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 WKUP_OSC0_XI and WKUP_OSC0_XO signals to shield the WKUP_OSC0_XI signal
from the WKUP_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 WKUP_OSC0_XI and WKUP_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.