Arduino with an AVR8 microcontroller can be debugged with debugWIRE. This is often done with a debugger from Atmel and Avr Studio. I found the dwire-debug project which contains an opensource ...

An open source, embedded, high performance g-code-parser and CNC milling controller written in optimized C that will run on a straight Arduino (modified to use two arduino uno via i2c) - Lukevdb01/ ...