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RASPBERRY PI PCF8523 RTC
Adafruit Industries LLC
Adding a Real Time Clock to Raspberry Pi
2020-01-21 | By Adafruit Industries
License: See Original Project Raspberry Pi
Courtesy of Adafruit
Guide by Lady Ada
Overview
This tutorial requires a Raspberry Pi running a kernel with the RTC module and DS1307 module included. Current Raspbian distros have this, but others may not!
The Raspberry Pi is designed to be an ultra-low cost computer, so a lot of things we are used to on a computer have been left out. For example, your laptop and computer have a little coin-battery-powered 'Real Time Clock' (RTC) module, which keeps time even when the power is off, or the battery removed. To keep costs low and the size small, an RTC is not included with the Raspberry Pi. Instead, the Pi is intended to be connected to the Internet via Ethernet or WiFi, updating the time automatically from the global NTP (Network Time Protocol) servers.
For stand-alone projects with no network connection, you will not be able to keep the time when the power goes out. So, in this project we will show you how to add a low-cost battery-backed RTC to your Pi to keep time!
Wiring the RTC
To keep costs low, the Raspberry Pi does not include a Real Time Clock module. Instead, users are expected to have it always connected to WiFi or Ethernet and keep time by checking the network. Since we want to include an external module, we'll have to wire one up.
We have three different RTC we suggest, PCF8523 is inexpensive, DS1307 is most common, and DS3231 is most precise. Any of them will do!
While the DS1307 is historically the most common, it’s not the best RTC chipset, we've found!
- Adafruit PiRTC - PCF8523 Real Time Clock for Raspberry Pi
- Adafruit PCF8523 Real Time Clock Assembled Breakout Board
- Adafruit DS1307 Real Time Clock Assembled Breakout Board
- Adafruit DS3231 Precision RTC Breakout
Don't forget to also install a CR1220 coin cell. In particular the DS1307 won’t work at all without it and none of the RTCs will keep time when the Pi is off and no coin battery is in place.
Wiring is simple:
- Connect VCC on the breakout board to the 5.0V pin of the Pi (if using DS1307)
Connect VCC on the breakout board to the 3.3V pin of the Pi (if using PCF8523 or DS3231)
2. Connect GND on the breakout board to the GND pin of the Pi
3. Connect SDA on the breakout board to the SDA pin of the Pi
4. Connect SCL on the breakout board to the SCL pin of the Pi
Set Up & Test I2C
Set up I2C on your Pi
You'll also need to set up i2c on your Pi, to do so, run sudo raspi-config and under Advanced select I2C and turn it on.
For more details, check out our tutorial on Raspberry Pi i2c setup and testing at
http://learn.adafruit.com/adafruits-raspberry-pi-lesson-4-gpio-setup/configuring-i2c
Reboot once you've done that with sudo reboot.
Verify Wiring (I2C scan)
Verify your wiring by running
sudo apt-get install python-smbus i2c-tools
to install the helper software and then sudo i2cdetect -y 1 at the command line, you should see ID #68 show up - that's the address of the DS1307, PCF8523 or DS3231!
If you have a much older Pi 1, you will have to run sudo i2cdetect -y 0 as the I2C bus address changed from 0 to 1.
Once you have the Kernel driver running, i2cdetect will skip over 0x68 and display UU instead, this means its working!
Set RTC Time
Now that we have the module wired up and verified that you can see the module with i2cdetect, we can set up the module.
Don't forget to set up I2C in the previous step!
Raspbian Jessie (Systemd)
Thanks to kd8twg for the hints!
You can add support for the RTC by adding a device tree overlay. Run
sudo nano /boot/config.txt
to edit the pi configuration and add whichever matches your RTC chip:
dtoverlay=i2c-rtc,ds1307
or
dtoverlay=i2c-rtc,pcf8523
or
dtoverlay=i2c-rtc,ds3231
to the end of the file.
Save it and run sudo reboot to start again. Log in and run sudo i2cdetect -y 1 to see the UU show up where 0x68 should be.
Disable the "fake hwclock" which interferes with the 'real' hwclock
- sudo apt-get -y remove fake-hwclock
- sudo update-rc.d -f fake-hwclock remove
- sudo systemctl disable fake-hwclock
Now with the fake-hw clock off, you can start the original 'hardware clock' script.
Run sudo nano /lib/udev/hwclock-set and comment out these three lines:
#if [ -e /run/systemd/system ] ; then
# exit 0
#fi
Also comment out the two lines
/sbin/hwclock --rtc=$dev --systz --badyear
and
/sbin/hwclock --rtc=$dev --systz
Sync time from Pi to RTC
When you first plug in the RTC module, it's going to have the wrong time because it has to be set once. You can always read the time directly from the RTC with sudo hwclock -D -r
You can see, the date at first is invalid! You can set the correct time easily. First run date to verify the time is correct. Plug in Ethernet or WiFi to let the Pi sync the right time from the Internet. Once that's done, run sudo hwclock -w to write the time, and another sudo hwclock -r to read the time.
Once the time is set, make sure the coin cell battery is inserted so that the time is saved. You only have to set the time once.
That's it! Next time you boot the time will automatically be synced from the RTC module.
Raspbian Wheezy or other pre-systemd Linux
First, load up the RTC module by running
sudo modprobe i2c-bcm2708
sudo modprobe i2c-dev
sudo modprobe rtc-ds1307
Then, as root (type in sudo bash) run
echo ds1307 0x68 > /sys/class/i2c-adapter/i2c-1/new_device
If you happen to have an old Rev 1 Pi, type in
echo ds1307 0x68 > /sys/class/i2c-adapter/i2c-0/new_device
You can then type in exit to drop out of the root shell.
Then check the time with sudo hwclock -r which will read the time from the DS1307 module. If this is the first time the module has been used, it will report back Jan 1 2000, and you'll need to set the time.
First, you'll need to get the right time set on the Pi, the easiest way is to connect it up to Ethernet or WiFi - it will automatically set the time from the network. Once the time is correct (check with the date command), run sudo hwclock -w to write the system time to the RTC.
You can then verify it with sudo hwclock -r
Next, you'll want to add the RTC kernel module to the /etc/modules list, so it’s loaded when the machine boots. Run sudo nano /etc/modules and add rtc-ds1307 at the end of the file (the image below says rtc-1307 but it’s a typo.)
Older pre-Jessie raspbian is a little different. First up, you'll want to create the DS1307 device creation at boot, edit /etc/rc.local by running
sudo nano /etc/rc.local
and add:
echo ds1307 0x68 > /sys/class/i2c-adapter/i2c-0/new_device (for v1 raspberry pi)
echo ds1307 0x68 > /sys/class/i2c-adapter/i2c-1/new_device (for v2 raspberry pi)
sudo hwclock -s (both versions)
before exit 0 (we forgot the hwclock -s part in the screenshot below)
That's it! Next time you boot the time will automatically be synced from the RTC module.
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