Figure 5-10 shows the recommended crystal circuit. It is recommended that preproduction printed-circuit board (PCB) designs include the two optional resistors Rbias and Rd in case they are required for proper oscillator operation when combined with production crystal circuit components. In most cases, Rbias is not required and Rd is a 0-Ω resistor. These resistors may be removed from production PCB designs after evaluating oscillator performance with production crystal circuit components installed on preproduction PCBs.
The XTALIN terminal has a 15-kΩ to 40-kΩ internal pulldown resistor which is enabled when OSC0 is disabled. This internal resistor prevents the XTALIN terminal from floating to an invalid logic level which may increase leakage current through the oscillator input buffer.