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If you should accidentally disable the reset pin while programming an AVR chip, you'll lose the ability to rewrite to it using common ISP programming devices. To make the chip writable once again, you'll need a 'high voltage programmer' that runs off of 12V. With an Arduino and some spare components, MightyOhm shows you how to make one -

The Arduino has just enough IO to implement the entire HV protocol plus a "go" button. So far I have only implemented setting LFUSE and HFUSE in software, but there is no reason why the code couldn't be extended to support chip erase and programming the entire flash as well.

The fuse programming process is simple:

• Upload the HVFuse sketch to the Arduino [available here]
  
• Install the shield and apply +12VDC to the terminals on the left
  
• Wait for the red LED to turn on
  
• Install the ATmega to be repaired
  
• Push the button
  
• As soon as the LED turns off, the AVR is fixed and ready to be put back into service!

- Arduino-based AVR High Voltage Programmer on Flickr

ProtoShield for Arduino Kit
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