Thermal management (Axel)

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Axel-04.png Applies to Axel Ultra
Axel-lite 02.png Applies to Axel Lite
Axel-02.png Applies to AXEL ESATTA

Software thermal protection

Axel Embedded Linux Kit (XELK)

In XELK, two software thermal protection mechanisms are implemented. Both are based on the processor's temperature sensor. It is worth remembering that maximum junction temperature (also denoted as Tj in the rest of the document) is:

  • 95°C for commercial parts
  • 105°C for extended commercial and industrial parts
  • 125°C for automotive parts.


At U-Boot level, automatic boot procedure is halted until Tj is smaller than (following temperatures might seem excessively low but they take into account the intrinsic poor precision of the temperature sensor to provide a reasonable safe margin):

  • 80°C for commercial parts
  • 90 for extended commercial and industrial parts
  • 110°C for automotive parts.

Linux kernel

At Linux level, a more sophisticated protection mechanism is implemented. Please refer to the following sections for more details.

XELK 2.x.y series

Practically speaking, for XELK 2.x.y series this mechanism makes use of two different thresholds, denoted as trip_point0 (also known as passive threshold) and trip_point1 (also known as critical threshold. When Tj reaches trip_point0, Linux kernel scales down processor frequency:

System is too hot. GPU3D will work at 1/64 clock.

If Tj reaches trip_point1, a complete shutdown is triggered:

thermal thermal_zone0: critical temperature reached(100 C),shutting down

By default, temperature thresholds are set up as follows:

  • trip_point0: 85°C
  • trip_point1: 100°C.

Trip point thresholds can be accessed via sysfs interface from user space. The following example shows how to read the values of the trip points:

root@axel-lite:~# cat /sys/devices/virtual/thermal/thermal_zone0/trip_point_0_temp
root@axel-lite:~# cat /sys/devices/virtual/thermal/thermal_zone0/trip_point_1_temp

This example shows how to set trip points 0 and 1 to 95°C and 100°C respectively:

echo 95000 > /sys/class/thermal/thermal_zone0/trip_point_0_temp
echo 100000 > /sys/class/thermal/thermal_zone0/trip_point_1_temp
200px-Emblem-important.svg.png Please note that
  • it is possible to set up trip points greater than the maximum junction temperature indicated by manufacturer datasheet
  • exceeding maximum junction temperature indicated by manufacturer datasheet may cause permanent damage

To read current processor temperature please issue this command (reported temperature is 36.676°C in the example):

root@axel-lite:~# cat /sys/class/thermal/thermal_zone0/temp

For more details about this functionality, please refer to Documentation/thermal/sysfs-api.txt in the kernel sources.

XELK 3.x.y series

XELK 3.x.y kernels implement the same mechanism of XELK 2.x.y series. However a notable difference exists, that is default temperature thresholds depend on the silicon grade, as follows:

  • for commercial parts
    • passive threshold: 85°C
    • critical threshold: 90°C
  • for extended commercial and industrial parts
    • passive threshold: 95°C
    • critical threshold: 100°C
  • for automotive parts
    • passive threshold: 115°C
    • critical threshold: 120°C.

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