18.104.22.168 Input Clocks and PLLs
GPIO18* and its mux options can be used only when the system is clocked by INTOSC and X1 has an external pulldown resistor.
In addition to the internal 0-pin oscillators, three types of external clock sources are supported:
- A single-ended 3.3-V external clock. The clock signal should be connected to X1, as shown in Figure 5-12, with the XTALCR.SE bit set to 1.
Figure 5-12 Single-ended 3.3-V External Clock
- An external crystal. The crystal should be connected across X1 and X2 with its load capacitors connected to VSS as shown in Figure 5-13.
Figure 5-13 External Crystal
- An external resonator. The resonator should be connected across X1 and X2 with its ground connected to VSS as shown in Figure 5-14.
Figure 5-14 External Resonator