Specifications

Chapter 2 - Microcontroller PIC16F84
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2.1 Clock generator - oscillator
Oscillator circuit is used for providing a microcontroller with a clock. Clock is needed so that
microcontroller could execute a program or program instructions.
Types of oscillators
PIC16F84 can work with four different configurations of an oscillator. Since configurations with
crystal oscillator and resistor-capacitor (RC) are the ones that are used most frequently, these are
the only ones we will mention here. Microcontroller type with a crystal oscillator has in its
designation XT, and a microcontroller with resistor-capacitor pair has a designation RC. This is
important because you need to mention the type of oscillator when buying a microcontroller.
XT Oscillator
Crystal oscillator is kept in metal housing
with two pins where you have written down
the frequency at which crystal oscillates. One
ceramic capacitor of 30pF whose other end is
connected to the ground needs to be
connected with each pin.
Oscillator and capacitors can be packed in
joint case with three pins. Such element is
called ceramic resonator and is represented
in charts like the one below. Center pins of
the element is the ground, while end pins are
connected with OSC1 and OSC2 pins on the
microcontroller. When designing a device,
the rule is to place an oscillator nearer a
microcontroller, so as to avoid any
interference on lines on which microcontroller
is receiving a clock.
RC Oscillator
In applications where great time precision is not necessary, RC oscillator offers additional savings
during purchase. Resonant frequency of RC oscillator depends on supply voltage rate, resistance
R, capacity C and working temperature. It should be mentioned here that resonant frequency is
also influenced by normal variations in process parameters, by tolerance of external R and C
components, etc.
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