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 crystal’s Equivalent Series Resistance (ESR) must be taken into consideration. Both values are specified by the crystal vendor.

The SAM D21 oscillator is optimized for very low power consumption, hence close attention should be made when selecting crystals, see the table below for maximum ESR recommendations on 9pF and 12.5pF crystals.

The Low-frequency Crystal Oscillator provides an internal load capacitance of typical values available in Table , 32kHz Crystal Oscillator Characteristics. This internal load capacitance and PCB capacitance can allow to use a Crystal inferior to 12.5pF load capacitance without external capacitors as shown in the following figure.

Table 1. Maximum ESR Recommendation for 32.768kHz Crystal
Crystal C_L (pF)	Max ESR [kΩ]
12.5	313

Note: Maximum ESR is typical value based on characterization. These values are not covered by test limits in production.

Figure 1. External Real Time Oscillator without Load Capacitor

However, to improve Crystal accuracy and Safety Factor, it can be recommended by crystal datasheet to add external capacitors as shown in the next figure.

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

Figure 2. External Real Time Oscillator with Load Capacitor

Table 2. 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

These values are given only as typical examples.
Decoupling capacitor 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.