NXP PCF2131 RTC driver

Driver for the NXP PCF2131 real time clock used by Sfera Labs products.

Compile and Install

For installation on Ubuntu read this.

Make sure your system is updated:

sudo apt update
sudo apt upgrade

If you are using a Raspberry Pi (CM) 4 and a 32-bit OS, add to /boot/firmware/config.txt (/boot/config.txt in older versions) the following line: [why?]

arm_64bit=0

Reboot:

sudo reboot

After reboot, install git and the Raspberry Pi kernel headers:

sudo apt install git raspberrypi-kernel-headers

Clone this repo:

git clone --depth 1 https://github.com/sfera-labs/rtc-pcf2131

Make and install:

cd rtc-pcf2131
make clean
make
sudo make install

Compile the Device Tree and install it:

dtc -@ -Hepapr -I dts -O dtb -o rtc-pcf2131.dtbo rtc-pcf2131.dts
sudo cp rtc-pcf2131.dtbo /boot/overlays/

Add to /boot/firmware/config.txt (/boot/config.txt in older versions) the following line:

dtoverlay=rtc-pcf2131

Reboot:

sudo reboot

After reboot, test the installation by checking that no error is returnd when running:

sudo hwclock -w
sudo hwclock -r

The system will automatically syncronize the RTC date/time when able to connect to an NTP server and, vice versa, the RTC stored values will be used when connection is not available.

Advanced usage

The device corresponding to the rtc will be available at /dev/rtc0 and its sysfs interface under /sys/class/rtc/rtc0/.

The file pcf2131_example.c contains a C example to read the RTC date/time and other information (such as low backup battery voltage) using the ioctl() system call.

Compile it with:

gcc -s -Wall pcf2131_example.c -o pcf2131_example

Run it:

$ sudo ./pcf2131_example 
Date/time: 2023-1-3 14:04:42
Voltage low flags: 0x2
> Backup voltage is low