Kernel driver lm90

Supported chips:

Author: Jean Delvare <>


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, many of which are supported by this driver.

The family of chips supported by this driver is derived from MAX1617. This chip as well as various compatible chips support a local and a remote temperature sensor with 8 bit accuracy. Later chips provide improved accuracy and other additional features such as hysteresis and temperature offset registers.

Note that there is no easy way to differentiate between the MAX6657, MAX6658 and MAX6659 variants. The extra features of the MAX6659 are only supported by this driver if the chip is located at address 0x4d or 0x4e, or if the chip type is explicitly selected as max6659. The MAX6680 and MAX6681 only differ in their pinout, therefore they obviously can’t (and don’t need to) be distinguished.

The different chipsets of the family are not strictly identical, although very similar. For reference, here comes a non-exhaustive list of specific features:

  • 8 bit sensor resolution

ADM1020, ADM1021, GL523SM, MAX1617, NE1617, NE1617A, THMC10:
  • 8 bit sensor resolution

  • Low temperature limits

NCT210, NE1618:
  • 11 bit sensor resolution for remote temperature sensor

  • Low temperature limits

ADM1021A, ADM1023:
  • Temperature offset register for remote temperature sensor

  • 11 bit resolution for remote temperature sensor

  • Low temperature limits

  • 11 bit resolution for remote temperature sensor

  • Temperature offset register for remote temperature sensor

  • Low and critical temperature limits

  • Configurable conversion rate

  • 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

  • Same as LM89

  • External temperature shifted by 16 degrees down

  • 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, ADT7461A, NCT1008:
  • Extended temperature range (breaks compatibility)

  • Lower resolution for remote temperature

  • SMBus PEC support for Write Byte and Receive Byte transactions.

  • 10 bit temperature resolution

ADT7481, ADT7482, ADT7483:
  • Temperature offset register

  • SMBus PEC support

  • 10 bit temperature resolution for external sensors

  • Two remote sensors

  • Selectable address (ADT7483)

  • No critical limit register

  • Conversion rate not configurable

  • Better local resolution (10 bit)

  • 10 bit external sensor resolution

MAX6646, MAX6647, MAX6649:
  • Better local resolution

  • Extended range unsigned external temperature

MAX6648, MAX6692:
  • Better local resolution

  • Unsigned temperature

MAX6654, MAX6690:
  • Better local resolution

  • Selectable address

  • Remote sensor type selection

  • Extended temperature range

  • Extended resolution only available when conversion rate <= 1 Hz

MAX6657 and MAX6658:
  • Better local resolution

  • Remote sensor type selection

  • Better local resolution

  • Selectable address

  • Second critical temperature limit

  • Remote sensor type selection

MAX6680 and MAX6681:
  • Selectable address

  • Remote sensor type selection

MAX6695 and MAX6696:
  • Better local resolution

  • Selectable address (max6696)

  • Second critical temperature limit

  • Two remote sensors

  • The G variant is lead-free, otherwise similar to the W.

  • Filter and alert configuration register at 0xBF

  • Moving average (depending on conversion rate)

  • Successor of the W83L771W/G, same features.

  • The AWG and ASG variants only differ in package format.

  • Diode ideality factor configuration (remote sensor) at 0xE3

  • Better local resolution

All temperature values are given in degrees Celsius. Resolution is 1.0 degree for the local temperature, 0.125 degree for the remote temperature, except for the MAX6654, MAX6657, MAX6658 and MAX6659 which have a resolution of 0.125 degree for both temperatures.

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 configured with the update_interval attribute; reading them more often will do no harm, but will return ‘old’ values.

SMBus Alert Support

This driver has basic support for SMBus alert. When an alert is received, the status register is read and the faulty temperature channel is logged.

The Analog Devices chips (ADM1032, ADT7461 and ADT7461A) and ON Semiconductor chips (NCT1008) do not implement the SMBus alert protocol properly so additional care is needed: the ALERT output is disabled when an alert is received, and is re-enabled only when the alarm is gone. Otherwise the chip would block alerts from other chips in the bus as long as the alarm is active.

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 is disabled by default and can be enabled through sysfs. Just write 1 to the “pec” file and PEC will be enabled. Write 0 to that file to disable PEC again.