OSCILLATORS — Oscillator control

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.

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.

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

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

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

It is possible to use external capacitors after disabling the internal capacitor.

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

C1 and C2 are the external capacitors. C_pcb1 and C_pcb2 are stray capacitance on the PCB. C_pin is the pin input capacitance on pins XC1 and XC2.

When using external capacitors, disable the internal capacitor by setting OSCILLATORS.XOSC32M.CONFIG.INTCAP to 0.

Several crystals are supported by the 32 MHz crystal oscillator.

The following figure shows a simple model of a crystal. It has R-L-C series components, called equivalent series resistance (ESR), motional capacitance (C_0=N ), and motional inductance (L_M ). The capacitor in parallel, C₀ , is called the shunt capacitance, and models the package capacitance.

The crystal needs to have parameters ESR, C₀ , and C_L selected to ensure the crystal oscillator is stable.

The following figure shows the maximum allowable combinations of ESR, C₀ and C_L for a given crystal. A crystal is supported if that crystal's parameters fall directly on the line or below it. Crystals that are above the line are not supported.

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.

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.

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

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

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

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

C1 and C2 are ceramic SMD capacitors connected between each crystal terminal and ground. C_pcb1 and C_pcb2 are stray capacitance on the PCB. C_pin is the pin input capacitance on pins XL1 and XL2. The load capacitors C1 and C2 must have the same value.

When using external capacitors, the internal capacitor is disabled by setting OSCILLATORS.XOSC32KI.INTCAP to 0.

The 32.768 kHz crystal oscillator (LFXO) is designed to work with external sources.

The device can use a rail-to-rail clock, where the signal should be applied to the XL1 pin with the XL2 pin left unconnected. To enable rail-to-rail clock, set XOSC32KI.BYPASS=Enabled.

Using an external source requires that CLOCK.LFCLK.SRC=LFXO.

The CPU clock frequency is configurable on boot in the register PLL.FREQ (Retained).

The device supports 64 or 128 MHz frequency.

The device starts at 64 MHz. For higher frequencies, it must be configured when the CPU starts and before any peripherals that use the high-frequency clock are enabled. Changing the frequency on a running system or to an unsupported value causes undefined system behavior and the device can malfunction.