Adafruit’s METRO boards are available with the Arduino UNO R3 pinout with and without headers, or as a breadboard compatible mini size. At the heart is an ATmega328 with 32 KB of Flash and 2 KB of RAM, running at 16 MHz and preloaded with the Optiboot bootloader. The METRO has a hardware USB-to-Serial converter that can be used by any computer to listen and send data to the METRO, and can also be used to launch and update code via the bootloader.

The full-sized version has 19 GPIO pins and the mini has 20 GPIO, of which six are analog in as well, and two of which are reserved for the USB-Serial converter. There are also six PWMs available on three timers (one 16-bit; two x 8-bit). There is a hardware SPI port, hardware I²C port, and hardware UART-to-USB. The logic level is 5 V but by cutting and soldering closed a jumper, it is easily converted to 3.3 V logic. Adafruit’s METRO boards also provide four indicator LEDs for easy debugging: one green power LED, two RX/TX LEDs for the UART, and a red LED connected to pin PB5 / digital #13.