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Why should I use an external resonator with an ATmega microcontroller if it has one built in?

I'm specifically referring to many of the 8 MHz Arduino boards. A lot, if not most, have an external ceramic resonator, but the chip has a built-in one so what's the benefit of using an external one?

Arduino Pro Mini

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The internal RC oscillator is not very precise beyond its calibration specifications (3V, 25degC). It can be tuned for more precision but only if you actually have an external timing source to compare it against. Resonators are naturally a bit better, but some applications require crystal accuracy or better.

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The internal oscillator is normally not as accurate than an external one. I have never had any problems with it, but of couse this won't help if in your specific case it's not accurate enough.

The other reason is that the internal RC is limited to 8 MHz. If look at the board you have mentioned in your question, there is a version for 16 MHz. So possibly from a design/manufacturing point of view it's easier to have one PCB, and possibly the same fuse settings for the boot loader. So it might simplify the manufacturing, and it possibly prevents complaints from customers who have problems with the internal one.

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