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.

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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.

Usage Notes

Basics

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

import busio
import adafruit_pcf8523
import time

All the Adafruit RTC libraries take an instantiated and active I2C object (from the busio 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:

from board import *

You can also use pins defined by the onboard microcontroller through the microcontroller.pin module.

Now, to initialize the I2C bus:

myI2C = busio.I2C(SCL, SDA)

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

rtc = adafruit_pcf8523.PCF8523(myI2C)

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

API Reference

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)[source]

Interface to the PCF8523 RTC.

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.

datetime

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

datetime_register

Current date and time.

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).