🛠 La première RTC des Compute Modules : câbler et vérifier la VBAT du CM5

Objectives

The CM5 is the first Compute Module with an embedded RTC: the VBAT pin (2.5–3.5 V, typically a CR2032) maintains the time when powered off, with a consumption of 1.7 µA when powered / 6 µA when unpowered — the datasheet states “a typical CR2032 lasts more than 3 years unpowered”. The CM4, on the other hand, had no RTC on the module. This tutorial covers: installing the battery, setting the time, writing it to the RTC, and verifying that it survives a total power cut — an essential practice for any offline or enclosed setup.

Hardware

• 1 × CM5 on a CM5IO Board (the RTC battery holder is provided on the IO Board)
• 1 × new CR2032 coin cell battery (respect the +/− orientation of the battery holder)
• Display or SSH session
• 5 minutes of mains power disconnection for the retention test

Full code

# 1) Insérer la CR2032, démarrer, vérifier la présence de la RTC
ls -l /dev/rtc*
cat /sys/class/rtc/rtc0/name
dmesg | grep -i rtc | head -3

# 2) Régler l'heure système puis l'écrire dans la RTC
timedatectl                       # état courant (NTP activé ou non)
sudo timedatectl set-ntp false
sudo date -s '2026-10-14 08:30:00'
sudo hwclock -w -f /dev/rtc0      # système -> RTC
sudo hwclock -r -f /dev/rtc0      # relecture : doit correspondre

# 3) Test de conservation : couper l'alimentation 5 minutes (pas le porte-pile),
#    rallumer SANS réseau, et vérifier :
timedatectl | head -3

# 4) Remettre la synchro réseau une fois en ligne
sudo timedatectl set-ntp true

Expected verification

/dev/rtc0 exists and hwclock -r reads back the written time; after 5 minutes without network, timedatectl displays a consistent time (drift on the order of a second); the kernel logs the RTC at boot (dmesg | grep -i rtc). For battery-powered designs, note the quiescent load of a few µA that VBAT imposes permanently (note on pin 76).

Common errors

• Setting date without running hwclock -w: the system time is lost when power is cut; only writing to the RTC preserves it.
• Forgetting to disable NTP during the test: network synchronization “corrects” the time and masks a dead or incorrectly wired RTC.
• Battery inserted backwards or oxidized contact: the time resets to the kernel build date on every cold boot.
• Comparing with the CM4: the CM4 has no RTC at all (it is the carrier board that adds one, e.g., PCF8523 on I2C); hwclock scripts inherited from the CM4 may target a different device.