| External clock |
| ============== |
| |
| The external clock is provided to :ref:`clock <clock>` pin (C9). |
| |
| The external clock functions as the source clock for the entire processor. |
| On startup, the processor runs at the same rate as the external clock. |
| The processor program may access the :doc:`housekeeping-spi` to set the processor into PLL mode or DCO free-running mode. |
| In PLL mode, the external clock is multiplied by the feedback divider value to obtain the core clock. |
| In DCO mode, the processor is driven by a trimmed free-running ring oscillator. |