49.6.3 32.768 kHz Crystal Oscillator

The low frequency crystal oscillator is optimized for use with a 32.768 kHz 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.

The PIC32CM JH00/JH01 oscillator is optimized for very low-power consumption, therefore 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 the following figure.

Figure 49-9. 32.768 kHz Crystal Oscillator without Load Capacitor

To improve accuracy and Safety Factor, the crystal data sheet can recommend adding external capacitors as shown in the following figure.

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

Figure 49-10. 32.768 kHz Crystal Oscillator with Load Capacitor

Table 49-6. 32.768 kHz Crystal Oscillator Checklist
Signal Name	Recommended Pin Connection	Description
XIN32	Load capacitor. ^(1,2)	Crystal frequency range specified in the XOSC32K Electrical Characteristics (XOSC32_1 parameter)
XOUT32	Load capacitor. ^(1,2)

1. The capacitors should be placed close to the device.

2. Use the equation in the XOSC32K Electrical Specifications chapter to calculate the load capacitor.

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 Pinout section.

For more information, refer to: