🛠 Surveiller température et fréquence CPU

Objectives

Set up monitoring of the temperature and core frequency of the Raspberry Pi 3 Model B. Know the thresholds for the 3B: progressive throttling of the Arm cores between 80 °C and 85 °C, hardware throttling of CPU + GPU at 85 °C (no soft software limit on this model). Interpret the get_throttled flag and log the history to size a case or a fan.

Hardware

• Raspberry Pi 3 Model B (note: pre-2019 boards use the BCM2837 without a heatsink, more sensitive to heat than the BCM2837B0 with a metal lid)
• Raspberry Pi OS Lite, included tools: vcgencmd, /sys/class/thermal
• Optional: heatsinks or a small USB fan to test the cooling effect
• Test load: stress (sudo apt install -y stress)

Full code

#!/usr/bin/env bash
# thermal_log.sh — échantillonne température, fréquence et drapeaux de bridage
# seuils du 3B : 80 °C ralentissement progressif, 85 °C bridage dur CPU+GPU

while true; do
  temp_c=$(vcgencmd measure_temp | grep -oE '[0-9]+\.[0-9]+')
  temp_sysfs=$(($(cat /sys/class/thermal/thermal_zone0/temp) / 1000))
  freq=$(cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq)
  throttled=$(vcgencmd get_throttled | cut -d= -f2)
  echo "$(date -Is) temp_C=$temp_c temp_sysfs=$temp_sysfs freq_kHz=$freq throttled=$throttled"
  sleep 5
done | tee -a /var/log/thermal.log

# Dans un autre terminal, provoquer la montée en température :
#   stress --cpu 4 --timeout 300
# Decoder un drapeau (ex. 0x50000 = sous-tension + bridage) :
#   printf '%x\n' $((0x50000 & (1<<0)))   # bit 0 : sous-tension

Expected verification

At idle, vcgencmd measure_temp and the sysfs file give the same temperature (±1 °C), for example 40–45 °C. During stress --cpu 4, the scaling_cur_freq frequency drops below 1200000 kHz when the temperature exceeds 80 °C, and vcgencmd get_throttled changes to a non-zero value with the throttling bit set. After stopping the load, temperature and frequency drop back down. The log allows tracing the curve and comparing open/closed cases.

Common errors

• Panicking over a non-zero throttled value: it is a history; restart or read the bits to distinguish undervoltage (bit 0) from thermal throttling.
• Confusing undervoltage and overheating: a thin micro-USB cable triggers the same throttling; check the 2.5 A power supply first.
• Expecting 1.4 GHz: the 3B is capped at 1.2 GHz (the B0 allows it, but not the 3B firmware).
• Reading a single snapshot: throttling is transient; log every 5 s with the script.
• Closed case without airflow on a pre-2019 board without a lid: throttling is guaranteed under sustained load — this is precisely what motivated the switch to the BCM2837B0.