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A phase locked loop in a microcontroller's clock tree is used to

  • A.multiply a low frequency crystal up to the core clock frequency
  • B.divide the core clock down to suit the various peripheral blocks
  • C.detect the failure of the external crystal
  • D.keep the real time clock running while the core sleeps

Answer: A

A cheap 8 or 16 MHz crystal can then drive a core at 168 MHz, which avoids both an expensive high frequency crystal and the radiated emissions of a fast external clock. Dividers handle the peripheral clocks and a separate monitor watches for crystal failure.

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