Low-frequency (32.768 kHz) crystal oscillator (LFXO)

For clock accuracy higher than LFRC, the 32.768 kHz crystal oscillator (LFXO) must be used.

To use the LFXO, a 32.768 kHz crystal must be connected between the XL1 and XL2 pins, as shown in the following figure.

Figure 6. Circuit diagram of the low-frequency crystal oscillator

The device can be used with external capacitors C1 and C2 or the built-in configurable internal capacitors C_INT.

When using internal capacitors, the load capacitance (CL) is the total capacitance seen by the crystal across its terminals. It is calculated by the following equation.

Figure 7. Load capacitance equation for internal capacitors

C_INT is the value of the internal capacitors. C_pcb1 and C_pcb2 are stray capacitance on the PCB.

The internal capacitors must be configured before starting the low-frequency crystal oscillator (LFXO). To enable the internal capacitors, determine the correct field for OSCILLATORS.XOSC32KI.INTCAP using the following equation.

INTCAP = round( (2*CAPACITANCE - 12) * (FICR->XOSC32KTRIM.SLOPE + 0.765625 * 512)/512 + FICR->XOSC32KTRIM.OFFSET/64 )

The equation has the following variables:

CAPACITANCE is the desired capacitor value in pF, holding any value between 3 pF and 18 pF in 0.65 pF steps.
FICR->XOSC32KTRIM are factory trim values which are device specific.

When LFXO starts, it will use the internal capacitor together with the external crystal.