ATmega UPDI Programmer

This is a firmware, that when uploaded on an atmega328p, or a similar AVR MCU (including experimental support for atmega1280/2560, and for Logic Green LGT8F328P/D AVR clones with 32 pins or more), enables it to interface with avrdude using the jtagice Mk2 protocol via a serial link. In particular, you can use an Arduino Uno/Nano to host this firmware (experimental support for Arduino Mega and LGT based Arduino clones). It provides a bridge to program the new attiny817 family of MCUs, that use the UPDI interface:

avrdude -> HW Serial interface -> Programmer MCU (e.g. Mega328P) -> SW Serial on PD6 -> Target MCU (e.g. tiny817)

Currently, I have not tested this software with a level shifter, however, since the UPDI pin is high voltage tolerant, it's ok to have V_prog > V_target, but not the reverse. Warning: only the UPDI pins of devices that support high voltage programming should be assumed to be high voltage tolerant. This is not the case for the Mega AVR-0 series!