34.5.3 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 PIC32C oscillator is optimized for very low power consumption, hence users must pay close attention when selecting crystals. See table 31-16 for maximum ESR recommendations on 12.5 pF crystals.

The low-frequency crystal oscillator provides an internal load capacitance of typical values available in Table 31-16, 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.

Figure 34-8. 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 following figure.

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

Figure 34-9. External Real Time Oscillator with Load Capacitor

Table 34-6. External Real Time Oscillator Checklist
Signal Name	Recommended Pin Connection	Description
XIN32	Load capacitor C_LEXT⁽¹⁾	Timer oscillator input
XOUT32	Load capacitor C_LEXT⁽¹⁾	Timer oscillator output

Decoupling capacitor must 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 section Oscillator Pinout.