36.5.3 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 crystal’s Equivalent Series Resistance (ESR) must be considered. Both of these values are specified by the crystal vendor.

The SAM D20 oscillator is optimized for very low-power consumption, hence the user must pay close attention while selecting crystals. Refer to the table below for maximum ESR recommendations on 9 pF and 12.5 pF crystals.

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

Table 36-6. Maximum ESR Recommendation for 32.768 kHz Crystal
Crystal C_L (pF)	Maximum ESR [kΩ]
12.5	313

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

Figure 36-7. External Real-Time Oscillator without Load Capacitor

However, to improve crystal accuracy and safety factor, it has been recommended to add external capacitors as shown in the figure below.

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

Figure 36-8. External Real-Time Oscillator with Load Capacitor

Table 36-7. 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 provided in Crystal Oscillator Characteristics to calculate C_LEXT.
Decoupling capacitor must 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 the Oscillator Pinout section.