- 5V - this is the 5V power from the USB input.
- 3V - this is the 3.3V power output from the voltage regulator.
- GND - this is the common ground for all power and logic.
- RST - reset pin, pulled high internally, set low to reset
- G0 to G3 - can be used as either digital inputs or outputs, logic level is 3.3V by default but can be changed to 5V
The GPIO pins do not have internal pull-up or pull-down support! If you need pull resistors, add them externally.
- SCL - the I2C clock signal, there's a 5.1K pullup resistor on this pin to whatever the logic level is (default 3.3V)
- SDA - the I2C data signal, there's a 5.1K pullup resistor on this pin to whatever the logic level is (default 3.3V)
- TX - transmit (out from board)
- RX - receive (in to board)
The uart is totally separate from the GPIO pins, it's controlled as a USB CDC device not as the USB HID interface so it shows up as a serial COM/tty port like any other USB-to-serial converter
The default logic level is 3.3V - this is for digital, analog, I2C and UART data. You can change it to 5V by cutting the jumper on the bottom from 3V and soldering it to the 5V side: