The Seeedstudio XIAO SAMD21 is a small board with 14 pins that is highly compatible with Arduino’s SAMD21 based microcontroller boards (Nano 33 IoT, Zero, and the MKR series of boards). It contains only the microcontroller, a crystal oscillator (which the Nano 33 IoT does not have), voltage regulator and a “modern” USB-C connector for programming, serial interface, and (if desired) power. It is certainly minimalist, and an ideal choice if no radio (WiFi/Bluetooth) is needed because of its rock-bottom price, currently as low as $5.40 when ordered direct.
It is a 3.3 volt part, which can be a disadvantage, but if that isn’t a problem it had 11 data pins that are pretty much universal. All 11 can be used for analog inputs (ADC) or digital I/O. The DAC is available on D0/A0. Pin interrupts (attachInterrupt function) are available on all pins and PWM (analogOut) on all but D0/A0.
There are SERCOMs configured for SPI, UART, and I2C on pins D4 through D10, but, of course, the USB interface provides a serial port as well.
The only difference I found in the library support code is that one pin, D4 when used with attachInterrupt is at a higher (Non-Maskable Interrupt) priority so can be used when minimum possible latency is required.
There is no reset button, but you can add one if you need it or short together a pair of landing pads for a quick alternative.
I really love this part!