View source for ATmega128 external clock recovery

If you accidentally programmed the Fuses to use the "External Clock" instead of "External crystal" in the SUT_CKSEL setting, the  microcontroller is temporarily rendered useless.

== Fix ==
To recover the micro with stuck in "External clock" mode, is to connect a 1-3MHz function generator to pin 24 on the ATmega128. The micro should then be revived on the next power cycle.

=== Connection ===
# Remove the crystal and the capacitor connected to pin 24, named XTAL1.
# Get another ATmega based microcontroller, I used a Arduino Uno with an ATmega323, download and program it with the code below
# Connect the other micro by connecting ground to ground on the ATmega128, usually pins 53 or 22, 
# And connect PD3, digital pin 3 on Arduino Uno, to pin 24 on the ATmega128, check the datasheet to find the correct pin 
# Connect the AVR programmer, I used a USBTinyAVR programmer that mimics the official AVRISP programmer

=== Program ===
# Open AVR Studio 4 and click the program icon in the tool bar to initialize a connection, select AVRISP and Auto, or the correct type and port for the programmer
# Once connected to the programmer, set the ISP speed to around 100-125kHz, I have 112.5kHz, click write
# If the entering programming fails, as it usually does, try resetting the other controller and the ATmega128, on that order
# Once the programming mode is successful, go to the Fuses tab, unpick CLOPT if selected, and pick "Int. RC Osc. 1MHz; Start-up time: 6 CK + 64 ms", click program
# The micro is now recovered and the external clock can be removed, reset to try

=== Function generator ===
==== Arduino ===
Paste into the Arduino editor, compile and upload.

 void setup()
 {
     cli();
     DDRD = 0xff;
 }
 
 void loop()
 {
  while (1) {
    // uncomment the delay and comment out cli() to try with a led to find the correct pin
    PORTD |= 0x8;
    //delay(100);
    PORTD &= ~0x8;
    //delay(100);
  } 
 }

==== AVR ====
Untested code, but it should work fine, compile and upload in AVR Studio or Eclipse. 
 #include <avr/io.h>
 void main(void)
 {
  cli();
  DDRD = 0xff;
  while (1) {
   PORTD |= 0x8;
   PORTD &= ~0x8;
  }
 }

== References ==
* [http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1230286016/30 Maximum pin toggle speed]
* [http://arduino.cc/en/Hacking/Atmega168Hardware Arduino ATmega 168 hardware]