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CPU termal throttling
#1
Avoid destruction:

There is a security mechanism in the BCM2837 CPU implemented preventing it from self destruction at high temperatures. For this a single temperature sensor in the CPU is monitoring the CPUs core temperature all the time (one sensor for all 4 cores) to be able to reduce the whole CPU's speed just in case and hence the waste heat it is producing.

How it works:

At core temperatures higher than 80°C the CPU "stepwise" starts lowering its standard 1.2Ghz (full speed) clock rate down to 600MHz (half speed) . At a level of 85°C and beyond the CPU stops reducing its speed and remains operating at 600Mhz (all 4 cores). Since the CPU  is only able to run at full speed or at half speed at a specifc time it uses a trick to vary the effective clock rate between the temperature range of 80°C and 85°C: a switching cycle decides on times the CPU is clocked at 1.2Ghz and on times it is clocked at 600Mhz and chopping the time periods in pieces. This mechanism works quite linear in the 5°C temperature difference range. So while at 80°C there is no switching resulting in 100% 1.2Ghz, at a level of 82.5°C it is running 50% of the time at 1.2Ghz and 50% at 600Mhz giving an effective clock rate of 900Mhz, while at 85°C and more it is 100% 600Mhz.

Standard RPI3, reduced speed at  24°C already:

A standard Raspberry Pi 3 that you run on your desk at a room temperature of 24°C (even with a passive heat sink on the CPU) reaches the core temperature of 80°C easily running a sysbench test with all 4 CPUs as described here. So imagine you would use such a RPi3 in an installation cabinet under industrial conditions where 40°C and more inside a cabinet are most common. Then an RPi3 would run just at 60% of its power.

netPI, no reduced speed till 50°C:

This is different with netPI. Of course netPI uses the same CPU, but netPI has a better cooling concept using an aluminium heat sink on top of the CPU that conducts its heat to the metallic housing. Additionally netPI consists of an 8 layer PCB while a RPi3 just has 6 and also the PCB is twice as big ensuring a better heat dissipation in general. All these countermeasures lead netPI keeping the CPU clock frequency to maximum 1.2GHz for all 4 cores up to an ambient air temperature of 50°C.
You never fail until you stop trying.“, Albert Einstein (1879 - 1955)

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