raspberry-pi-rtc

Raspberry Pi RTC

Versions

Version	Last Changelog	Ready?
V1	Initial design	✅

Notes

This RTC HAT was designed to avoid any conflict between Raspberry Pi touchscreen and RTC. All the RTC you can find are connected on SDA3/SCL3 of the Raspberry 4. But the touchscreen is already connected to this port, and there is some random issues.

So, this small board is designed to connect a DS3231 on I2C6 of the Raspberry 4 (pin 15 and 16).

How to setup RTC on I2C6 of Raspberry 4

Make sure that the Raspberry Pi is up to date:

sudo apt update
sudo apt upgrade
sudo reboot
sudo rpi-update

• Append to /boot/config.txt:

dtoverlay=i2c6

• Make a new DTC from the existing RTC one (named i2c6-rtc);

sudo dtc -I dtb -O dts /boot/overlays/i2c-rtc.dtbo -o /boot/overlays/i2c6-rtc.dts

• Edit it to change i2c_arm to i2c6:

sudo sed -i "s/i2c_arm/i2c6/g" /boot/overlays/i2c6-rtc.dts

• Recompile it:

sudo dtc -I dts -O dtb /boot/overlays/i2c6-rtc.dts -o /boot/overlays/i2c6-rtc.dtbo

• Append to /boot/config.txt:

dtoverlay=i2c6-rtc,ds3231

After reboot, check dmesg:

dmesg | grep rtc
[    5.883603] rtc : registered as rtc0

If battery is low:

[  42.117539] rtc: low voltage detected, time is unreliable

Now write current time into the RTC, make sure your Raspberry Pi clock is synchronized, using date, then sudo hwclock -w to write current date into the RTC.

On Arch Linux, you have to create a script on startup to read RTC time during the boot:

cat > /etc/systemd/system/rtc.service << ENDRTCSERVICE
[Unit]
Description=RTClock
Before=network.target

[Service]
ExecStart=/usr/bin/hwclock -s
Type=oneshot
[Install]
WantedBy=multi-user.target
ENDRTCSERVICE

Then enable this service:

systemctl enable rtc
systemctl start rtc

Reboot, then check everything is OK:

journalctl -u rtc