The DS3231 real time clock chip

This is a remarkably inexpensive and easily interfaced battery-backed RTC. It is an ideal way rapidly to calibrate the Pyboard's RTC which can then achieve similar levels of accuracy (+- ~2 mins/year). The chip can also provide accurate time to platforms lacking a good RTC (notably the ESP8266).

Three drivers are provided:

ds3231_gen.py General purpose portable driver supporting alarms.
ds3231_port.py Portable driver: main purpose is to test accuracy of a platform's RTC.
ds3231_pb.py A Pyboard-specific driver with RTC calibration facility. For Pyboard 1.x and Pyboard D.

Breakout boards are widely available. The interface is I2C. Pullups to 3.3V (typically 10KΩ) should be provided on the SCL and SDA lines if these are not supplied on the breakout board.

Drivers 2 and 3 use edge detection to achieve millisecond-level precision from the DS3231. This enables relatively rapid accuracy testing of the platform's RTC, and fast calibration of the Pyboard's RTC. To quantify this, a sufficiently precise value of calibration may be acquired in 5-10 minutes.

Datetime tuples

MicroPython currently enjoys three formats. The first is that used by the time module in localtime and gmtime. This is
(year, month, mday, hour, minute, second, weekday, yearday)
with the meaning of each field as described in the MP official docs and CPython docs. These drivers use this format as it is the CPython standard.

The micropython RTC class uses this format in its __init__ method:
(year, month, day[, hour[, minute[, second[, microsecond[, tzinfo]]]]])
and this format in its datetime method:
(year, month, mday, weekday, hours, minutes, seconds, subseconds)
It is to be hoped that some standardisation will occur. Please check official docs for any changes.

Main README

1. General purpose driver ds3231_gen

This uses datetime tuples to set and read time values. These are of form
(year, month, day, hour, minute, second, weekday, yearday)
as used by time.localtime.

1.1 The DS3231 class

Constructor:

This takes one mandatory argument, an initialised I2C bus.

Public methods:

get_time(). Returns the DS3231 time as a datetime tuple with yearday=0.
set_time(tt=None). Sets the DS3231 time. By default it uses the platform's syatem time, otherwise the passed datetime tuple. If passing a tuple, see the note below.
__str__() Returns a dump of the device's registers for debug in a "pretty print" format.
temperature() A float, temperature in °C. Datasheet specifies +-3°C accuracy. It really is that bad.

Public bound variables:

alarm1 Alarm instances (see below). Can be set to 1s precision.
alarm2 Can be set to 1min precision.

Alarm Public methods

set(when, day=0, hr=0, min=0, sec=0) Arg when is one of the module constants listed below. Alarm operation is started.
clear() Clears the alarm status and releases the alarm pin. The alarm will occur again the next time the parameters match.
__call__() No args. Return True if alarm has occurred.
enable(run) If run is False the alarm is cleared and will enter a stopped state; in that state the alarm will not occur again. If True a stopped alarm is restarted and will occur on the next match.

Alarm bound variables

alno Alarm no. (1 or 2).

Module constants

These are the allowable options for the alarm's when arg, along with the relevant Alarm.set() args:
EVERY_SECOND Only supported by alarm1.
EVERY_MINUTE sec
EVERY_HOUR min, sec
EVERY_DAY hr, min, sec
EVERY_WEEK day (weekday 0..6), hr, min, sec
EVERY_MONTH day (month day 1..month end), hr, min, sec

In all cases sec values are ignored by alarm2: alarms occur on minute boundaries. This is a hardware restriction.

Setting DS3231 time

Where this is to be set using a datetime tuple rather than from system time, it is necessary to pass the correct value of weekday. This can be acquired with this function. It can be passed a tuple with dt[6] == 0 and will return a corrected tuple:

import time
def dt_tuple(dt):
    return time.localtime(time.mktime(dt))  # Populate weekday field

Alarms

Comments assume that a backup battery is in use, in which case alarms continue to operate.

The battery ensures that alarm settings are stored through a power outage. If an alarm occurs during an outage the pin will be driven low and will stay low until power is restored and clear or disable are issued.

If an alarm is set and a power outage occurs, when power is restored the alarm will continue to operate at the specified frequency. Setting an alarm:

from machine import SoftI2C, Pin
from ds3231_gen import *
i2c = SoftI2C(scl=Pin(16, Pin.OPEN_DRAIN, value=1), sda=Pin(17, Pin.OPEN_DRAIN, value=1))
d = DS3231(i2c)
dt.alarm1.set(EVERY_MINUTE, sec=30)  # Alarm on the half minute

If a power outage occurs here the following code demonstrates that alarms continue to occur at one minute intervals:

from machine import SoftI2C, Pin
from ds3231_gen import *
i2c = SoftI2C(scl=Pin(16, Pin.OPEN_DRAIN, value=1), sda=Pin(17, Pin.OPEN_DRAIN, value=1))
d = DS3231(i2c)
while True:
    d.alarm1.clear()  # Clear pending alarm
    while not d.alarm1():  # Wait for alarm
        pass
    time.sleep(0.3)  # Pin stays low for 300ms

Note that the DS3231 alarm2 does not have a seconds register: sec values will be ignored and EVERY_SECOND is unsupported.

Re the INT\ (alarm) pin the datasheet (P9) states "The pullup voltage can be up to 5.5V, regardless of the voltage on Vcc". Note that some breakout boards have a pullup resistor between this pin and Vcc.

Setting system RTC

Note that the DS3231 driver uses the CPython standard datetime tuple:
(year, month, mday, hour, minute, second, weekday, yearday)
Currently the RTC datetime method uses a different format.

The system RTC may be set from the DS3231 as follows:

from machine import SoftI2C, Pin, RTC
from ds3231_gen import *
i2c = SoftI2C(scl=Pin(16, Pin.OPEN_DRAIN, value=1), sda=Pin(17, Pin.OPEN_DRAIN, value=1))
d = DS3231(i2c)
rtc = RTC()
YY, MM, DD, hh, mm, ss, wday, _ = d.get_time()
rtc.datetime((YY, MM, DD, wday, hh, mm, ss, 0))

The following can be used to set the RTC to better than +-1s accuracy. Though the DS3231 has only 1s resolution, setting the RTC on a transition of the seconds value minimises error.

t = d.get_time()  # As per above, d is the DS3231 instance
while t == d.get_time()[5]:  # Wait for change in seconds
    pass
YY, MM, DD, hh, mm, ss, wday, _ = t  # Set time now
rtc.datetime((YY, MM, DD, wday, hh, mm, ss, 0))

Application: micropower systems

The DS3231 alarm pin may be used to control the application of power to the MicroPython host and peripheral devices. This may be done with a PNP transistor driven (via suitable resistors) from the alarm output. When in an alarm state, power is applied to the system. The DS3231 is set to alarm at the required interval - say at 00:03:00 every Sunday. On startup the system takes readings and logs them or reports them via MQTT. Its final act is to issue

ds3231.alarm1.clear()

which turns off the MOSFET and the transistor and powers down the system. Current in the OFF state will be very much less than 1μA.

2. Portable driver ds3231_port

This can use soft I2C so any pins may be used.

It uses the RTC.datetime() method to set and to query the platform RTC. The meaning of the subseconds field is hardware dependent so this is ignored. The RTC is checked against the DS3231 by timing the transition of the seconds field of each clock (using system time to measure the relative timing of the edges).

This example ran on a WeMos D1 Mini ESP8266 board, also a generic ESP32.

from ds3231_port import DS3231
from machine import Pin, SoftI2C
# Pins with pullups on ESP8266: clk=WeMos D3(P0) data=WeMos D4(P2)
i2c = SoftI2C(Pin(0, Pin.OPEN_DRAIN), Pin(2, Pin.OPEN_DRAIN))
ds3231 = DS3231(i2c)
ds3231.get_time()

Testing the onboard RTC:

ds3231.rtc_test()  # Takes 10 minutes

In my testing the ESP8266 RTC was out by 5%. The ESP32 was out by 6.7ppm or about 12 minutes/yr. A PiPico was out by 1.7ppm, 3.2mins/yr. Hardware samples will vary.

2.1 The DS3231 class

Constructor:
This takes one mandatory argument, an initialised I2C bus.

Public methods:

get_time(set_rtc=False). If set_rtc is True it sets the platform's RTC from the DS3231. It returns the DS3231 time as a tuple in the same format as utime.localtime() except that yday (day of year) is 0. So the format is (year, month, day, hour, minute, second, wday, 0).
Note that on ports/platforms which don't support an RTC, if set_rtc is True, the local time will be set from the DS3231.
save_time() No args. Sets the DS3231 time from the platform's local time.
rtc_test(runtime=600, ppm=False, verbose=True). This tests the platform's RTC time against the DS3231 returning the error in parts per million (if ppm is True) or seconds per year. A positive value indicates that the DS3231 clock leads the platform RTC.
The runtime value in seconds defines the duration of the test. The default of 10 minutes provides high accuracy but shorter durations will suffice on devices with poor RTC's (e.g. ESP8266).
If machine.RTC is unsupported a RuntimeError will be thrown.

3. The Pyboard driver

The principal reason to use this driver is to calibrate the Pyboard's RTC. This supports the Pyboard 1.x and Pyboard D. Note that the RTC on the Pyboard D is much more accurate than that on the Pyboard 1.x but can still be in error by up to 20ppm. It can benefit from calibration. For this to work reliably on the D a firmware build later than V1.12 is required: use a daily build if a later release is not yet available.

Note that, while the code will run on the Pyboard Lite, this device cannot be calibrated. This is because its RTC uses an inaccurate RC oscillator whose frequency is usually beyond the range of the chip's calibration capability. Even if this is not the case, the lack of stability of RC oscillators makes calibration a pointless exercise.

The sample below assumes that the DS3231 is connected to the hardware I2C port via I2C(2) but any I2C may be used including soft I2C. Ensure that the Pyboard RTC is set to the correct time and date.

Usage to calibrate the Pyboard's RTC. Takes 5 minutes.

from ds3231_pb import DS3231
import machine
i2c = machine.I2C(2)  # Connected on 'Y' side Y9 clk Y10 data
ds3231 = DS3231(i2c)
ds3231.save_time()  # Set DS3231 to match Pyboard RTC
ds3231.calibrate()

Calibration data is stored in battery-backed memory. So if a backup cell is used the RTC will run accurately in the event of a power outage.

3.1 The DS3231 class

Constructor:
This takes one mandatory argument, an I2C bus instantiated using the machine library.

Public methods:

get_time(set_rtc=False). If set_rtc is True it sets the Pyboard's RTC from the DS3231. It returns the DS3231 time as a tuple in the same format as utime.localtime() except that yday (day of year) is 0.
Namely (year, month, day, hour, minute, second, wday, 0).
save_time() No args. Sets the DS3231 time from the Pyboard's RTC.
calibrate(minutes=5). The time to run. This calculates the calibration factor and applies it to the Pyboard. It returns the calibration factor which may be stored in a file if the calibration needs to survive an outage of all power sources.
getcal(minutes=5, cal=0, verbose=True) Measures the performance of the Pyboard RTC against the DS3231. If cal is specified, the calibration factor is applied before the test is run. The default is to zero the calibration and return the required factor.

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