12.3.4.2.1 High-Frequency Crystal Oscillator (XOSCHF)
This oscillator supports two input options:
- A crystal is connected to the XTALHF1 and XTALHF2 pins
- An external clock running at up to 32 MHz connected to XTALHF1
The input option must be configured by writing to the Source Select (SELHF) bit in the XOSCHF Control A (CLKCTRL.XOSCHFCTRLA) register.
The maximum frequency of the crystal must be configured by writing to the Frequency Range (FRQRANGE) bit field in XOSCHFCTRLA. This is to ensure sufficient power is delivered to the oscillator to drive the crystal.
The XOSCHF is enabled by writing a ‘1’ to the ENABLE bit in XOSCHFCTRLA.
When enabled, the configuration of the general purpose input/output (GPIO) pins used by
the XOSCHF is overridden as XTALHF1 and XTALHF2 pins. The oscillator needs to be enabled
to start running when requested.
The start-up time of a given crystal oscillator can be accommodated by writing to the Crystal Start-Up Time (CSUTHF) bit field in XOSCHFCTRLA.
When XOSCHF is configured to use an external clock on XTALHF1, the start-up time is fixed to two cycles.