Introduction

The datasheet for the DS3231 explains that this part is an "Extremely Accurate I²C-Integrated RTC/TCXO/Crystal". And, hey, it does exactly what it says on the tin! This Real Time Clock (RTC) is the most precise you can get in a small, low power package.

Most RTCs use an external 32kHz timing crystal that is used to keep time with low current draw. And that's all well and good, but those crystals have slight drift, particularly when the temperature changes (the temperature changes the oscillation frequency very very very slightly but it does add up!) This RTC is in a beefy package because the crystal is inside the chip! And right next to the integrated crystal is a temperature sensor. That sensor compensates for the frequency changes by adding or removing clock ticks so that the timekeeping stays on schedule.

This is the finest RTC you can get, and now we have it in a compact, breadboard-friendly breakout. With a coin cell plugged into the back, you can get years of precision timekeeping, even when main power is lost. Great for datalogging and clocks, or anything where you need to really know the time.

Dependencies

This driver depends on:

• Adafruit CircuitPython
• Bus Device
• Register

Please ensure all dependencies are available on the CircuitPython filesystem. This is easily achieved by downloading the Adafruit library and driver bundle.

Usage Notes

Basics

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

import busio
import adafruit_ds3231
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_ds3231.DS3231(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 alarm1 or alarm2 to a tuple with a time.struct_time object and string representing the frequency such as "hourly":

rtc.alarm1 = (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 corresponding alarm1_status or alarm2_status attributes. Once True, set it back to False to reset.

if rtc.alarm1_status:
    print("wake up!")
    rtc.alarm1_status = False

Contributing

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

Building locally

To build this library locally you'll need to install the circuitpython-build-tools package.

python3 -m venv .env
source .env/bin/activate
pip install circuitpython-build-tools

Once installed, make sure you are in the virtual environment:

source .env/bin/activate

Then run the build:

circuitpython-build-bundles --filename_prefix adafruit-circuitpython-ds3231 --library_location .

Sphinx documentation

Sphinx is used to build the documentation based on rST files and comments in the code. First, install dependencies (feel free to reuse the virtual environment from above):

python3 -m venv .env
source .env/bin/activate
pip install Sphinx sphinx-rtd-theme

Now, once you have the virtual environment activated:

cd docs
sphinx-build -E -W -b html . _build/html

This will output the documentation to docs/_build/html. Open the index.html in your browser to view them. It will also (due to -W) error out on any warning like Travis will. This is a good way to locally verify it will pass.