156 lines
6.2 KiB
Text
156 lines
6.2 KiB
Text
Kernel driver lm90
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==================
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Supported chips:
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* National Semiconductor LM90
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Prefix: 'lm90'
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Addresses scanned: I2C 0x4c
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Datasheet: Publicly available at the National Semiconductor website
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http://www.national.com/pf/LM/LM90.html
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* National Semiconductor LM89
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Prefix: 'lm99'
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Addresses scanned: I2C 0x4c and 0x4d
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Datasheet: Publicly available at the National Semiconductor website
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http://www.national.com/pf/LM/LM89.html
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* National Semiconductor LM99
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Prefix: 'lm99'
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Addresses scanned: I2C 0x4c and 0x4d
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Datasheet: Publicly available at the National Semiconductor website
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http://www.national.com/pf/LM/LM99.html
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* National Semiconductor LM86
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Prefix: 'lm86'
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Addresses scanned: I2C 0x4c
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Datasheet: Publicly available at the National Semiconductor website
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http://www.national.com/pf/LM/LM86.html
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* Analog Devices ADM1032
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Prefix: 'adm1032'
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Addresses scanned: I2C 0x4c and 0x4d
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Datasheet: Publicly available at the Analog Devices website
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http://www.analog.com/en/prod/0,2877,ADM1032,00.html
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* Analog Devices ADT7461
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Prefix: 'adt7461'
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Addresses scanned: I2C 0x4c and 0x4d
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Datasheet: Publicly available at the Analog Devices website
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http://www.analog.com/en/prod/0,2877,ADT7461,00.html
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Note: Only if in ADM1032 compatibility mode
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* Maxim MAX6657
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Prefix: 'max6657'
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Addresses scanned: I2C 0x4c
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Datasheet: Publicly available at the Maxim website
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http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2578
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* Maxim MAX6658
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Prefix: 'max6657'
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Addresses scanned: I2C 0x4c
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Datasheet: Publicly available at the Maxim website
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http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2578
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* Maxim MAX6659
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Prefix: 'max6657'
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Addresses scanned: I2C 0x4c, 0x4d (unsupported 0x4e)
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Datasheet: Publicly available at the Maxim website
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http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2578
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Author: Jean Delvare <khali@linux-fr.org>
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Description
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-----------
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The LM90 is a digital temperature sensor. It senses its own temperature as
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well as the temperature of up to one external diode. It is compatible
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with many other devices such as the LM86, the LM89, the LM99, the ADM1032,
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the MAX6657, MAX6658 and the MAX6659 all of which are supported by this driver.
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Note that there is no easy way to differentiate between the last three
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variants. The extra address and features of the MAX6659 are not supported by
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this driver. Additionally, the ADT7461 is supported if found in ADM1032
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compatibility mode.
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The specificity of this family of chipsets over the ADM1021/LM84
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family is that it features critical limits with hysteresis, and an
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increased resolution of the remote temperature measurement.
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The different chipsets of the family are not strictly identical, although
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very similar. This driver doesn't handle any specific feature for now,
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with the exception of SMBus PEC. For reference, here comes a non-exhaustive
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list of specific features:
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LM90:
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* Filter and alert configuration register at 0xBF.
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* ALERT is triggered by temperatures over critical limits.
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LM86 and LM89:
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* Same as LM90
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* Better external channel accuracy
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LM99:
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* Same as LM89
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* External temperature shifted by 16 degrees down
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ADM1032:
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* Consecutive alert register at 0x22.
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* Conversion averaging.
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* Up to 64 conversions/s.
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* ALERT is triggered by open remote sensor.
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* SMBus PEC support for Write Byte and Receive Byte transactions.
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ADT7461
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* Extended temperature range (breaks compatibility)
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* Lower resolution for remote temperature
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MAX6657 and MAX6658:
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* Remote sensor type selection
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MAX6659
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* Selectable address
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* Second critical temperature limit
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* Remote sensor type selection
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All temperature values are given in degrees Celsius. Resolution
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is 1.0 degree for the local temperature, 0.125 degree for the remote
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temperature.
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Each sensor has its own high and low limits, plus a critical limit.
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Additionally, there is a relative hysteresis value common to both critical
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values. To make life easier to user-space applications, two absolute values
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are exported, one for each channel, but these values are of course linked.
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Only the local hysteresis can be set from user-space, and the same delta
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applies to the remote hysteresis.
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The lm90 driver will not update its values more frequently than every
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other second; reading them more often will do no harm, but will return
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'old' values.
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PEC Support
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-----------
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The ADM1032 is the only chip of the family which supports PEC. It does
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not support PEC on all transactions though, so some care must be taken.
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When reading a register value, the PEC byte is computed and sent by the
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ADM1032 chip. However, in the case of a combined transaction (SMBus Read
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Byte), the ADM1032 computes the CRC value over only the second half of
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the message rather than its entirety, because it thinks the first half
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of the message belongs to a different transaction. As a result, the CRC
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value differs from what the SMBus master expects, and all reads fail.
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For this reason, the lm90 driver will enable PEC for the ADM1032 only if
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the bus supports the SMBus Send Byte and Receive Byte transaction types.
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These transactions will be used to read register values, instead of
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SMBus Read Byte, and PEC will work properly.
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Additionally, the ADM1032 doesn't support SMBus Send Byte with PEC.
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Instead, it will try to write the PEC value to the register (because the
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SMBus Send Byte transaction with PEC is similar to a Write Byte transaction
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without PEC), which is not what we want. Thus, PEC is explicitly disabled
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on SMBus Send Byte transactions in the lm90 driver.
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PEC on byte data transactions represents a significant increase in bandwidth
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usage (+33% for writes, +25% for reads) in normal conditions. With the need
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to use two SMBus transaction for reads, this overhead jumps to +50%. Worse,
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two transactions will typically mean twice as much delay waiting for
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transaction completion, effectively doubling the register cache refresh time.
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I guess reliability comes at a price, but it's quite expensive this time.
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So, as not everyone might enjoy the slowdown, PEC can be disabled through
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sysfs. Just write 0 to the "pec" file and PEC will be disabled. Write 1
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to that file to enable PEC again.
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