Dan Creagen says:
This is a frequently asked question which is available in the phantom FAQ at reference 22.7.3. I asked it myself when I first started playing with PICs.The question generates tons of advice - some of it conflicting - because the correct answer is dependent on whether you are doing this for the hell of it, or you are wanting to deploy the final product to unsuspecting users.
The easy answer: Don't do it. That way you don't have to worry about whether the weird problems are due to your software or the chip being overdriven. You eliminate one variable. The users in the field (if there are any) won't get hosed by an overdriven design. 20/20 won't come knocking at your door right when you finally become successful, etc.
The answer that you want: Do it. Lots of people have overdriven them as an experiment and most did not have problems. There are all kinds of tales about how much you can overdrive them and be safe. Why, I once superglued a cryo unit to the top of a 16C84 and ran it to 120 mHz ! ;-). Seriously, 8 mHz is probably going to work. Just don't deploy it in the field if you have a conscience. How do you do it? Be sure the bypass caps are correct for the crystal. Make sure timing dependent software is adjusted for the new freq. Plug in - turn on.
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