I recently ordered a DS3231 Real Time Clock module so that I could complete a new Arduino library. The module shown in the photo (ZS-042) was used to test the new library’s functionality. The larger chip is the RTC, but I was curious about the ‘other’ smaller chip that was on the module.
It turns out that the extra IC is an Atmel AT24C32 EEPROM. The IC has 32k bits of data storage accessible in 32 byte ‘pages’, read through the I2C interface. This bonus feature is not advertised anywhere and I doubt that most people who buy these clock modules even realise they have also bought an EEPROM.
Needless to say I am now in the process of writing a small library to allows access to this additional hardware. Not bad for $1.27 on eBay!