Arduino Uno board (as TTL programmer) programming ATmega328

Hello there,

For several weeks I am struggling to make Xbee S2 -> ATmega328p wireless sketch upload to work.

I have already tried solutions which should have worked (I can wirelessly reset the chip) but I always get classic error "avrdude: stk500_getsync() attempt 1 of 10: not in sync: resp=0x8c" (the "8c" at the end differes every time)

As a matter of investigation I wanted to make a step back and try to make at least manual reset solution to work which is used for example on Arduino Pro Mini when you have to reset the chip at the specific time. This is where I think might be the problem.

I turned one Arduino Uno (chinese clone with CH340G) board into a TTL programmer (as a simulation of Xbee programming the chip through RX/TX pins). I did that by connecting RST with GND and then connecting it into another Arduino Uno board 5V ->5V GND->GND RX->TX TX->RX and even RX->RX and TX->TX.

When I hit the upload button I hold the reset button and wait till it says "Uploading". I release the reset button and my target Arduino to be programmed blinks 3 times with its RX pin but then nothing happens and my already present Blink sketch starts running.

I have also tried it with a customized bootloader with wait time around 8 seconds after which the sketch starts running to avoid timing issues but with the same result. It just does not start uploading the sketch.

I have tried to make a loopback test and I don't receive the echo in the serial monitor, but RX and TX blinks when I hit the enter.

Could anyone help me to navigate at least with some guesses where the issue might be? I appreciate your help, it's really frustrating after more than hundred of hours working on the same problem, but you surely know what I mean :cry: :smiling_imp: