lines
285	which are hardwired and used to indicate to the power regulator (usually on
286	the mainboard close to the processor socket/slot) what voltage to supply to
287	the processor.
288
289	<p>The LM78 can be interfaced to a system via the ISA bus and/or the
290	SMBus.
291
292	<p>Most other sensor chips have comparable functionality. Each supported
293	chip is documented in the <samp><span class="file">doc/chips</span></samp> directory.
294
295	<div class="node">
296	<p><hr>
297	<a name="Section-1.3"></a>
298	<a name="Section-1_002e3"></a>
299	Previous: <a rel="previous" accesskey="p" href="#Section-1_002e2">Section 1.2</a>,
300	Up: <a rel="up" accesskey="u" href="#Overview">Overview</a>
301
302	</div>
303
304	<h3 class="section">1.3 Where do I find out more about any of these chips?</h3>
305
306	<p>Most semiconductor companies have comprehensive documentation,
307	including complete datasheets, on their websites. Analog Devices,
308	Dallas Semiconductor, Maxim, and National Semiconductor have the widest selection
309	of sensor chips. Their websites are:
310
311	<ul>
312	<li><a href="http://www.analog.com">http://www.analog.com</a>
313	<li><a href="http://www.dalsemi.com">http://www.dalsemi.com</a>
314	<li><a href="http://www.maxim-ic.com">http://www.maxim-ic.com</a>
315	<li><a href="http://www.national.com">http://www.national.com</a>
316	</ul>
317
318	<p>Please see the file <a href="http://www.lm-sensors.org/wiki/UsefulLinks">http://www.lm-sensors.org/wiki/UsefulLinks</a>
319	for links to other companies' websites.
320
321	<div class="node">
322	<p><hr>
323	<a name="Basics"></a>
324	Next: <a rel="next" accesskey="n" href="#Installation">Installation</a>,
325	Previous: <a rel="previous" accesskey="p" href="#Overview">Overview</a>,
326	Up: <a rel="up" accesskey="u" href="#Top">Top</a>
327
328	</div>
329
330	<h2 class="chapter">2 Sensor and Bus Basics</h2>
331
332	<ul class="menu">
333	<li><a accesskey="1" href="#Section-2_002e1">Section 2.1</a>: What sensors are available on my PC?
334	<li><a accesskey="2" href="#Section-2_002e2">Section 2.2</a>: What can a sensor chip like the "LM78" do?
335	<li><a accesskey="3" href="#Section-2_002e3">Section 2.3</a>: Where do I find out more about any of these chips?
336	<li><a accesskey="4" href="#Section-2_002e4">Section 2.4</a>: What sensors are available on my PC?
337	<li><a accesskey="5" href="#Section-2_002e5">Section 2.5</a>: What can a sensor chip like the "LM78" do?
338	<li><a accesskey="6" href="#Section-2_002e6">Section 2.6</a>: Where do I find out more about any of these chips?
339	</ul>
340
341	<div class="node">
342	<p><hr>
343	<a name="Section-2.1"></a>
344	<a name="Section-2_002e1"></a>
345	Next: <a rel="next" accesskey="n" href="#Section-2_002e2">Section 2.2</a>,
346	Up: <a rel="up" accesskey="u" href="#Basics">Basics</a>
347
348	</div>
349
350	<h3 class="section">2.1 How are these sensors read?</h3>
351
352	<p>Sensor chips reside on either the ISA bus, the SMBus, or both.
353	See the file <samp><span class="file">doc/chips/SUMMARY</span></samp> in our package for a list.
354
355	<p>To communicate with chips on the ISA bus, the software uses
356	simple I/O reads and writes.
357
358	<p>To communicate with chips on the SMBus, the software must
359	use an SMBus interface device, explained below.
360
361	<div class="node">
362	<p><hr>
363	<a name="Section-2.2"></a>
364	<a name="Section-2_002e2"></a>
365	Next: <a rel="next" accesskey="n" href="#Section-2_002e3">Section 2.3</a>,
366	Previous: <a rel="previous" accesskey="p" href="#Section-2_002e1">Section 2.1</a>,
367	Up: <a rel="up" accesskey="u" href="#Basics">Basics</a>
368
369	</div>
370
371	<h3 class="section">2.2 What is the SMBus? And the I2C bus?</h3>
372
373	<p>The SMBus is the "System Management Bus". More specifically, it is a
374	2-wire, low-speed serial communication bus used for basic health monitoring
375	and hardware management. It is a specific implementation of the more
376	general I2C (pronunciation: I-squared-C) bus. In fact, both I2C devices
377	and SMBus devices may be connected to the same (I2C) bus.
378
379	<p>The SMBus (or I2C bus) starts at the host controller, used for
380	starting transactions on the SMBus. From the host interface, the
381	devices communicated with are the <dfn>slave</dfn> devices. Each slave device has a
382	unique 7-bit address which the host uses to refer to that device.
383
384	<p>For each supported SMBus host, there is a separate kernel module
385	which implements the communication protocol with the host. Some SMBus hosts
386	really operate on the SMBus level; these hosts can not cope with pure I2C
387	devices. Other hosts are in fact I2C hosts: in this case, we implement
388	the SMBus protocol in terms of I2C operations. But these hosts can also
389	talk to pure I2C devices.
390
391	<div class="node">
392	<p><hr>
393	<a name="Section-2.3"></a>
394	<a name="Section-2_002e3"></a>
395	Next: <a rel="next" accesskey="n" href="#Section-2_002e4">Section 2.4</a>,
396	Previous: <a rel="previous" accesskey="p" href="#Section-2_002e2">Section 2.2</a>,
397	Up: <a rel="up" accesskey="u" href="#Basics">Basics</a>
398
399	</div>
400
401	<h3 class="section">2.3 I don't have an ISA bus!</h3>
402
403	<p>We promise, you do, even if you don't have any old ISA slots.
404	The "ISA Bus" exists in your computer even if you don't have ISA slots;
405	it is simply a memory-mapped area, 64KB in size (0x0000 - 0xFFFF)
406	where many "legacy" functions, such as keyboard and interrupt controllers,
407	are found. It isn't necessarily a separate physical bus.
408	See the file <samp><span class="file">/proc/ioports</span></samp> for a list of devices living on
409	the "ISA Bus" in your system. If you don't like the term "ISA Bus"
410	think "I/O Space".
411
412	<div class="node">
413	<p><hr>
414	<a name="Section-2.4"></a>
415	<a name="Section-2_002e4"></a>
416	Next: <a rel="next" accesskey="n" href="#Section-2_002e5">Section 2.5</a>,
417	Previous: <a rel="previous" accesskey="p" href="#Section-2_002e3">Section 2.3</a>,
418	Up: <a rel="up" accesskey="u" href="#Basics">Basics</a>
419
420	</div>
421
422	<h3 class="section">2.4 What sensors do processors have?</h3>
423
424	<p>Most new processors contain a thermal diode on the die itself.
425	The electical properties of all diodes and transistors vary
426	slightly with temperature. The thermal diode is exceptionally accurate
427	because it is directly on the die. Newer temperature sensor chips,
428	like the Analog Devices ADM1021 and clones, and the Winbond chips,
429	have circuitry for measuring the the electrical properties of
430	an external diode and converting this data to a temperature.
431	Any sensor chip listed in <samp><span class="file">doc/chips/SUMMARY</span></samp> in our package which
432	has support for more than one temperature supports external temperature sensing.
433
434	<p>Older motherboards and processors without this feature generally use
435	an LM75 placed close to the processor. This is much less accurate.
436
437	<p>The Pentium 2 'boxed' processor usually has an LM75 very close to the
438	base of the box. It can be read through the SMBus to report the approximate
439	temperature of the processor. The processor also contains an internal
440	temperature sensor (of low accuracy) used as a fail-safe to disable the
441	processor in case it gets much too hot (usually around 130 degrees C). And,
442	the Pentium 2 also has a hard-wired signal (VID lines) on it's SEC (single
443	edge connector) which indicates what power supply is required to operate the
444	processor.
445
446	<p>The P6 (Pentium-Pro) may have an LM75 in or just under the socket.
447	P6's also have VID lines.
448
449	<p>Pentiums and Pentium w/ MMX do not have VID lines, and sometimes have
450	LM75's under the sockets (depends on the mainboard, and how 'modern' the
451	mainboard is).
452
453	<p>The P2 Xeon was the first Intel processor to include the SMBus
454	interface on the P2 Xeon SEC.
455
456	<div class="node">
457	<p><hr>
458	<a name="Section-2.5"></a>
459	<a name="Section-2_002e5"></a>
460	Next: <a rel="next" accesskey="n" href="#Section-2_002e6">Section 2.6</a>,
461	Previous: <a rel="previous" accesskey="p" href="#Section-2_002e4">Section 2.4</a>,
462	Up: <a rel="up" accesskey="u" href="#Basics">Basics</a>
463
464	</div>
465
466	<h3 class="section">2.5 How often are the sensor values updated?</h3>
467
468	<p>The LM78, and most other sensor chips like it, reads its sensors one
469	by one. A complete scanning sweep will take about 1.5 seconds. The LM78 stops
470	readings sensors if you try to access it, so if you access it very often
471	(by reading sensor values; writing new limits is safe) it will not find the
472	time to update its sensor values at all! Fortunately, the kernel module takes
473	care not to do this, and only reads new values each 1.5 seconds. If you
474	read the values again, you will get the 'old' values again.
475
476	<div class="node">
477	<p><hr>
478	<a name="Section-2.6"></a>
479	<a name="Section-2_002e6"></a>
480	Previous: <a rel="previous" accesskey="p" href="#Section-2_002e5">Section 2.5</a>,
481	Up: <a rel="up" accesskey="u" href="#Basics">Basics</a>
482
483	</div>
484
485	<h3 class="section">2.6 How are alarms triggered?</h3>
486
487	<p>It is possible to monitor each sensor and have an alarm go off if
488	it crosses some pre-determined limits. There are two sorts of interrupts
489	which can be generated by sensor chips if this happens (it depends a bit on
490	the actual chip if both are supported; the LM80, for example, has only
491	IRQ interrupts): IRQ interrupts and SMI interrupts. IRQ stands for
492	Interrupt Request and are the interrupt lines you can find in <samp><span class="file">/proc/interrupts</span></samp>.
493	SMI stands for System Management Interrupt, and is a special interrupt which
494	puts the processor in a secure environment independent of any other things
495	running. SMI is currently not supported by the Linux kernel. IRQs are
496	supported, of course.
497
498	<p>Even if no interrupt is generated, some bits in a status register
499	will be set until the register is read the next time. If the alarm condition
500	persists after that, the bits will be set on the next scanning sweep, etc.
501
502	<p>Most drivers in our package do not support interrupts at this time.
503
504	<div class="node">
505	<p><hr>