I was looking Arduino Mega's schematic and I noticed a resistor between the MCU and its resonator. R3 in this image:

enter image description here

Why is it there and what is its function?

  • forum.arduino.cc/… – Gerben Dec 22 '16 at 19:45
  • @Gerben - This addresses R1, not R3. – Wesley Lee Dec 22 '16 at 19:58
  • Oops. Didn't even see R3. That's indeed a weird location for a resistor. – Gerben Dec 22 '16 at 20:08
up vote 3 down vote accepted

That resistor limits the drive through the crystal. If the power applied to the crystal is too high then the slope and phase shift at the resonant frequency will not be optimal and at really high drive levels could cause mechanical damage to the crystal (like that shattered wine glass trick).

See good instruction PDF here, section 2.1.

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