Use a Raspberry Pi Zero as a host to do In-Circuit Serial Programming of AVR’s. IOFF buffers are used to level convert to the targets voltage. It also allows the serial port to be used for serial bootload and has some switches to control looping of the upload.
# Alt0 pin function
# raspi-config can set enable_uart
As the R-Pi Zero W boots up the D1 green LED is dimly lit. Once the overlay is applied D1 turns off, the R-Pi pin has 1.6V at power on and does sink some current. It is acceptable for this application, but that is not how the AVR's or PIC's that I have used power up.
I did a script so that I can push the bootload button (S1) and run the make bootload rule. I run that script after setting the working folder to the location with the make rule (e.g., change directory to the folder with Makefile).
Next I have a bootload rule in the RPUno i2c-debug application's Makefile.