1. introduction

./pic/pcf8523_1.jpg ./pic/pcf8523_2.jpg

2. installation

vi /boot/config.txt
///
dtoverlay=i2c-rtc,pcf8523
vi /etc/modules-load.d/rtc.conf
i2c-bcm2708
i2c-dev
rtc-pcf8523
reboot

3. testing

yum install i2c-tools
i2cdetect -y 1
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- UU -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --
Tip if UU → 68, check kernel module rtc_pcf8523 
cat /proc/driver/rtc
24hr            : yes

4. configuration

ntpdate ntp.belnet.be
date -s "$(date '+%Y%m%d %T')"
cat /proc/driver/rtc
rtc_time        : 17:16:20
rtc_date        : 2017-03-16
24hr            : yes
hwclock -s

5. debugging

i2cdetect -l
i2c-1   i2c             bcm2835 (i2c@7e804000)                  I2C adapter
cat /etc/adjtime
hwclock -r
hwclock -w
i2cdump -y -f -r 0-6 1 0x68 c
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f    0123456789abcdef
00: 80 00 08 53 14 14 23                               ?.?S??#

The command i2cdump -y -f -r 0-6 1 0x68 c can be used to dump the first 7 registers of the ds1307 which contain the time

(the -f is needed only if you have the rtc driver installed; it overrides the reservation).

6. references

https://www.kiwi-electronics.nl/adafruit-pirtc-pcf8523-real-time-clock-for-raspberry-pi

https://spellfoundry.com/docs/setting-up-the-real-time-clock-on-raspbian-jessie-or-stretch/

https://learn.adafruit.com/adding-a-real-time-clock-to-raspberry-pi/set-up-and-test-i2c

https://cdn-learn.adafruit.com/downloads/pdf/adding-a-real-time-clock-to-raspberry-pi.pdf