Olaf Rempel

The AVR tiny does not have a dedicated TWI peripheral like most of the AVR mega but uses USI peripheral for I2C. Should be possible to implement, but currently not...

Thanks for the info. I've recently re-implemented my butterfly protocol bridge and integrated it into ispprog (https://github.com/orempel/ispprog). Currently it can only be used with multiboot_tool (https://github.com/orempel/multiboot_tool), but writing a patch...

No, twiboot will stay GPL-2 as it is already used in other projects as well.

Moin, without seeing your code changes I would guess that the HW timer is not running correctly and thus never setting "cmd = CMD_BOOT_APPLICATION" in TIMER0_OVF_vect. Might be an overflow...