Debug

Jan found a newish in circuit debugger that is very cheap to build and works like a charm:

https://github.com/dcwbrown/dwire-debug

He uses it on a attiny441. It is quite liberal, as long as there is a dwire facility on the chip.

The is a facility to use it with gdb.

The only issue is that you have to program a fuse with avr-dude to enable dwire, but even that is not hard to incorporate. 5 serial port lines do everything.