You can drive a stepper motor via UART using the Arduino Mega and ADI Trinamic's TMC5072-EVAL. There's a blog post showing how to connect the evaluation board via Single Wire UART to an Arduino Mega for basic operation. The wiring is limited to the basic functionality for communication via Single Wire UART.
Read more on how to use the setup in the blog post 'How to drive a stepper motor via UART with your Arduino Mega using a TMC5072-EVAL'.
Please note that to use the 5V version of the Arduino Mega, you have to resolder the resistor from position R3 to R8, enabling the 5V logic level for the TMC5072 cDriver
chip.