Kernel driver jc42

Supported chips:

Common for all chips:

Prefix: ‘jc42’

Addresses scanned: I2C 0x18 - 0x1f


Guenter Roeck <>


This driver implements support for JEDEC JC 42.4 compliant temperature sensors, which are used on many DDR3 memory modules for mobile devices and servers. Some systems use the sensor to prevent memory overheating by automatically throttling the memory controller.

The driver auto-detects the chips listed above, but can be manually instantiated to support other JC 42.4 compliant chips.

Example: the following will load the driver for a generic JC 42.4 compliant temperature sensor at address 0x18 on I2C bus #1:

# modprobe jc42
# echo jc42 0x18 > /sys/bus/i2c/devices/i2c-1/new_device

A JC 42.4 compliant chip supports a single temperature sensor. Minimum, maximum, and critical temperature can be configured. There are alarms for high, low, and critical thresholds.

There is also an hysteresis to control the thresholds for resetting alarms. Per JC 42.4 specification, the hysteresis threshold can be configured to 0, 1.5, 3.0, and 6.0 degrees C. Configured hysteresis values will be rounded to those limits. The chip supports only a single register to configure the hysteresis, which applies to all limits. This register can be written by writing into temp1_crit_hyst. Other hysteresis attributes are read-only.

If the BIOS has configured the sensor for automatic temperature management, it is likely that it has locked the registers, i.e., that the temperature limits cannot be changed.

Sysfs entries


Temperature (RO)


Minimum temperature (RO or RW)


Maximum temperature (RO or RW)


Critical high temperature (RO or RW)


Critical hysteresis temperature (RO or RW)


Maximum hysteresis temperature (RO)


Temperature low alarm


Temperature high alarm


Temperature critical alarm