--- /dev/null
+Kernel driver lm90
+==================
+
+Supported chips:
+ * National Semiconductor LM90
+ Prefix: 'lm90'
+ Addresses scanned: I2C 0x4c
+ Datasheet: Publicly available at the National Semiconductor website
+ http://www.national.com/pf/LM/LM90.html
+ * National Semiconductor LM89
+ Prefix: 'lm99'
+ Addresses scanned: I2C 0x4c and 0x4d
+ Datasheet: Publicly available at the National Semiconductor website
+ http://www.national.com/pf/LM/LM89.html
+ * National Semiconductor LM99
+ Prefix: 'lm99'
+ Addresses scanned: I2C 0x4c and 0x4d
+ Datasheet: Publicly available at the National Semiconductor website
+ http://www.national.com/pf/LM/LM99.html
+ * National Semiconductor LM86
+ Prefix: 'lm86'
+ Addresses scanned: I2C 0x4c
+ Datasheet: Publicly available at the National Semiconductor website
+ http://www.national.com/pf/LM/LM86.html
+ * Analog Devices ADM1032
+ Prefix: 'adm1032'
+ Addresses scanned: I2C 0x4c and 0x4d
+ Datasheet: Publicly available at the Analog Devices website
+ http://www.analog.com/en/prod/0,2877,ADM1032,00.html
+ * Analog Devices ADT7461
+ Prefix: 'adt7461'
+ Addresses scanned: I2C 0x4c and 0x4d
+ Datasheet: Publicly available at the Analog Devices website
+ http://www.analog.com/en/prod/0,2877,ADT7461,00.html
+ Note: Only if in ADM1032 compatibility mode
+ * Maxim MAX6657
+ Prefix: 'max6657'
+ Addresses scanned: I2C 0x4c
+ Datasheet: Publicly available at the Maxim website
+ http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2578
+ * Maxim MAX6658
+ Prefix: 'max6657'
+ Addresses scanned: I2C 0x4c
+ Datasheet: Publicly available at the Maxim website
+ http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2578
+ * Maxim MAX6659
+ Prefix: 'max6657'
+ Addresses scanned: I2C 0x4c, 0x4d (unsupported 0x4e)
+ Datasheet: Publicly available at the Maxim website
+ http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2578
+
+
+Author: Jean Delvare <khali@linux-fr.org>
+
+
+Description
+-----------
+
+The LM90 is a digital temperature sensor. It senses its own temperature as
+well as the temperature of up to one external diode. It is compatible
+with many other devices such as the LM86, the LM89, the LM99, the ADM1032,
+the MAX6657, MAX6658 and the MAX6659 all of which are supported by this driver.
+Note that there is no easy way to differentiate between the last three
+variants. The extra address and features of the MAX6659 are not supported by
+this driver. Additionally, the ADT7461 is supported if found in ADM1032
+compatibility mode.
+
+The specificity of this family of chipsets over the ADM1021/LM84
+family is that it features critical limits with hysteresis, and an
+increased resolution of the remote temperature measurement.
+
+The different chipsets of the family are not strictly identical, although
+very similar. This driver doesn't handle any specific feature for now,
+with the exception of SMBus PEC. For reference, here comes a non-exhaustive
+list of specific features:
+
+LM90:
+ * Filter and alert configuration register at 0xBF.
+ * ALERT is triggered by temperatures over critical limits.
+
+LM86 and LM89:
+ * Same as LM90
+ * Better external channel accuracy
+
+LM99:
+ * Same as LM89
+ * External temperature shifted by 16 degrees down
+
+ADM1032:
+ * Consecutive alert register at 0x22.
+ * Conversion averaging.
+ * Up to 64 conversions/s.
+ * ALERT is triggered by open remote sensor.
+ * SMBus PEC support for Write Byte and Receive Byte transactions.
+
+ADT7461
+ * Extended temperature range (breaks compatibility)
+ * Lower resolution for remote temperature
+
+MAX6657 and MAX6658:
+ * Remote sensor type selection
+
+MAX6659
+ * Selectable address
+ * Second critical temperature limit
+ * Remote sensor type selection
+
+All temperature values are given in degrees Celsius. Resolution
+is 1.0 degree for the local temperature, 0.125 degree for the remote
+temperature.
+
+Each sensor has its own high and low limits, plus a critical limit.
+Additionally, there is a relative hysteresis value common to both critical
+values. To make life easier to user-space applications, two absolute values
+are exported, one for each channel, but these values are of course linked.
+Only the local hysteresis can be set from user-space, and the same delta
+applies to the remote hysteresis.
+
+The lm90 driver will not update its values more frequently than every
+other second; reading them more often will do no harm, but will return
+'old' values.
+
+PEC Support
+-----------
+
+The ADM1032 is the only chip of the family which supports PEC. It does
+not support PEC on all transactions though, so some care must be taken.
+
+When reading a register value, the PEC byte is computed and sent by the
+ADM1032 chip. However, in the case of a combined transaction (SMBus Read
+Byte), the ADM1032 computes the CRC value over only the second half of
+the message rather than its entirety, because it thinks the first half
+of the message belongs to a different transaction. As a result, the CRC
+value differs from what the SMBus master expects, and all reads fail.
+
+For this reason, the lm90 driver will enable PEC for the ADM1032 only if
+the bus supports the SMBus Send Byte and Receive Byte transaction types.
+These transactions will be used to read register values, instead of
+SMBus Read Byte, and PEC will work properly.
+
+Additionally, the ADM1032 doesn't support SMBus Send Byte with PEC.
+Instead, it will try to write the PEC value to the register (because the
+SMBus Send Byte transaction with PEC is similar to a Write Byte transaction
+without PEC), which is not what we want. Thus, PEC is explicitely disabled
+on SMBus Send Byte transactions in the lm90 driver.
+
+PEC on byte data transactions represents a significant increase in bandwidth
+usage (+33% for writes, +25% for reads) in normal conditions. With the need
+to use two SMBus transaction for reads, this overhead jumps to +50%. Worse,
+two transactions will typically mean twice as much delay waiting for
+transaction completion, effectively doubling the register cache refresh time.
+I guess reliability comes at a price, but it's quite expensive this time.
+
+So, as not everyone might enjoy the slowdown, PEC can be disabled through
+sysfs. Just write 0 to the "pec" file and PEC will be disabled. Write 1
+to that file to enable PEC again.
git://git.onelab.eu / linux-2.6.git / blobdiff