The difference is that I have a Uno Wifi Rev2 on the right instead of a Uno R3. I've uploaded the ArduinoISP sketch onto the Uno R3 on the left. Here is the
avrdude command that I am using:
$ avrdude -v -patmega4809 -cavrisp -P /dev/ttyACM1 -b19200 -U program.elf ... avrdude: AVR device initialized and ready to accept instructions Reading | | 0% 0.00savr_read(): error reading address 0x0000 read operation not supported for memory "signature" avrdude: error reading signature data for part "ATmega4809", rc=-2 avrdude: error reading signature data, rc=-2
Full log: https://pastebin.com/LUfhsYeD
I am wondering if I am missing some difference between the UnoR3 and Uno Wifi Rev2... there doesn't seem to be much documentation about external programming of the Uno Wifi Rev2's Atmega 4809.
If it helps, my usecase for trying to program my Uno Wifi Rev2 externally is a need to reprogram the 32u4 as a HID. Once I program the 32u4, I won't be able to program the 4809 via USB (afaik).
If anyone knows what I'm doing wrong or the proper way to program the Uno Wifi Rev2 externally, your help would be greatly appreciated.