Even pulses coming
from the oscillator enable
harmonic and synchronous operation of all circuits of the microcontroller. The oscillator
module is usually configured to use quartz crystal or ceramic resonator for
frequency stabilization. Furthermore, it can also operate without elements for
frequency stabilization (like RC oscillator). It is important to say that
instructions are not executed at the rate imposed by the oscillator itself, but
several times slower. It happens because each instruction is executed in
several steps. In some microcontrollers, the same number of cycles is needed to
execute any instruction, while in others, the execution time is not the same
for all instructions. Accordingly, if the system uses quartz crystal with a
frequency of 20 MHz, execution time of an instruction is not 50nS, but 200, 400
or 800 nS, depending on the
type of Microcontroller Unit (MCU)!
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