Finally, We’ve found the cause of the heat problem in Raspberry Pi CM4. I’ll write my note here.
Background
I’ve worked on a Raspberry Pi product for more than 2 years. Then I struggled with the heat problem that the temperature of CPU goes to 85 ℃ at most randomly. Once a Raspberry Pi hits 85 ℃, the functionalities are very unstable and it could stop at the worst case. Strangely, the CPU usage was around 5% and kept less than 10% at most. We did many things to avoid the problem but we couldn’t find the root cause.
One day, it was 2 months ago, I met a guy who was really geeked into Raspberry Pi and asked for his help. Then my team tackled the problem seriously last month.
Findings
First of all, he suggested we take thermal pictures of our Raspberry Pis. We noticed the heat source was the voltage regulator. We should have bought the thermal camera earlier and then I think we could have got the cause much earlier.
Then we dug the component deeper. Finally, we found the cause. It was the voltage regulator itself. We thought there might be Raspberry Pi’s firmware to reduce heat and tested a lot. However, we found there are two types of Raspberry Pi CM4 with two different regulators in the market.
The MxL is old and the dialog is new. Then the temperature of dialogregulator is less than MxL regulator by 10 ~ 15 ℃. I asked the Raspberry Pi team about the difference directly and they told me “If these parts are SC0667 or SC0667B, These are both Rev5 versions (Dialog PMIC). If you had CM4102000 SC0275, These were the previous Rev3 Parts (MXL PMIC).” Also, they told me they aren’t supplying the Rev3 Raspberry Pi anymore.
Details: https://pip.raspberrypi.com/categories/645/documents/RP-001606-PC/RP-001606-PC-2.pdf
I guess the old and new ones would be mixed in the market now because of the shortage of semiconductors and the recent short supply of Raspberry Pi. Unfortunately, we bought the many old ones.
The new Raspberry Pi 5 also uses the same regulator and I guess the heat efficiency is proved in the Raspberry Pi team.
That’s it!