4.7.1 Crystal Pins (XTI/XTO)

The XTI and XTO crystal oscillator pins are used to connect a 25.0 MHz crystal clock source for MII operation. The crystal oscillator should be in a fundamental, parallel resonant mode. Figure 4-4 depicts the external circuitry connected to the LAN8670/2 oscillator circuit. Since the internal inverter/amplifier is operated in its linear region, external series resistors should not be used as they will lower the gain and could cause start-up problems. The device contains an internal ~1 MΩ resistance in parallel with the crystal amplifier to initiate oscillation, therefore an external resistance between XTO and XTI should not be used. Several factors must be considered when selecting a crystal including load capacitance, oscillator margin, cut, and operating temperature. The crystal frequency must be 25.0 MHz.