It's called the Program and Debug interface and uses two wires (PDI/O). That's for xMega though. There's also debugWire (one wire). But many chips also support

It's called the Program and Debug interface and uses two wires (PDI/O). That's for xMega though. There's also debugWire (one wire). But many chips also support JTAG as well (for both programming and debugging as opposed to using the ISP). https://microchipsupport.force.com/s/article/On-chip-debugging-with-AVR https://microchipsupport.force.com/s/article/On-chip-debuggi... The official 328 breakout board uses debugWire I think, but you get a nice USB interface. I recommend the dev boards, they're very cheap and quite capable (all pins broken out, prototyping areas, usually a few LEDs and you can use the debugger). They also have a footprint for arduino Shields! https://www.microchip.com/en-us/development-tool/ATMEGA328P-XMINI https://www.microchip.com/en-us/development-tool/ATMEGA328P-... Watching variables is flaky in my experience, but hardware breakpoints and memory inspection work fine.