Seeeduino XIAO is the smallest Arduino compatible board in Seeeduino Family. It is an Arduino microcontroller that is embedded with the Microchip SAMD21. The Seeeduino XIAO has many interfaces for such a tiny board: 11 digital/analog pins, 10 PWM pins, a DAC output, a SWD bonding pad, a I2C interface, a UART and an SPI interface.
The Seeeduino XIAO carries the powerful ATSAMD21G18A-MU with 256KB Flash and 32KB SRAM. This little board has a good performance but needs less power.
More information is available on the website of Seeed Studio and wiki.