Datasheet

Power Pins

GND - this is the common ground for all power and logic

BAT - this is the positive voltage to/from the JST jack for the optional Lipoly battery

USB - this is the positive voltage to/from the micro USB jack if connected

EN - this is the 3.3V regulator's enable pin. It's pulled up, so connect to ground to disable the 3.3V regulator

3V - this is the output from the 3.3V regulator, it can supply 500mA peak

Logic pins

This is the general purpose I/O pin set for the microcontroller.

All logic is 3.3V

Nearly all pins can do PWM output

All pins can be interrupt inputs

#0 / RX - GPIO #0, also receive (input) pin for Serial1 (hardware UART), also can be analog input

#1 / TX - GPIO #1, also transmit (output) pin for Serial1, also can be analog input