Introduction to Adafruit’s PCF8523 Real Time Clock (RTC) Library

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This is a great battery-backed real time clock (RTC) that allows your microcontroller project to keep track of time even if it is reprogrammed, or if the power is lost. Perfect for datalogging, clock-building, time stamping, timers and alarms, etc. Equipped with PCF8523 RTC - it can run from 3.3V or 5V power & logic!

The PCF8523 is simple and inexpensive but not a high precision device. It may lose or gain up to two seconds a day. For a high-precision, temperature compensated alternative, please check out the DS3231 precision RTC. If you need a DS1307 for compatibility reasons, check out our DS1307 RTC breakout.

PCF8523 Breakout Board

Dependencies

This driver depends on the Register and Bus Device libraries. Please ensure they are also available on the CircuitPython filesystem. This is easily achieved by downloading a library and driver bundle.

Installing from PyPI

On supported GNU/Linux systems like the Raspberry Pi, you can install the driver locally from PyPI. To install for current user:

pip3 install adafruit-circuitpython-pcf8523

To install system-wide (this may be required in some cases):

sudo pip3 install adafruit-circuitpython-pcf8523

To install in a virtual environment in your current project:

mkdir project-name && cd project-name
python3 -m venv .env
source .env/bin/activate
pip3 install adafruit-circuitpython-pcf8523

Usage Notes

Basics

Of course, you must import the library to use it:

import time
import adafruit_pcf8523

All the Adafruit RTC libraries take an instantiated and active I2C object (from the board library) as an argument to their constructor. The way to create an I2C object depends on the board you are using. For boards with labeled SCL and SDA pins, you can:

import board

Now, to initialize the I2C bus:

i2c = board.I2C()

Once you have created the I2C interface object, you can use it to instantiate the RTC object:

rtc = adafruit_pcf8523.PCF8523(i2c)

Date and time

To set the time, you need to set datetime` to a time.struct_time object:

rtc.datetime = time.struct_time((2017,1,9,15,6,0,0,9,-1))

After the RTC is set, you retrieve the time by reading the datetime attribute and access the standard attributes of a struct_time such as tm_year, tm_hour and tm_min.

t = rtc.datetime
print(t)
print(t.tm_hour, t.tm_min)

Alarm

To set the time, you need to set alarm to a tuple with a time.struct_time object and string representing the frequency such as “hourly”:

rtc.alarm = (time.struct_time((2017,1,9,15,6,0,0,9,-1)), "daily")

After the RTC is set, you retrieve the alarm status by reading the alarm_status attribute. Once True, set it back to False to reset.

if rtc.alarm_status:
    print("wake up!")
    rtc.alarm_status = False

Contributing

Contributions are welcome! Please read our Code of Conduct before contributing to help this project stay welcoming.

Documentation

For information on building library documentation, please check out this guide.

Table of Contents

Demo

examples/pcf8523_simpletest.py
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# SPDX-FileCopyrightText: 2021 ladyada for Adafruit Industries
# SPDX-License-Identifier: MIT

# Simple demo of reading and writing the time for the PCF8523 real-time clock.
# Change the if False to if True below to set the time, otherwise it will just
# print the current date and time every second.  Notice also comments to adjust
# for working with hardware vs. software I2C.

import time
import board
import adafruit_pcf8523

i2c = board.I2C()
rtc = adafruit_pcf8523.PCF8523(i2c)

# Lookup table for names of days (nicer printing).
days = ("Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday")


# pylint: disable-msg=using-constant-test
if False:  # change to True if you want to set the time!
    #                     year, mon, date, hour, min, sec, wday, yday, isdst
    t = time.struct_time((2017, 10, 29, 10, 31, 0, 0, -1, -1))
    # you must set year, mon, date, hour, min, sec and weekday
    # yearday is not supported, isdst can be set but we don't do anything with it at this time
    print("Setting time to:", t)  # uncomment for debugging
    rtc.datetime = t
    print()
# pylint: enable-msg=using-constant-test

# Main loop:
while True:
    t = rtc.datetime
    # print(t)     # uncomment for debugging
    print(
        "The date is {} {}/{}/{}".format(
            days[int(t.tm_wday)], t.tm_mday, t.tm_mon, t.tm_year
        )
    )
    print("The time is {}:{:02}:{:02}".format(t.tm_hour, t.tm_min, t.tm_sec))
    time.sleep(1)  # wait a second

adafruit_pcf8523 - PCF8523 Real Time Clock module

This library supports the use of the PCF8523-based RTC in CircuitPython. It contains a base RTC class used by all Adafruit RTC libraries. This base class is inherited by the chip-specific subclasses.

Functions are included for reading and writing registers and manipulating datetime objects.

Author(s): Philip R. Moyer and Radomir Dopieralski for Adafruit Industries. Date: November 2016 Affiliation: Adafruit Industries

Implementation Notes

Hardware:

Software and Dependencies:

Notes:

  1. Milliseconds are not supported by this RTC.
  2. Datasheet: http://cache.nxp.com/documents/data_sheet/PCF8523.pdf
class adafruit_pcf8523.PCF8523(i2c_bus)

Interface to the PCF8523 RTC.

Parameters:i2c_bus (I2C) – The I2C bus the device is connected to

Quickstart: Importing and using the device

Here is an example of using the PCF8523 class. First you will need to import the libraries to use the sensor

import time
import board
import adafruit_pcf8523

Once this is done you can define your board.I2C object and define your sensor object

i2c = board.I2C()  # uses board.SCL and board.SDA
rtc = adafruit_pcf8523.PCF8523(i2c)

Now you can give the current time to the device.

t = time.struct_time((2017, 10, 29, 15, 14, 15, 0, -1, -1))
rtc.datetime = t

You can access the current time accessing the datetime attribute.

current_time = rtc.datetime
alarm

Alarm time for the first alarm.

alarm_interrupt

True if the interrupt pin will output when alarm is alarming.

alarm_status

True if alarm is alarming. Set to False to reset.

battery_low

True if the battery is low and should be replaced.

calibration

Calibration offset to apply, from -64 to +63. See the PCF8523 datasheet figure 18 for the offset calibration calculation workflow.

calibration_schedule_per_minute

False to apply the calibration offset every 2 hours (1 LSB = 4.340ppm); True to offset every minute (1 LSB = 4.069ppm). The default, False, consumes less power. See datasheet figures 28-31 for details.

datetime

Gets the current date and time or sets the current date and time then starts the clock.

datetime_register

Current date and time.

high_capacitance

True for high oscillator capacitance (12.5pF), otherwise lower (7pF)

lost_power

True if the device has lost power since the time was set.

power_management

Power management state that dictates battery switchover, power sources and low battery detection. Defaults to BATTERY_SWITCHOVER_OFF (0b000).

Indices and tables