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I'm selecting the caps for the crystal oscillator of the wiznet W5500. The standard Load Capacitance for 25Mhz crystals is 18pF. I found out that the hardware guideline claims for a Load Capacitance crystal (CL) of 18pF, and taking into accout the formula to select CL0 and CL1, and assuming Stray Capacitance (CS) of 4pF...

CL1 = CL0 = 2 (CL - CS) = 2 (18 - 4) = 28 pF

If you take a low CS of 3pf, it gives 30 pF, and if takes a high value of CS like 5pf, it takes 26pF.

Why, then, in all reference schematics I found (including wiznet schematic reference), CL0 and CL1 are 18pF ???

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  • \$\begingroup\$ You have a point. Wiznet's crystal selection guide for the W5100S (a closely related part) mentions typical values for CS in the range 5-7pF. Even assuming CS = 7pF, you get CL0 = CL1 = 22pF for CL = 18pF. Suggest you post this question to the appropriate Wiznet forum. See also this question - clearly the crystal spec is important for reliable operation of these parts. \$\endgroup\$ – amb Aug 11 '18 at 11:48
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Why, then, in all reference schematics I found (including wiznet schematic reference), CL0 and CL1 are 18pF ???

Double-check the BOM. I see a lot of crystals with <10pF CL, especially in small SMD packages. The 18pF CL0/1 seem about right for those when you include trace and pin capacitances.

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  • \$\begingroup\$ In the hardware reference of wiznet w5500, they recommend you to use a crystal with 25 mhz with CL 18pf (we can suppose they recomend us the HC-49S package, because 18pf is the standard on this size). So, taking into account that values, the CL0 and CL1, should be between 26 and 30 pF, but all schematics samples from wiznet website, show CL0 and CL1 as 18pF. \$\endgroup\$ – Fran H Aug 11 '18 at 15:43

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