This project implements an Atmel In System Programmer (ISP) programmer using only a low cost (≈$5-$17) Pro Micro (Sparkfun DEV-12640 or equivalent clone) (or Leonardo) module with an ATMega32U4 ...
This sketch can be used to let one Arduino program a sketch or bootloader into another one, using the ISP programming pins. This sketch is based on OptiLoader, but modified for use with Adafruit's ...