W5500 Crytsal specification

Question

Are there any examples of crystals that work reliably with the W5500. No reference schematic defines with which crystals it works. Most of them use 18pF as load capacitors. According to the formula CL = C1 / 2 - Cs the load capacitance of the crystal would have to be between 14pF and 16pF (assuming 5pF to 7pF stray capacitance). On the other hand the Crystal Selection Guide Application Note for the W5100S (which is supposedly also valid for the W5500) states that a crystal with 14pF load capacitance and 7pF shunt capacitance (which most crystals seem to have) can have at most 25 Ohms ESR to satisfy the gain margin criteria for stable operation. Neither DigiKey nor Mouser sell crystals that match these criteria, so I am somewhat at a loss at the moment. Can somebody please list parts that would work? Preferable with matching load capacitor values? Or did I misunderstand something?

Eugeny · Answer

Sympatron: can have at most 25 Ohms ESR to satisfy the gain margin criteria for stable operation Please give examples of your calculations for the crystals you think most appropriate from the list you have.

Eugeny · Answer

Gain margin has relation to the oscillation startup. If condition is not satisfied, oscillation will not start, and device will not work. Sympatron: I was wondering which crystals WIZnet is using in their designs to require 18pF load capacitors, It is only possible to know that crystal they use has 18 pF minimal load. There’s stray capacitance added (traces, pads, XI/XO input capacitance, internal crystal capacitance). The nearest realistic value is 20 pF for the crystal. Values of caps, logically, will depend on the PCB design, length of the traces, their width, and even on devices located near oscillator circuit. Using math is good and must be used as a guidance. a starting point, and then, if you want better results, you tune the values. You must use parallel resonant crystals. I did not see WIZnet publishing list of part numbers for crystals they use / recommend.