51.6.3 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. See Table 51-6 for maximum ESR recommendations on 12.5pF 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 51-9.

Table 51-6. 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 51-9. External Real Time Oscillator without Load Capacitor

To improve accuracy and Safety Factor, the crystal data sheet can recommend adding external capacitors as shown in Figure 51-10.

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

Figure 51-10. External Real Time Oscillator with Load Capacitor

Table 51-7. External Real Time Oscillator Checklist
Signal Name	Recommended Pin Connection	Description
XIN32	Load capacitor C_LEXT⁽¹⁾⁽²⁾	Timer oscillator input
XOUT32	Load capacitor C_LEXT⁽¹⁾⁽²⁾	Timer oscillator output

Note:

Use the equation in Crystal Oscillator Characteristics to calculate C_LEXT.
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.