Crystal ESR for Atmega32U4


I’m trying to select the right Crystal for my atmega32U4 device.
My question is about crystal ESR. How much it can be?

I can choose the values between 50 to 300 Ohm.
Is the small values better than big ones?

The atmega32U4 datasheet does not say anything about this.

Thank you very much.

Standard crystals should work, just avoid ones cut for overtone operation..

I found the following lines from the Atmel documentation:

"The equivalent series resistance (ESR) is an electrical representation of the
mechanical losses, and at the series resonant frequency, fs, it is equal to R1 in the
electrical model. The ESR is a very important parameter, and can be found in the
crystal datasheet. The ESR will usually be dependent of the crystal’s physical size,
and small crystals (especially small SMD crystals) typically have higher losses and
ESR values than larger crystals.
Higher ESR values will load the inverting amplifier more, and too high an ESR may
cause unstable oscillator operation. Unity gain will not be achieved, and the
Barkhausen criterion will not be fulfilled"

So this means that small ESR value is better.

SMD or thruhole?
This smaller crystal 16 MHz works nice

You can see it here next to pins 11-12-13

The pads were laid out for a standard HC49S size part,

and the wrong part got ordered be mistake - which worked out well as this part takes up much less board space!