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I have built a custom board that has an ATmega328p connected to an ESP8266 module via UART. I am currently programming the ATmega328p using a USBtiny programmer via an SPI header. The ESP8266 module is programmed via an FTDI cable connected the the UART.

Firmware for both are created in Arduino IDE.

I have set up a webserver on the ESP that allows for files to be uploaded to the SPIFFS via FTP. I would like to be able to upload a hex file for the ATmega and then have the ESP program the ATmega via the UART connection. A GPIO on the ESP controls a transistor that can pull the reset pin on the ATmega to ground.

I know I need a bootloader on the ATmega (e.g. optiboot), but I don't know what I need to make the ESP do to trigger the bootloader and flash the ATmega. I'm thinking I need something like:

void initialiseAtmegaFlash () {
  // Trigger the bootloader to accept incoming hex over UART
}

void flashHexToAtmega (File atmegaHex) {
  for (uint32_t i = 0; i < atmegaHex.size(); ++i) {
    // Handle timing of sending byte by byte over serial... or chunk by chunk?
  }
}
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To answer my own question in case someone else ends up here, the library Juraj suggested (https://github.com/ciminaghi/libdfu) looks like it does the job. I also found another project doing something similar (https://github.com/rene-win/esp_avr_programmer). Also the STK500 docs linked to by Majenko (http://ww1.microchip.com/downloads/en/AppNotes/doc2591.pdf) are really useful for understanding the nuts and bolts!

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