Here's an interesting video of the Caravel chip with the SPI master enabled and looped back to the housekeeping SPI (a configuration unique to MPW-one and ChipIgnite-1). The processor is switching its own clock from the external oscillator to the on-board oscillator, and then setting the on-board oscillator to DCO mode and adjusting its own clock timing by modifying the DCO trim setting. The on-board oscillator runs from about 50 to 100 MHz, and the output is being run through a divide-by-4 output divider, so the processor is changing its own core clock rate cyclically from about 12.5MHz to 25MHz (the adjustment is not linear because some of the trim bits are coarse tuning and some are fine tuning and my simple program wasn't paying attention to which ones were which).