AXEL Lite SOM/AXEL Lite Hardware/Electrical Thermal and Mechanical Features/Thermal management and heat dissipation

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Version Issue Date Notes
1.0.0 Sep 2020 New layout documentation


Thermal management[edit | edit source]

The AXEL Lite SOM is designed to support the maximum available temperature range declared by the manufacturer.

The customer shall define and conduct a reasonable number of tests and verification in order to qualify the DUT capabilities to manage the heat dissipation.

Any heatsink, fan etc shall be defined case by case.

DAVE Embedded Systems' team is available for any additional information, please contact sales@dave.eu.

Software thermal protection[edit | edit source]

In DESK-MX6-L, 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.

U-Boot[edit | edit source]

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[edit | edit source]

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

The 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.

Practically speaking, for DESK-MX6-L 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
85000
root@axel-lite:~# cat /sys/devices/virtual/thermal/thermal_zone0/trip_point_1_temp
100000

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
36676

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