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1 | Kernel driver lm90 |
2 | ================== | |
3 | ||
4 | Supported chips: | |
5 | * National Semiconductor LM90 | |
6 | Prefix: 'lm90' | |
7 | Addresses scanned: I2C 0x4c | |
8 | Datasheet: Publicly available at the National Semiconductor website | |
9 | http://www.national.com/pf/LM/LM90.html | |
10 | * National Semiconductor LM89 | |
11 | Prefix: 'lm99' | |
12 | Addresses scanned: I2C 0x4c and 0x4d | |
13 | Datasheet: Publicly available at the National Semiconductor website | |
14 | http://www.national.com/pf/LM/LM89.html | |
15 | * National Semiconductor LM99 | |
16 | Prefix: 'lm99' | |
17 | Addresses scanned: I2C 0x4c and 0x4d | |
18 | Datasheet: Publicly available at the National Semiconductor website | |
19 | http://www.national.com/pf/LM/LM99.html | |
20 | * National Semiconductor LM86 | |
21 | Prefix: 'lm86' | |
22 | Addresses scanned: I2C 0x4c | |
23 | Datasheet: Publicly available at the National Semiconductor website | |
24 | http://www.national.com/pf/LM/LM86.html | |
25 | * Analog Devices ADM1032 | |
26 | Prefix: 'adm1032' | |
90209b42 | 27 | Addresses scanned: I2C 0x4c and 0x4d |
7f15b664 | 28 | Datasheet: Publicly available at the Analog Devices website |
90209b42 | 29 | http://www.analog.com/en/prod/0,2877,ADM1032,00.html |
7f15b664 M |
30 | * Analog Devices ADT7461 |
31 | Prefix: 'adt7461' | |
90209b42 | 32 | Addresses scanned: I2C 0x4c and 0x4d |
7f15b664 | 33 | Datasheet: Publicly available at the Analog Devices website |
90209b42 | 34 | http://www.analog.com/en/prod/0,2877,ADT7461,00.html |
7f15b664 M |
35 | Note: Only if in ADM1032 compatibility mode |
36 | * Maxim MAX6657 | |
37 | Prefix: 'max6657' | |
38 | Addresses scanned: I2C 0x4c | |
39 | Datasheet: Publicly available at the Maxim website | |
40 | http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2578 | |
41 | * Maxim MAX6658 | |
42 | Prefix: 'max6657' | |
43 | Addresses scanned: I2C 0x4c | |
44 | Datasheet: Publicly available at the Maxim website | |
45 | http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2578 | |
46 | * Maxim MAX6659 | |
47 | Prefix: 'max6657' | |
48 | Addresses scanned: I2C 0x4c, 0x4d (unsupported 0x4e) | |
49 | Datasheet: Publicly available at the Maxim website | |
50 | http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2578 | |
51 | ||
52 | ||
53 | Author: Jean Delvare <khali@linux-fr.org> | |
54 | ||
55 | ||
56 | Description | |
57 | ----------- | |
58 | ||
59 | The LM90 is a digital temperature sensor. It senses its own temperature as | |
60 | well as the temperature of up to one external diode. It is compatible | |
61 | with many other devices such as the LM86, the LM89, the LM99, the ADM1032, | |
62 | the MAX6657, MAX6658 and the MAX6659 all of which are supported by this driver. | |
63 | Note that there is no easy way to differentiate between the last three | |
64 | variants. The extra address and features of the MAX6659 are not supported by | |
65 | this driver. Additionally, the ADT7461 is supported if found in ADM1032 | |
66 | compatibility mode. | |
67 | ||
68 | The specificity of this family of chipsets over the ADM1021/LM84 | |
69 | family is that it features critical limits with hysteresis, and an | |
70 | increased resolution of the remote temperature measurement. | |
71 | ||
72 | The different chipsets of the family are not strictly identical, although | |
73 | very similar. This driver doesn't handle any specific feature for now, | |
c3df5806 JD |
74 | with the exception of SMBus PEC. For reference, here comes a non-exhaustive |
75 | list of specific features: | |
7f15b664 M |
76 | |
77 | LM90: | |
78 | * Filter and alert configuration register at 0xBF. | |
79 | * ALERT is triggered by temperatures over critical limits. | |
80 | ||
81 | LM86 and LM89: | |
82 | * Same as LM90 | |
83 | * Better external channel accuracy | |
84 | ||
85 | LM99: | |
86 | * Same as LM89 | |
87 | * External temperature shifted by 16 degrees down | |
88 | ||
89 | ADM1032: | |
90 | * Consecutive alert register at 0x22. | |
91 | * Conversion averaging. | |
92 | * Up to 64 conversions/s. | |
93 | * ALERT is triggered by open remote sensor. | |
c3df5806 | 94 | * SMBus PEC support for Write Byte and Receive Byte transactions. |
7f15b664 M |
95 | |
96 | ADT7461 | |
97 | * Extended temperature range (breaks compatibility) | |
98 | * Lower resolution for remote temperature | |
99 | ||
100 | MAX6657 and MAX6658: | |
101 | * Remote sensor type selection | |
102 | ||
103 | MAX6659 | |
104 | * Selectable address | |
105 | * Second critical temperature limit | |
106 | * Remote sensor type selection | |
107 | ||
108 | All temperature values are given in degrees Celsius. Resolution | |
109 | is 1.0 degree for the local temperature, 0.125 degree for the remote | |
110 | temperature. | |
111 | ||
112 | Each sensor has its own high and low limits, plus a critical limit. | |
113 | Additionally, there is a relative hysteresis value common to both critical | |
114 | values. To make life easier to user-space applications, two absolute values | |
115 | are exported, one for each channel, but these values are of course linked. | |
116 | Only the local hysteresis can be set from user-space, and the same delta | |
117 | applies to the remote hysteresis. | |
118 | ||
119 | The lm90 driver will not update its values more frequently than every | |
120 | other second; reading them more often will do no harm, but will return | |
121 | 'old' values. | |
122 | ||
c3df5806 JD |
123 | PEC Support |
124 | ----------- | |
125 | ||
126 | The ADM1032 is the only chip of the family which supports PEC. It does | |
127 | not support PEC on all transactions though, so some care must be taken. | |
128 | ||
129 | When reading a register value, the PEC byte is computed and sent by the | |
130 | ADM1032 chip. However, in the case of a combined transaction (SMBus Read | |
131 | Byte), the ADM1032 computes the CRC value over only the second half of | |
132 | the message rather than its entirety, because it thinks the first half | |
133 | of the message belongs to a different transaction. As a result, the CRC | |
134 | value differs from what the SMBus master expects, and all reads fail. | |
135 | ||
136 | For this reason, the lm90 driver will enable PEC for the ADM1032 only if | |
137 | the bus supports the SMBus Send Byte and Receive Byte transaction types. | |
138 | These transactions will be used to read register values, instead of | |
139 | SMBus Read Byte, and PEC will work properly. | |
140 | ||
141 | Additionally, the ADM1032 doesn't support SMBus Send Byte with PEC. | |
142 | Instead, it will try to write the PEC value to the register (because the | |
143 | SMBus Send Byte transaction with PEC is similar to a Write Byte transaction | |
144 | without PEC), which is not what we want. Thus, PEC is explicitely disabled | |
145 | on SMBus Send Byte transactions in the lm90 driver. | |
146 | ||
147 | PEC on byte data transactions represents a significant increase in bandwidth | |
148 | usage (+33% for writes, +25% for reads) in normal conditions. With the need | |
149 | to use two SMBus transaction for reads, this overhead jumps to +50%. Worse, | |
150 | two transactions will typically mean twice as much delay waiting for | |
151 | transaction completion, effectively doubling the register cache refresh time. | |
152 | I guess reliability comes at a price, but it's quite expensive this time. | |
153 | ||
154 | So, as not everyone might enjoy the slowdown, PEC can be disabled through | |
155 | sysfs. Just write 0 to the "pec" file and PEC will be disabled. Write 1 | |
156 | to that file to enable PEC again. |