Evern though every pin from the Feather is 'doubled up' with an inner header, not all of the pins are actually used!
On the bottom row, the 3.3V (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)
In the top right, SDA (rightmost) and SCL (to the left of SDA) are used to talk to the RTC 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
These pins are in the same location on every Feather
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 open drain - you must enable the internal pullup on whatever digital pin it is connected to!
Starting from the left you've got
- SPI Clock (SCK) - output from feather to wing
- SPI Master Out Slave In (MOSI) - output from feather to wing
- SPI Master In Slave Out (MISO) - input from wing to feather
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 pin is pulled high
The SDCS pin is the chip select line.
- On ESP8266, the SD CS pin is on GPIO 15
- On ESP32 it's GPIO 33
- On WICED it's GPIO PB5
- On the nRF52832 it's GPIO 11
- On Atmel M0, M4, 328p or 32u4 it's on GPIO 10
- On Teensy 3.x it's on GPIO 10
You can cut the trace to the default pin and change this to any pin by soldering a wire to any available pad.