DS3231 RTC Module

The DS3231 RTC is a low cost, battery backup, Real Time Clock, that connects to an I2C bus, providing accurate time information.

Many DS3231 modules, like this one, https://www.ebay.com/itm/314250880191, have two I2C devices on-board,
a DS3231 RTC and a AT24C32 EEPROM, allowing the storage of information into non-volatile memory.

Documents

DS3231: https://www.analog.com/media/en/technical-documentation/data-sheets/ds3231.pdf
The time (and temperature), are accessed via 19 byte-wide registers, see Address Map on page 11.

AT24C32: https://ww1.microchip.com/downloads/en/devicedoc/doc0336.pdf
This 32Kbit EEPROM (4096 x 8), will hold 4096 bytes of data.

Charging Circuit

These modules typically have a battery charging circuit to recharge a LIR2032, lithium coin cell.
This can cause a problem if using CR2032, a cheaper non-rechargable coin cell.
This page describes the problem:
  https://forum.arduino.cc/t/ds3231-cr2032-vs-lir2032-warning-is-your-module-killing-the-battery/557346
  My preferred fix is to desolder the diode (or resistor), and lift one end from the board, thus
  defeating the charging circuit.  This method allows the circuit to be restored easily in the future.

DS3231, DS3231SN, DS3231M

A variant of the DS3231/DS3231SN chip is the DS3231M
The "M" variant using a MEMS oscillator which is less accurate (around 5ppm)
compared to the standard "SN" version that uses a crystal oscillator,
providing higher precision (around 2ppm). The "M" is more resistant to shock and vibration.

The "M" variant only produces a 1Hz output frequency on the INT/SQW pin.
Although the frequency output can be enabled/disabled, the frequency is fixed at 1Hz.

Notes / Other Links

https://thecavepearlproject.org/2014/05/21/using-a-cheap-3-ds3231-rtc-at24c32-eeprom-from-ebay/
https://forum.arduino.cc/t/ds3231-and-sqw-unable-to-change-output-frequency-beyond-1hz/479262/10