RaspberryPi:RTC

RTC on the Pi

The Raspberry Pi can make use of a kernel driver to handle an RTC (Real time clock). This page was written for use with the MCP7940 IC. Adaptations may be required for different RTC ICs.

The Pi doesn’t come with an RTC, and instead uses a daemon service called fake-hwclock to handle the time. But, seeing that we have an RTC, we can use that instead. And by supplying an external current (from, say a CR2032, a common coin cell battery for RTCs), we can hold the time even while the Raspberry Pi is off and doesn’t have internet access.

Note: This guide was tested on a fresh install of Raspbian Stretch, but should work fine on Buster, too.

Build the Circuit

1. Build the RTC circuit and connect it to your Pi

2. SSH in to your Pi

3. Run the following. We’re using the MCP7940, which means we expect a result at address 0x6f

   $ gpio i2cdetect

Configure the Pi

1. Run the following:

   $ touch rtc.sh
   $ chmod +x rtc.sh

2. Using your favourite text editor, place the following script into rtc.sh
   Take note when a red arrow appears below, you need to ensure that the text after the arrow is on the same line as the line above it (i.e. a red arrow just shows a line is split, the text should be on one line)

   #!/bin/bash
   # UniPi driver setup
   # https://forum.unipi.technology/topic/171/mcp7491x-jessie/3
   # Modified for the MCP7940x
   
   if [ "$EUID" -ne 0 ]
       then echo "This script must be run as root"
       exit
   fi
   
   echo Backup files
   cp /boot/config.txt /boot/config.txt.bak
   echo Enabling I2C bus...
   
   if ! grep -q '^dtparam=i2c_arm=on' /boot/config.txt
       then echo 'dtparam=i2c_arm=on' >> /boot/config.txt
   fi
   
   if ! grep -q '^dtparam=i2c_baudrate=400000' /boot/config.txt
       then echo 'dtparam=i2c_baudrate=400000' >> /boot/config.txt
   fi
   echo Enabling the RTC chip...
   if ! grep -q '^dtoverlay=i2c-rtc,mcp7940x' /boot/config.txt
       then echo 'dtoverlay=i2c-rtc,mcp7940x' >> /boot/config.txt
   fi
   
   echo Getting i2c development tools
   apt-get install -y i2c-tools
   if ! grep -q '^i2c-dev' /etc/modules
       then echo 'i2c-dev' >> /etc/modules
   fi
   
   echo Removing fake-hwclock...
   apt-get purge -y fake-hwclock
   update-rc.d -f fake-hwclock remove
   apt-get autoremove -y
   
   sed -i '/^if \[ -e \/run\/systemd\/system/,/^fi/s/^/#/'  /lib/udev/hwclock-set 
   
   echo ' '
   echo 'UniPi installed.'
   echo ' '
   echo '!!! REBOOT IS REQUIRED !!!'
   echo ' '
   
   read -p "Is it OK to reboot now? [y/N] " -n 1 -r
   echo ' '
   if [[ $REPLY =~ ^[Yy]$ ]]
   then
       reboot
   else
     echo 'Reboot to finish configuring drivers'
   fi
   echo ' '
       

3. Run the script as sudo, as follows:

   $ sudo ./rtc.sh

4. The script will prompt you to reboot. Do so.

Test the Hardware

1. Once rebooted, SSH in to your Pi

2. Run the following. We’re using the MCP7940, which means we expect a result at address 0x6f

    $ gpio i2cdetect

3. instead of the address “6f”, you should see “UU”

   

   gpio i2cdetect showing the device at 0x6f is in use

Test the Software

And now for the fun stuff:

1. Check if we can see the hwclock. Depending on whether or not you’re connected to the internet, you should see an arbitrary time

   $ sudo hwclock -r

2. Assuming you have internet connectivity, your Pi should fetch the time through NTP

3. Check the various times the Pi has access to:

   $ timedatectl

   Depending on whether or not you’ve connected to the internet, your results can look quite varied:


RTC with time set and no internet connection

RTC without time set and no internet connection

4. Connect your Pi to the internet and reboot so you can get the NTP time as shown in Figure [fig:rtcwint].

5. Set the System time to the RTC:

   $ sudo hwclock -w
   $ timedatectl

   

   Once you’ve connected to the internet and set the RTC