This FeatherWing will make it easy to add data logging to any Feather Board you might have. You get both an I2C real-time clock (PCF8523) with 32 KHz crystal and battery backup, and a microSD socket that connects to the SPI port pins (+ extra pin for CS).
Note: FeatherWing doesn't come with a microSD card.
A CR1220 coin cell is required to use the RTC battery-backup capabilities. If you're not using the RTC part of the FeatherWing, a battery is not required.
To talk to the microSD card socket Arduino's default SD library is recommended. Some light soldering is required to attach the headers onto the Wing.
On the bottom row, the 3.3 V (second from left) and GND (fourth from left) pin are used to power the SD card and RTC (to take a load off the coin cell battery when main power is available)
RTC & I2C Pins
In the top right, SDA (rightmost) and SCL (to the left of SDA) are used to talk to the RTC chip.
There's also a breakout for INT which is the output pin from the RTC. It can be used as an interrupt output or it could also be used to generate a square wave.
Note that this pin is an open drain - you must enable the internal pull-up on whatever digital pin it is connected to.
SD & SPI Pins
Starting from the left you've got
These pins are in the same location on every Feather. They are used for communicating with the SD card. When the SD card is not inserted, these pins are completely free. MISO is tri-stated whenever the SD CS (chip select) pin is pulled high