We packed a full Metro's worth of goodies into this slim little PCB. There's everything you need to build a project in Arduino - lets check it out!

PDF of above pinout image available here.

USB & Serial converter

The USB plug is how you will need to program and debug your code. We use it to both power your project and also provide the USB serial interface. Check the Arduino IDE Usage page for how to install the drivers.

Microcontroller & Crystal

This is the brains of your Metro Mini. We use the ATmega328P at 16 MHz. This matches the same exact chip used in the Adafruit Metro and Arduino Uno, so you know that you'll have 100% compatibility.


There are 4 LEDs in the middle of the board. In no particular order:

  • ON is a green LED that is always lit when the board is powered, it is connected directly to the 5V power supply through a 1K resistor
  • L is the red 'pin 13' LED and is connected to (you guessed it) pin #13 through a 1K resistor
  • RX and TX are controlled by the USB serial chip. They will let you know when data is sent from or to the computer. Both are yellow. They are driven by the USB serial chip only, you cannot control them directly.

Power Pins & Regulators

The bottom row has 6 power pins that you will find handy!

  • USB is connected directly to the micro USB connector, and is great when you need a lot of 5V power for NeoPixels or servos, etc. If USB is not plugged in, this will not be powered
  • 3V this is the 3V regulator ouput. On the FTDI-using Metro Mini, this will be 50mA output. On the CP2104-using Metro Mini it will be 500mA max output. Note you can only get 500mA out when its plugged into USB
  • 5V is the 5V output. If powered from USB, it is powered from USB power through a Schottkey diode. If you are only powering from Vin, it will give you up to 150mA of current out
  • Vin is the battery input. You can provide 5V to 16V input. Note that we linearly regulate this down, so using a higher voltage will cause your board to heat up more. 5-8V is a good place to stay.

You can connect both USB and Vin power, and the metro will automatically switch between the two to maintain the highest voltage output on 5V out.

Digital I/O Pins

On the right hand side are all the digital I/O pins you expect. These match the same names/usage of the Adafruit Metro and Arduino UNO. They are all 5V logic output/input unless you've set the Mini to be 3V logic (see at the end)

The two pins labeled 0 and 1 are the two Serial pins that are used to send data to and from the Arduino to the USB-Serial translator chip

Don't connect anything to Digital 0 or 1 unless you are super sure because it will affect your Metro's ability to communicate!

  • Digital 2 through Digital 12 are normal every day digital pins. Pins # 3, 5, 6, 9, 10, 11 can do PWM output
  • Digital 13 is a little special because it is also connected to the L LED. You can use this pin without affecting the Arduino just be aware that the L LED will also blink at the same time.

SPI Port

The ATmega328p has a hardware SPI port, but you can only use the following pins:

  • SPI Clock on D13
  • SPI MISO on D12
  • SPI MOSI on D11

The default chip select pin is D10. If you aren't using that pin for CS, it has to be a digital output. So we recommend using that pin for your first SPI-device CS

I2C Port

The ATmega328p has a hardware I2C port, but you can only use the following pins:

  • I2C Data on A4
  • I2C Clock on A5


The ATmega328p has a hardware UART port, but it is used for the Serial connection and programming the chip bootloader so it isn't recommended!

  • UART TX on D0
  • UART RX on D1

For connecting to Serial-needing devices we recommend SoftwareSerial which can use any two pins

Analog In + Digital I/O Pins

The analog input pins are special pins that can read sensors. They can also be used as digital input/output pins, they really are the most versatile pins!

Each analog pin can read a voltage between 0 and 5V. When used in digital mode, they are all 5V logic output/input. Unless you've set the Mini to be 3V logic (see at the end) in which case the analog read-range and logic level is 3.3V

Other Pins

There's a Reset pin and button, they are connected together. When RST is connected to ground, it will reset your Metro Mini. Note it will not reset the serial connection so you can keep that open when resetting. The button will do the same as connecting RST to ground, but its fun to press.

Aref is the analog reference, which can be connected to other voltages when you want to change the range or quality of your analog readings.

Voltage Select Jumper

On the bottom of the board, there is a Vio jumper, which you can see ties the middle pad to 5V. If you want to use your Metro Mini in 3V logic mode - which can really simplify connecting to many chips, sensors, and displays - cut the little trace to the left, and solder the middle pad and right pad together. Use a multimeter to check that you really cut that trace, as shorting 3V and 5V together is very bad and may damage your Mini!

This guide was first published on Aug 13, 2017. It was last updated on Aug 13, 2017.

This page (Pinouts) was last updated on Aug 13, 2017.

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