Thermal management (Axel)

Info Box Applies to Axel Ultra Applies to Axel Lite 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 105°C for industrial and extended comm.

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.

At Linux level, a more sophisticated protection mechanism - that can be accessed via sysfs - is implemented. For more details about this functionality, please refer to Documentation/thermal/sysfs-api.txt in the kernel sources.

Practically speaking, this mechanism makes use of two different thresholds, denoted as trip_point0 and trip_point1. 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

The following example shows how to read the values of the trip points:

root@imx6qsabresd:~# cat /sys/devices/virtual/thermal/thermal_zone0/trip_point_0_temp
85000
root@imx6qsabresd:~# 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

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 SoC frequency management, see also this page.