Documentation/hwmon/lm90

   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: 'lm89' (no auto-detection)
  12     Addresses scanned: I2C 0x4c and 0x4d
  13     Datasheet: Publicly available at the National Semiconductor website
  14                http://www.national.com/mpf/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/mpf/LM/LM86.html
  25   * Analog Devices ADM1032
  26     Prefix: 'adm1032'
  27     Addresses scanned: I2C 0x4c and 0x4d
  28     Datasheet: Publicly available at the ON Semiconductor website
  29                http://www.onsemi.com/PowerSolutions/product.do?id=ADM1032
  30   * Analog Devices ADT7461
  31     Prefix: 'adt7461'
  32     Addresses scanned: I2C 0x4c and 0x4d
  33     Datasheet: Publicly available at the ON Semiconductor website
  34                http://www.onsemi.com/PowerSolutions/product.do?id=ADT7461
  35   * Maxim MAX6646
  36     Prefix: 'max6646'
  37     Addresses scanned: I2C 0x4d
  38     Datasheet: Publicly available at the Maxim website
  39                http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3497
  40   * Maxim MAX6647
  41     Prefix: 'max6646'
  42     Addresses scanned: I2C 0x4e
  43     Datasheet: Publicly available at the Maxim website
  44                http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3497
  45   * Maxim MAX6648
  46     Prefix: 'max6646'
  47     Addresses scanned: I2C 0x4c
  48     Datasheet: Publicly available at the Maxim website
  49                http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3500
  50   * Maxim MAX6649
  51     Prefix: 'max6646'
  52     Addresses scanned: I2C 0x4c
  53     Datasheet: Publicly available at the Maxim website
  54                http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3497
  55   * Maxim MAX6657
  56     Prefix: 'max6657'
  57     Addresses scanned: I2C 0x4c
  58     Datasheet: Publicly available at the Maxim website
  59                http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2578
  60   * Maxim MAX6658
  61     Prefix: 'max6657'
  62     Addresses scanned: I2C 0x4c
  63     Datasheet: Publicly available at the Maxim website
  64                http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2578
  65   * Maxim MAX6659
  66     Prefix: 'max6657'
  67     Addresses scanned: I2C 0x4c, 0x4d (unsupported 0x4e)
  68     Datasheet: Publicly available at the Maxim website
  69                http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2578
  70   * Maxim MAX6680
  71     Prefix: 'max6680'
  72     Addresses scanned: I2C 0x18, 0x19, 0x1a, 0x29, 0x2a, 0x2b,
  73                            0x4c, 0x4d and 0x4e
  74     Datasheet: Publicly available at the Maxim website
  75                http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3370
  76   * Maxim MAX6681
  77     Prefix: 'max6680'
  78     Addresses scanned: I2C 0x18, 0x19, 0x1a, 0x29, 0x2a, 0x2b,
  79                            0x4c, 0x4d and 0x4e
  80     Datasheet: Publicly available at the Maxim website
  81                http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3370
  82   * Maxim MAX6692
  83     Prefix: 'max6646'
  84     Addresses scanned: I2C 0x4c
  85     Datasheet: Publicly available at the Maxim website
  86                http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3500
  87
  88
  89 Author: Jean Delvare <khali@linux-fr.org>
  90
  91
  92 Description
  93 -----------
  94
  95 The LM90 is a digital temperature sensor. It senses its own temperature as
  96 well as the temperature of up to one external diode. It is compatible
  97 with many other devices, many of which are supported by this driver.
  98
  99 Note that there is no easy way to differentiate between the MAX6657,
 100 MAX6658 and MAX6659 variants. The extra address and features of the
 101 MAX6659 are not supported by this driver. The MAX6680 and MAX6681 only
 102 differ in their pinout, therefore they obviously can't (and don't need to)
 103 be distinguished.
 104
 105 The specificity of this family of chipsets over the ADM1021/LM84
 106 family is that it features critical limits with hysteresis, and an
 107 increased resolution of the remote temperature measurement.
 108
 109 The different chipsets of the family are not strictly identical, although
 110 very similar. For reference, here comes a non-exhaustive list of specific
 111 features:
 112
 113 LM90:
 114   * Filter and alert configuration register at 0xBF.
 115   * ALERT is triggered by temperatures over critical limits.
 116
 117 LM86 and LM89:
 118   * Same as LM90
 119   * Better external channel accuracy
 120
 121 LM99:
 122   * Same as LM89
 123   * External temperature shifted by 16 degrees down
 124
 125 ADM1032:
 126   * Consecutive alert register at 0x22.
 127   * Conversion averaging.
 128   * Up to 64 conversions/s.
 129   * ALERT is triggered by open remote sensor.
 130   * SMBus PEC support for Write Byte and Receive Byte transactions.
 131
 132 ADT7461:
 133   * Extended temperature range (breaks compatibility)
 134   * Lower resolution for remote temperature
 135
 136 MAX6657 and MAX6658:
 137   * Better local resolution
 138   * Remote sensor type selection
 139
 140 MAX6659:
 141   * Better local resolution
 142   * Selectable address
 143   * Second critical temperature limit
 144   * Remote sensor type selection
 145
 146 MAX6680 and MAX6681:
 147   * Selectable address
 148   * Remote sensor type selection
 149
 150 All temperature values are given in degrees Celsius. Resolution
 151 is 1.0 degree for the local temperature, 0.125 degree for the remote
 152 temperature, except for the MAX6657, MAX6658 and MAX6659 which have a
 153 resolution of 0.125 degree for both temperatures.
 154
 155 Each sensor has its own high and low limits, plus a critical limit.
 156 Additionally, there is a relative hysteresis value common to both critical
 157 values. To make life easier to user-space applications, two absolute values
 158 are exported, one for each channel, but these values are of course linked.
 159 Only the local hysteresis can be set from user-space, and the same delta
 160 applies to the remote hysteresis.
 161
 162 The lm90 driver will not update its values more frequently than every
 163 other second; reading them more often will do no harm, but will return
 164 'old' values.
 165
 166 PEC Support
 167 -----------
 168
 169 The ADM1032 is the only chip of the family which supports PEC. It does
 170 not support PEC on all transactions though, so some care must be taken.
 171
 172 When reading a register value, the PEC byte is computed and sent by the
 173 ADM1032 chip. However, in the case of a combined transaction (SMBus Read
 174 Byte), the ADM1032 computes the CRC value over only the second half of
 175 the message rather than its entirety, because it thinks the first half
 176 of the message belongs to a different transaction. As a result, the CRC
 177 value differs from what the SMBus master expects, and all reads fail.
 178
 179 For this reason, the lm90 driver will enable PEC for the ADM1032 only if
 180 the bus supports the SMBus Send Byte and Receive Byte transaction types.
 181 These transactions will be used to read register values, instead of
 182 SMBus Read Byte, and PEC will work properly.
 183
 184 Additionally, the ADM1032 doesn't support SMBus Send Byte with PEC.
 185 Instead, it will try to write the PEC value to the register (because the
 186 SMBus Send Byte transaction with PEC is similar to a Write Byte transaction
 187 without PEC), which is not what we want. Thus, PEC is explicitly disabled
 188 on SMBus Send Byte transactions in the lm90 driver.
 189
 190 PEC on byte data transactions represents a significant increase in bandwidth
 191 usage (+33% for writes, +25% for reads) in normal conditions. With the need
 192 to use two SMBus transaction for reads, this overhead jumps to +50%. Worse,
 193 two transactions will typically mean twice as much delay waiting for
 194 transaction completion, effectively doubling the register cache refresh time.
 195 I guess reliability comes at a price, but it's quite expensive this time.
 196
 197 So, as not everyone might enjoy the slowdown, PEC can be disabled through
 198 sysfs. Just write 0 to the "pec" file and PEC will be disabled. Write 1
 199 to that file to enable PEC again.