8.8.1 32-kHz Oscillator Routing
When designing the printed-circuit board:
- Keep the crystal as close as possible to the crystal pins X1 and X2.
- Keep the trace lengths short and small to reduce capacitor loading and prevent unwanted noise pickup.
- Place a guard ring around the crystal and tie the ring to ground to help isolate the crystal from unwanted noise pickup.
- Keep all signals out from beneath the crystal and the X1 and X2 pins to prevent noise coupling.
- Finally, an additional local ground plane on an adjacent PCB layer can be added under the crystal to shield it from unwanted pickup from traces on other layers of the board. This plane must be isolated from the regular PCB ground plane and tied to the GND pin of the RTC. The plane must not be any larger than the perimeter of the guard ring. Make sure that this ground plane does not contribute to significant capacitance (a few pF) between the signal line and ground on the connections that run from X1 and X2 to the crystal.
Figure 8-31 Slow Clock PCB Requirements