Even though every pin from the Feather is 'doubled up' with an inner header, only 4 of those pins are actually used!
On the bottom row, the 3.3V and GND pin are used to power the RTC - to take a load off the coin cell battery.
In the top right, SDA and SCL (i2c bus) are used to talk to the chip.
- SCL - I2C clock pin, connect to your microcontrollers I2C clock line. This pin has a 10K pullup resistor to 3.3V
- SDA - I2C data pin, connect to your microcontrollers I2C data line. This pin has a 10K pullup resistor to 3.3V
The DS3231 has a few other pins you may want to use, they are broken out to a 3-pin header in the center.
- 32K - 32KHz oscillator output. Open drain, you need to attach a pullup to read this signal from a microcontroller pin (or use a microcontroller that can turn on it's internal pullup)
- SQW - optional square wave or interrupt output. Open drain, you need to attach a pullup to read this signal from a microcontroller pin (or use a microcontroller that can turn on it's internal pullup)
- RST - This one is a little different than most RST pins, rather than being just an input, it is designed to be used to reset an external device or indicate when main power is lost. Open drain, but has an internal 50K pullup. The pullup keeps this pin voltage high as long as Vin is present. When Vin drops and the chip switches to battery backup, the pin goes low.
The Feather RESET pin is connected to a button on the right of the wing, handy if you need to reset or restart your Feather! Note that this RESET is not connected to the RST pin on the DS3231
A CR1220 battery holder is on-board for long term RTC timekeeping. A new CR1220 should be able to power the DS3231 for at least 5 years. If you want to use that battery for powering something else (maybe a GPS module or a very low power BLE beacon?) you can connect to the BATT breakout on the very right.