MITO 8M Mini SOM/MITO 8M Mini Hardware/Electrical Thermal management and heat dissipation

History
Version	Issue Date	Notes
1.0.0	Feb 2021	First release

Thermal management

The MITO 8M Mini/Nano 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.

Temperature sensor on SOC

The internal temperature sensor of iMX8M Mini/Nano SOC has some limitations regarding the operative range (from 10 to 105°C) and the precision (±5 accuracy).

If the operative temperature is a crucial parameter on the system where MITO 8M Mini/Nano is integrated, we suggest adding one or more temperature sensors on the carrier board in order to overcome these limitations.

In the following Software thermal protection chapter, the implementation of reading and trip points management are referred at the internal SOC temperature sensor.

Heatsink suggestion

The MITO 8M Mini/Nano SOM is designed with four holes that allow to mount a heatsink directly on the CPU IC.

See Mito 8M Mini/Nano Board Layout section for more details.

SOM Thermography

The following image is taken in stress tests conditions (see this section) without any heatsinkin order to highlight the hot spots on this board. The hottests points here are:

• the CPU
• the PMIC
• the Ethernet PHY (iperf test was enable during this test)

Depending on your application it is may needed to dissipate heat not only from CPU but also other areas (the thermal analysis is important in conjunction with the temperature range of each component which may be 125°C or 150°C). Depending on the application running, this platform can be suitable for fanless approach.

Software thermal protection

In DESK-MX8-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 Consumer parts
• 105°C for Industrial parts

U-Boot

At U-Boot level, automatic boot procedure is halted until Tj is greater 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):

• 85°C for Consumer parts
• 95°C for Industrial parts

Linux kernel

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 Consumer parts
  • passive threshold:85°C
  • critical threshold: 90°C

• for Industrial parts
  • passive threshold: 95°C
  • critical threshold: 100°C

Practically speaking, for DESK-MX8-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.

If Tj reaches trip_point1, a complete shutdown is triggered:

thermal thermal_zone0: critical temperature reached(95 C),shutting down

To resume the CPU from the shutdown, the CPU_ONOFF signal must be externally managed. Please contact sales@dave.eu for any additional information.

By default, temperature thresholds are set up as follows:

• trip_point0: 85°C
• trip_point1: 95°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@mito:~# cat /sys/devices/virtual/thermal/thermal_zone0/trip_point_0_temp
85000
root@mito:~# cat /sys/devices/virtual/thermal/thermal_zone0/trip_point_1_temp
950000

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

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@mito:~# 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.

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