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 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 explicitly 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.