17.6.3 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 R30 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 17-8.
To improve accuracy and Safety Factor, the crystal datasheet can recommend adding external capacitors as shown in Figure 17-9.
To find suitable load capacitance for a 32.768kHz crystal, consult the crystal datasheet.
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.