External Real Time Oscillator

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

SAM L21 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 section. This capacitance and PCB capacitance can allow using a crystal inferior to 12.5pF load capacitance without external capacitors as shown in Figure 1.

Figure 1. External Real Time Oscillator without Load Capacitor

To improve accuracy and Safety Factor, the crystal datasheet can recommend adding external capacitors as shown in Figure 2.

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

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 22pF⁽¹⁾⁽²⁾	Timer oscillator input
XOUT32	Load capacitor 22pF⁽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: In order 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 the Oscillator Pinout section.