45.5.3 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 taken into consideration. Both values are specified by the crystal vendor.

The SAM D21 oscillator is optimized for very-low-power consumption, so pay close attention 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 , 32 kHz Crystal Oscillator Characteristics. This internal load capacitance and PCB capacitance can use a crystal inferior to 12.5pF load capacitance without external capacitors as shown in the following figure.

Table 45-6. Maximum ESR Recommendation for 32.768 kHz Crystal
Crystal CL (pF)Max ESR [kΩ]
12.5313

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

Figure 45-8. External Real Time Oscillator without Load Capacitor

However, to improve crystal accuracy and safety factor, the data sheet recommends adding external capacitors as shown in the next figure.

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

Figure 45-9. External Real Time Oscillator with Load Capacitor
Table 45-7. External Real Time Oscillator Checklist
Signal NameRecommended Pin ConnectionDescription
XIN32Load capacitor CLEXT(1)(2)Timer oscillator input
XOUT32Load capacitor CLEXT(1)(2)Timer oscillator output
  1. Use the equation in Crystal Oscillator Characteristics to calculate CLEXT.
  2. 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.