I hava an Arduino Duemilanove but I think that its FTDI chip is crashed.
I have an Arduino Mega too, so can I use it to program my Duemilanove?
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Yes, this is not an uncommon activity. This is called using an Arduino as an In-System Programmer (ISP)
This page has the information and Arduino program you need. You'll need to adjust for the Arduino Mega pin differences, but that's fairly easy. http://arduino.cc/en/Tutorial/ArduinoISP
Where the ArduinoISP page references pin 13, on the Mega it's pin 52. Where the ArduinoISP page references pin 12, on the Mega it's pin 50. Where the ArduinoISP page references pin 11, on the Mega it's pin 51. Where the ArduinoISP page references pin 10, on the Mega it's pin 53.
Otherwise you should be able to connect the Mega to the patient in the same way as described, load the ArduinoISP sketch into the Mega, preferably from the GitHub repository linked in the article (https://github.com/rsbohn/arduinoisp).