🛠 Ajouter une RTC au CM4 (PCF8523 sur I2C1 via dt-blob)

Objectives

The CM4 module has neither a battery-backed RTC nor an embedded real-time clock (contrary to what one might think: the datasheet lists none — it is the carrier board that provides one, with the IO Board having an RTC connector with a battery holder). This tutorial adds a PCF8523 RTC on I2C1 using the dt-blob workflow from the Peripheral Software Guide — the official method for configuring pins on a custom carrier board from the earliest stages of boot — then wires the system time to the RTC.

Hardware

• 1 × CM4 on IO Board (or custom carrier board with PCF8523 RTC on I2C1)
• 1 × PCF8523 RTC module (3.3 V) + CR1220/CR2032 coin cell battery depending on the module
• Wiring to SDA1=GPIO44, SCL1=GPIO45 (on the IO Board's HAT footprint), 3.3 V and GND
• sudo apt install -y device-tree-compiler i2c-tools

Complete code

# 1) Exposer I2C1 sur GPIO44/45 via dt-blob (méthode Peripheral Software Guide)
cat > minimal-cm-dt-blob.dts <<'EOF'
/dts-v1/; /plugin/;
/ {
  pin-config {
    pin@p44 { function = "i2c1"; termination = "pull_up"; };  // SDA1
    pin@p45 { function = "i2c1"; termination = "pull_up"; };  // SCL1
  };
};
EOF
sudo dtc -I dts -O dtb -o /boot/firmware/dt-blob.bin minimal-cm-dt-blob.dts

# 2) Activer le driver RTC et purger la fausse horloge
echo 'dtoverlay=i2c-rtc,pcf8523' | sudo tee -a /boot/firmware/config.txt
sudo apt remove -y fake-hwclock || true
sudo systemctl disable -q fake-hwclock.service 2>/dev/null || true
sudo reboot

# 3) Après reboot : vérifier bus, RTC et heure
sudo i2cdetect -y 1                     # 68 = PCF8523 présent
sudo hwclock -s                         # RTC -> heure système
date && timedatectl | head -3
sudo hwclock -r -f /dev/rtc0            # lit l'heure de la RTC

Expected verification

i2cdetect -y 1 displays address 0x68; /dev/rtc0 exists; after a 5-minute power cut, date returns the correct time without NTP. dmesg | grep -i pcf8523 confirms the driver probe.

Common errors

• fake-hwclock still active: it rewrites the system time at boot with its fixed date and masks the proper functioning of the RTC.
• Wrong bus: the PCF8523 on the IO Board is on I2C1 (GPIO44/45), not on I2C0/SDA0-GPIO0; forgetting the dt-blob or having i2c_bcm2835 not enabled leaves i2cdetect empty.
• Misspelled overlay (pcf8523 and not ds1307 for a PCF8523): the driver does not probe the correct chip.
• Missing battery or reversed contact: the time resets to 2000 on every cold boot.