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 20 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 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.