High-frequency (32 MHz) crystal oscillator (HFXO)

The high-frequency crystal oscillator (HFXO) is controlled by a 32 MHz external crystal.

The crystal oscillator is designed for use with an AT-cut quartz crystal in parallel resonant mode and is connected between pins XC1 and XC2. For correct oscillation frequency, the load capacitance must match the specification in the crystal datasheet. The following figure shows how the 32 MHz crystal is connected to the high frequency crystal oscillator.

Figure 1. Circuit diagram of the high-frequency crystal oscillator

The device can be used with external capacitors C1 and C2 or the internal capacitors C_INT, which are configurable.

For reliable operation, the crystal load capacitance, shunt capacitance, equivalent series resistance, and drive level must comply with the specifications in table 32 MHz crystal oscillator (HFXO). It is recommended to use a crystal with lower than maximum load capacitance and/or shunt capacitance. A low load capacitance reduces both start up time and current consumption.

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

Figure 2. 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 capacitor must be configured before starting the high-frequency crystal oscillator using the XOSTART task. To enable the internal capacitors, find the correct value for C_INT in the field OSCILLATORS.XOSC32M.CONFIG.INTCAP using the following equation.

INTCAP = (((CAPACITANCE-5.5)*(FICR->XOSC32MTRIM.SLOPE+791)) + 
           FICR->XOSC32MTRIM.OFFSET*4)/256

The equation has the following variables:

CAPACITANCE is the desired capacitor value of C_INT in pF, holding any value between 4.0 pF and 17.0 pF in 0.25 pF steps.
FICR->XOSC32MTRIM are factory trim values which vary between devices.

After HFXO starts, the device uses the internal capacitor together with the external crystal after configuration.