Specifications

mikroElektronika | Free Online Book | PIC Microcontrollers | Chapter 8: Other MCU's Circuits

Fig.8-8 Ceramic Resonator

Ceramic resonators are by their features similar to quartz crystals. This is why they are connected in the same way. Unlike

quartz crystals, they are cheaper and oscillators containing them have a bit worse characteristics. They are used for clock

frequencies ranging between 100 kHz and 20 MHz.

External oscillator in RC and RCIO mode

There are certainly many advantages in using elements for frequency stabilization, but sometimes they are really

unnecessary. It is mostly sufficient that the oscillator operates at frequency not precisely defined so that embedding of

such expensive elements means a waste of money. The simplest and cheapest solution in these situations is to use one

resistor and one capacitor for the operation of oscillator. There are two modes:

RC mode. In RC mode, the RC circuit is connected to the OSC1

pin as shown in figure. The OSC2 pin outputs the RC oscillator

frequency divided by 4. This signal may be used for calibration,

synchronization or other application requirements.

Fig. 8-9 RC Mode

RCIO mode. Similar to the previous case, the RC circuit is

connected to the OSC1 pin. This time, the available OSC2 pin is

used as additional general purpose I/O pin.

Fig. 8-10 RCIO Mode

In both cases, it is recommended to use components as shown in figure.

The frequency of such oscillator is calculated according to the formula f = 1/T in which:

f = frequency [Hz]

T = R*C = time constant [s]

R = resistor resistance [Ω]

C = capacitor capacity [F]

Internal Clock Modes

The internal oscillator circuit consists of two separate oscillators that can be selected as the system clock source:

The HFINTOSC oscillator is factory calibrated and operates at 8 MHz. Its frequency can be user-adjusted via software

using bits of the OSCTUNE register; and

The LFINTOSC oscillator is not factory calibrated and operates at 31kHz.

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