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What would happen if I would connect an external oscillator that has a higher frequency than the microcontroller (AVR) supports? Will it blow up, constantly reset, or what?

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This is generally called "overclocking" and it will not usually cause your micro to "blow up". It is possible to damage your chip however by running it above its rated frequency. The AVR may boot depending on how much you overclock it. I have run ATMega8 micros overclocked and they have been fine. The voltage and frequency ratings given in the data sheets are "safe" ratings and often you can run the chips higher. This is not guaranteed though, and different chips will have different maximums. You can also increase the operating voltage of the chip to improve stability when running at higher clock rates. Remember that increased voltages/frequency will cause increased heat and may damage your chip. Running a micro above rated frequency/voltage may cause errors while running, or cause other numbers in the data sheets to go out of spec.

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It should probably be noted that overclocking can cause the uC to become unreliable, and not just because of damage. Beyond the rated spec, each individual IC will have slightly different performance, and while it may work on one IC, swap it out for another seemingly identical one and it may not. It also makes the uC more vulnerable to glitching in the power supply and similar otherwise minor external events, and can make for failure at random times in the execution, with failure reports saying things like "sometimes it works, sometimes it doesn't" - a very painful symptom to diagnose. – Chintalagiri Shashank Mar 12 at 21:53
Yes I absolutely agree! – Devin Mar 15 at 15:30
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The same as overclocking any digital logic device:

  • its power consumption will increase
  • this may result in overheating. Lots of overheating will result in physical damage.
  • above a certain point there will be errors, not necessarily causing resets
  • peripherals may not work properly (ADC, UART)
  • small overclocking in favourable conditions will often be completely fine, BUT this has to be experimentally determined for a particular device, speed and temperature

Edit: here's a video purporting to show an Intel 486 running at 5GHz, ten times its design rating, while being cooled to -196C with liquid nitrogen.

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Pentium 4, not 486! The 486 was on the order of 100 MHz and less, while the P4 was up to 3.8 GHz. – exscape Mar 12 at 20:51

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