Specifications
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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)!
Power supply circuit
There are two things worth attention concerning the microcontroller power supply circuit:
Brown-out is a potentially dangerous state which occurs at the moment the microcontroller is being turned off or in situations when power
supply voltage drops to the limit due to electric noise. As the microcontroller consists of several circuits which have different operating voltage
levels, this state can cause its out-of-control performance. In order to prevent it, the microcontroller usually has built-in circuit for brown out
reset. This circuit immediately resets the whole electronics when the voltage level drops below the limit.
Reset pin is usually marked as MCLR (Master Clear Reset) and serves for external reset of the microcontroller by applying logic zero (0) or one
(1), depending on type of the microcontroller. In case the brown out circuit is not built in, a simple external circuit for brown out reset can be
connected to this pin.
Timers/Counters
The microcontroller oscillator uses quartz crystal for its operation. Even though it is not the simplest solution, there are many reasons to use it.
Namely, the frequency of such oscillator is precisely defined and very stable, the pulses it generates are always of the
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