Example of the command to use under linux to flash with avrdude (version 5.8):
avrdude -p x192a3 -P /dev/ttyUSB0 -c avr109 -b 115200 -U flash:w:test.hex
There is a temporisation at boot. After reset or cold start, the MCU waits 3 seconds max until it starts its application. There is no dedicated pin to set to enter flashing mode.
Source code and hex image: