External Real Time Oscillator

The low-frequency crystal oscillator is optimized for use with a 32.768 kHz watch crystal. When selecting crystals, load capacitance and the crystal’s Equivalent Series Resistance (ESR) must be taken into consideration. Both the values are specified by the crystal vendor.

SAM L10/L11 oscillator is optimized for very low power consumption, hence close attention should be made when selecting crystals.

The typical parasitic load capacitance values are available in the Electrical Characteristics chapters. This capacitance and PCB capacitance can allow using a crystal inferior to 12.5 pF load capacitance without external capacitors as shown in the following figure.

Figure 1. External Real Time Oscillator without Load Capacitor

To improve accuracy and safety, it has recommended to add external capacitors, as recommend in the crystal data sheet. See figure below for additional information.

To find suitable load capacitance for a 32.768 kHz crystal, consult the crystal data sheet.

Figure 2. External Real Time Oscillator with Load Capacitor
Table 1. External Real Time Oscillator Checklist
Signal Name Recommended Pin Connection Description
XIN32 Load capacitor 12 pF(1,2) Timer oscillator input
XOUT32 Load capacitor 12 pF(1,2) Timer oscillator output

1. These values are only given as typical examples.

2. The capacitors should be placed close to the device for each supply pin pair in the signal group.

Note: To minimize the cycle-to-cycle jitter of the external oscillator, keep the neighboring pins as steady as possible. For neighboring pin details, refer to Oscillators Pinout.