ATmega328 is a very popular microcontroller. The processor has plenty of GPIO, Analog inputs, hardware UART SPI and I2C, timers and PWM galore - just enough for most simple projects. The METRO design is a derivative of Arduino UNO R3 reference design.
Adafruit METRO 328 without Headers - ATmega328 is the culmination of years of playing with AVRs. Adafruit wanted to make a development board that is easy to use and is hacker friendly. At the heart is an ATmega328P, with 32KB of flash and 2KB of RAM, running at 16 MHz. You can power Adafruit METRO 328 without Headers - ATmega328 with 7-9V polarity protected DC or the micro USB connector to any 5V USB source. The 2.1mm DC jack has an on/off switch next to it so you can turn off your setup easily. METRO will automatically switch between USB and DC.
Adafruit METRO 328 without Headers - ATmega328 has 19 GPIO pins, 6 of which are Analog in as well, and 2 of which are reserved for the USB-serial converter. There are also 6 PWMs available on 3 timers (1 x 16-bit, 2 x 8-bit). There's a hardware SPI port, hardware I2C port and hardware UART to USB. Logic level is 5V but by cutting and soldering closed a jumper, you can easily convert it to 3.3V logic
Adafruit METRO 328 without Headers - ATmega328 has hardware USB to Serial converter that can be used by any computer to listen/send data to the METRO, and can also be used to launch and update code via the bootloader
Adafruit METRO 328 without Headers - ATmega328 comes pre-loaded with the Optiboot bootloader, which is supported by Avrdude and only uses 512 bytes.
Adafruit METRO 328 without Headers - ATmega328 also works with all Adafruit shields. The board comes without the headers assembled.
Adafruit Web Site: https://www.adafruit.com/product/2466