Is it possible to build a hardware debugger for debugWIRE for less than €10? As it turns out, it is. You just have to make a few compromises and also do a bit of soldering and gluing.
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Surprise, Surprise!
When you develop a tool for a protocol that is undocumented, it is not surprising that you will encounter situations you will not have be anticipated. This was exactly what I experienced developing the hardware debugger dw-link, which connects debugWIRE MCUs to the GDB debugger. Although a substantial part of the debugWIRE protocol has been reverse-engineered, I encountered plenty of surprising situations: Split personality MCUs, stuck-at-one bits in program counters, secret I/O addresses, half-legal opcodes, and more.
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One Line Only
A new Arduino library has seen the light of day: SingleWireSerial. It supports single-wire, asynchronous serial, half-duplex communication. By using the input capture feature of the AVR MCUs, it is extremely accurate and supports bit rates up to 250 kbps robustly. And contrary to its title, one can even use it in a two-wire setting.