W6300 crystal selection

Question

It’s a question related to the specification of the crystal that is used for the internal system clock on the W6300. In the datasheet the crystal requirements are shown in Table 10 Quartz Crystal, section 8.1, page 136. It gives values for the crystal parameters and the following equations to calculate the required trans-conductance, gmcrit. It also directs the designer to reference https://docs.wiznet.io/img/products/w5100s/w5100s_crystal_selection_guide_v100k.pdf gmcrit = 4 × (ESR + RExt) × (2πF)2 × (C0 + CL)2 gainmargin = gm / gmcrit If I use the above calcualtions with the transconductance value of 8.43 given in the selection guide I can only use a very narrow range of crystals. In addition, the reference design W6300-EVB-PICO itself seems to include a crystal that fails this criteria, see https://github.com/Wiznet/Hardware-Files-of-WIZnet/tree/master/02_iEthernet/W6300/W6300-EVB-Pico_V100/Schematic How strictly do I have to keep to the above criteria in choosing the crystal? And which is right the datasheet or the reference design?

Alan · Answer

Thank you for your inquiry. The gm value of 8.43 mA/V stated in the linked W5100S Crystal Selection Guide applies to the W5100S oscillator circuit. Table 10 of the W6300 datasheet specifies gm = 16.7 mA/V (typ.), so this value should be used when reviewing crystal selection calculations for the W6300. For the same crystal and external circuit conditions, the calculated gm/gmcrit ratio is approximately 1.98 times the value obtained using 8.43 mA/V. Crystal selection should be evaluated against the requirements in the W6300 datasheet. The gain margin is a design criterion intended to ensure reliable startup across variations in temperature, supply voltage, and component characteristics, so sufficient margin should be maintained. The final evaluation should account for the actual load capacitance and series resistance on the PCB, and should verify startup reliability, oscillation frequency, and crystal drive level under the intended operating conditions. The W6300-EVB-Pico reference design provides an implementation example for component selection and circuit configuration. It should not be interpreted as a basis for relaxing the datasheet requirements. Although the publicly available BOM specifies a load capacitance of 18 pF for the crystal, it does not provide the ESR and C0 values required for the gain margin calculation. Therefore, the suitability of the EVB crystal cannot be determined solely from the public documentation and a calculation using the W5100S gm value. For a definitive assessment, the notation used for the W6300 gain margin requirement and the detailed specifications of the EVB crystal need to be confirmed. Please share the manufacturer and part number of the crystal you evaluated, together with the ESR, C0, CL, and external series resistance values used in your calculation, so that the conditions can be compared. Thank you.