Specifications

Chapter 2 - Microcontroller PIC16F84
Above diagram shows how RC oscillator is connected with PIC16F84. With value of resistor R being
below 2.2k, oscillator can become unstable, or it can even stop the oscillation. With very high
value of R (ex.1M) oscillator becomes very sensitive to noise and humidity. It is recommended
that value of resistor R should be between 3 and 100k. Even though oscillator will work without an
external capacitor(C=0pF), capacitor above 20pF should still be used for noise and stability. No
matter which oscillator is being used, in order to get a clock that microcontroller works upon, a
clock of the oscillator must be divided by 4. Oscillator clock divided by 4 can also be obtained on
OSC2/CLKOUT pin, and can be used for testing or synchronizing other logical circuits.
Following a supply, oscillator starts oscillating. Oscillation at first has an unstable period and
amplitude, but after some period of time it becomes stabilized.
To prevent such inaccurate clock from influencing microcontroller's performance, we need to keep
the microcontroller in reset state during stabilization of oscillator's clock. Above diagram shows a
typical shape of a signal which microcontroller gets from the quartz oscillator following a supply.
http://www.mikroelektronika.co.yu/english/product/books/PICbook/2_02Poglavlje.htm (2 of 3) [4/2/2003 16:17:41]