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'
27 Addresses scanned: I2C 0x4c and 0x4d
28 Datasheet: Publicly available at the Analog Devices website
29 http://www.analog.com/en/prod/0,2877,ADM1032,00.html
30 * Analog Devices ADT7461
31 Prefix: 'adt7461'
32 Addresses scanned: I2C 0x4c and 0x4d
33 Datasheet: Publicly available at the Analog Devices website
34 http://www.analog.com/en/prod/0,2877,ADT7461,00.html
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,
74 with the exception of SMBus PEC. For reference, here comes a non-exhaustive
75 list of specific features:
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.
94 * SMBus PEC support for Write Byte and Receive Byte transactions.
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
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.
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