Adafruit Feather 32u4 Adalogger Cheat sheet

The Adafruit feather are 3.3 volt devices

GPIO

https://learn.adafruit.com/adafruit-feather-32u4-adalogger/pinouts

#0,#1 rx, tx serial 1

PWM

8 GPIOs support PWN 5, 9, 10, 11, 12
2, 3 (GPIO used for I2C)

ADC

10 GPIOs support analog to digital convertion

Reserved GPIO

GPIO 8 is reserved for green LED, use to display SD Card activity
GPIO 4 SPI Select for the SD Card
GPIO7 To detect if there is an SD Card

I2C

2 sda, interrupt on #1
3 scl, interrupt on #0
need the 2 pull up

#5 PWM
#6 PWM and #Analog7
#9 PWM and #Analog9 -- Use by the LIPO Battery

#10 PWM and #Analog10
#11 PWM
#12 PWM and #Analog11
#13 PWM red on board led

#Analog 0 to 5

SPI

SCK, MOSI, MISO (could be used as GPIO if no SD Card, but carefull)
Used by the SD Card

Micro SD Card + Green LED

#4 Select
#7 MicroSD Card detect pin, open pni 7 as input_pullup, if 0 no card inserted
#8 Green Led

USB 

It can also act like a mouse, keyboard, USB MIDI device, et

Arduino IDE Preference Link

ATTINY84
https://raw.githubusercontent.com/damellis/attiny/ide-1.6.x-boards-manager/package_damellis_attiny_index.json

FEATHER 32u4
https://adafruit.github.io/arduino-board-index/package_adafruit_index.json