🛠 Surveillance thermique et fréquence des quatre Cortex-A53

Objectives

Monitor temperature and frequency of the Raspberry Pi 3 A+ under load. Watch the terminology: the board does NOT have a big.LITTLE architecture — the BCM2837B0 features 4 strictly identical Cortex-A53 cores @ 1.4 GHz; therefore, we monitor a single cluster, its thermal sensor, and throttling.

Hardware

• Raspberry Pi 3 A+ running Raspberry Pi OS, heatsink or ventilated case recommended (65 × 56 mm, no stock heatsink)
• sudo apt install stress-ng vcgencmd (vcgencmd is provided by Raspberry Pi OS)
• Manufacturer operating range: 0–50 °C ambient
• >

Full Code

#!/usr/bin/env bash
# surveille_temp.sh — température + fréquence + drapeaux de bridage
while true; do
    t=$(</sys/class/thermal/thermal_zone0/temp)          # milli-°C
    f=$(</sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq)  # kHz
    printf '%s  %d.%d C  %6.0f MHz  %s\n' "$(date +%T) " \
        $((t/1000)) $((t%1000)) $((f/1000)) "$(vcgencmd get_throttled)"
    sleep 2
done

# Terminal 1 : journal ; terminal 2 : contrainte des 4 cœurs
./surveille_temp.sh
stress-ng --cpu 4 --timeout 300   # puis observer la montée vers ~60-70 °C
# Historique des événements de bridage depuis le boot :
dmesg | grep -i -e thermal -e throttle

Expected Verification

At idle: 35–45 °C, 1400 MHz, throttled=0x0. At the end of the stress test: temperature plateaus and frequency drops below 1.4 GHz, with non-zero throttling flags (under-voltage or down-softmax bit). A fan or heatsink significantly lowers the plateau.

Common Errors

• Talking about "performance/efficiency cores": the 3 A+ is a homogeneous 4× Cortex-A53 cluster, no big.LITTLE.
• Using a 5 V/0.5 A phone charger: under load, this causes undervoltage (bit 0x1 of get_throttled) and throttling — 5 V/2.5 A is required.
• Confusing °C and milli-°C: /sys/.../temp returns 45123 for 45.1 °C.
• Closed case without ventilation: the datasheet requires well-ventilated operation; the SoC can take it, the plastic cannot.