Take a look at this picture:
I think most of use agree that the "big silvery" thing on the board (red) is the famous 16 MHz crystal of the Arduino. That's what I've believed until very recently.
I was bored so I looked at my Arduino Uno, and saw that it has what seems like another crystal (yellow). It looks like the one that is on the Pro Mini. It's also located near the XTAL pins. After I inspected the schematic a second time, it also appears that there are two crystals on the board.
Which one out of those two is actually connected to the XTAL pins and why are there two?
2 Answers 2
The ceramic resonator in yellow is connected to the main MCU, and the crystal in red is connected to the other MCU. USB requires tighter tolerances than is available from a resonator therefore a crystal must be used instead.
There are 2 that you can see, but there are internal resonators in both chips. You can use the ATMega328's one if it's on a breadboard (it's half the frequency tho), and much more temperature-dependent. Just need to give it a fresh bootloader.