3V Power Pins; Logic Pins - adafruit learning system ItsyBitsy 32u4 Manual

G - these are all Ground pins

3V Power Pins

In addition to the three standard power pins, the 3V ItsyBitsy has a few more pins available for power sourcing:
3V - this is the regulated output from the onboard regulator. You can draw 500mA whether powered by USB or
battery.
Vhi - this is a special pin! It is a dual-Schottkey-diode connected output from BAT and USB. This means this will
always have the higher-of-the-two voltages, but will always have power output. The voltage will about 5VDC
when powered by USB, but can range from 3.5-6VDC when powered from battery. It's not regulated, but it is
high-current, great for driving servos and NeoPixels.
EN - connected to the regulator enable, it will let you shut off power - when running on battery only . But at least
you don't have to cut a trace or wire to your battery. This pin does not affect power when using USB
G - these are all Ground pins

Logic pins

This is the general purpose I/O pin set for the microcontroller. All logic is 3.3V on the 3V Itsy, and 5V on the 5V Itsy.
You can usually use 3V logic as an input to 5V, but the 3V Itsy pins should not be connected to 5V!
Along the right edge
#0 / RX - GPIO #0, also receive (input) pin for Serial1 and Interrupt #2
#1 / TX - GPIO #1, also transmit (output) pin for Serial1 and Interrupt #3
#2 / SDA - GPIO #2, also the I2C (Wire) data pin. There's no pull up on this pin by default so when using with I2C,
you may need a 2.2K-10K pullup. Also Interrupt #1
#3 / SCL - GPIO #3, also the I2C (Wire) clock pin. There's no pull up on this pin by default so when using with I2C,
you may need a 2.2K-10K pullup. Can also do PWM output and act as Interrupt #0.
#5 - GPIO #5, can also do PWM output. On the 3.3V ItsyBitsy this is a special OUTPUT-only pin, that is level-
shifted up to Vhi voltage, so its perfect for driving NeoPixels that want a ~5V logic level input.
#7 - GPIO #7. Also hardware Interrupt #4
#9 - GPIO #9, also analog input A9 and can do PWM output.
#10 - GPIO #10, also analog input A10 and can do PWM output.
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