nownuri:
I need four UART channels.
What about an AdaFruit itsy bitsy M0 or a Sparkfun SAMD21 breakout? Similar form factor to Pro Micro, 3.3V, 48MHz, 32K ram, 256K flash.
One of the most unique features of the SAMD21 is SERCOM – a set of six configurable serial interfaces that can be turned into either a UART, I2C master, I2C slave, SPI master, or SPI slave. Each SERCOM provides for a lot of flexibility: the ports can be multiplexed, giving you a choice of which task each pin is assigned.