matseng wrote:From what I remember the series resistor is only used for not overdriving sensitive crystals, and most current common crystals aren't classified as "sensitive" today. And most modern microcontrollers are made for standard crystals without the series resistor. I can be totally wrong here - I'm not an expert on the subject.
You are not wrong, but there are other reasons as well.
The series resistor is also used to limit the Q of the crystal setup to limit the voltage upswing, and thereby protecting the clock input of the driver. When the tuning is right, then the upswing will cause the voltage to be significantly higher than the Vcc of the driver, which is dissipated by the protection diodes.
The series resistor adds a better phase-margin to the oscillator, so it will start more reliably. High frequency crystals are often 3rd overtone type, which requires a parallel resistor of significantly smaller value (order of 1k..10k). However, the parallel resistor is often not enough to dampen the system to a reasonable low level without creating a load on the driver that is too high. There you need to limit the drive-power, for which you add the series resistor. Without the dampening, the crystal refuses to swing at the overtone and will get stuck at the ground-frequency.
The app-notes I linked previously are very good at going into the details of the setup.