When designing the printed-circuit board:
- Place all crystal circuit components as close as possible to the respective device pins.
- Route the crystal circuit traces on the outer layer of the PCB and minimize trace lengths to reduce parasitic capacitance and minimize crosstalk from other signals.
- Place a continuous ground plane on the adjacent layer of the PCB such that it is under all crystal circuit components and crystal circuit traces.
- Route a ground guard around the crystal circuit components to shield it from any adjacent signals routed on the same layer as the crystal circuit traces. Insert multiple vias to stitch down the ground guard such that it does not have any unterminated stubs.
- Route a ground guard between the MCU_OSC0_XI and MCU_OSC0_XO signals to shield the MCU_OSC0_XI signal from the MCU_OSC0_X0 signal. Insert multiple vias to stitch down the ground guard such that it does not have any unterminated stubs.
- Connect all crystal circuit ground connections and ground guard connections directly to the adjacent layer ground plane, and the device VSS ground plane if they are implemented separately on different layers of the PCB.
Note: Implementing a ground guard between the MCU_OSC0_XI and MCU_OSC0_XO signals is critical to minimize shunt capacitance between the two signals. Routing these two signals adjacent to each other without a ground guard between them will effectively reduce the gain of the oscillator amplifier, which reduces its ability to start oscillation.