is it possible to upload a firmware to an ATMEGA328P aka Arduino Uno using a serial terminal like Putty or Coolterm?
I know that the Arduino is ready to receive a new firmware after a reset over USB, does the IDE (and avrdude) makes something different other than echoing the binary hex file over serial port?
Thanks
Theoretically, yes. It would be awkward, and you'd need to be able to send and receive arbitrary bytes within a strict time limit, before the bootloader gives up waiting, but you could do it.
The most common bootloaders for the atmega328 use Atmel's STK500 protocol, originally designed for their own AVR development boards but widely used elsewhere (such as on the Arduino). Documentation for this protocol can be found here.
Note that Optiboot, used on modern Arduinos, implements only a subset of this protocol.
