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PIC16F84A Datasheet(PDF) 40 Page - Microchip Technology |
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PIC16F84A Datasheet(HTML) 40 Page - Microchip Technology |
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40 / 124 page  PIC16F8X DS30430C-page 40 © 1998 Microchip Technology Inc. 8.2.3 EXTERNAL CRYSTAL OSCILLATOR CIRCUIT Either a prepackaged oscillator can be used or a simple oscillator circuit with TTL gates can be built. Prepackaged oscillators provide a wide operating range and better stability. A well-designed crystal oscillator will provide good performance with TTL gates. Two types of crystal oscillator circuits are available; one with series resonance, and one with parallel resonance. Figure 8-5 shows a parallel resonant oscillator circuit. The circuit is designed to use the fundamental frequency of the crystal. The 74AS04 inverter performs the 180-degree phase shift that a parallel oscillator requires. The 4.7 k Ω resistor provides negative feedback for stability. The 10 k Ω potentiometer biases the 74AS04 in the linear region. This could be used for external oscillator designs. FIGURE 8-5: EXTERNAL PARALLEL RESONANT CRYSTAL OSCILLATOR CIRCUIT Figure 8-6 shows a series resonant oscillator circuit. This circuit is also designed to use the fundamental frequency of the crystal. The inverter performs a 180-degree phase shift. The 330 k Ω resistors provide the negative feedback to bias the inverters in their linear region. FIGURE 8-6: EXTERNAL SERIES RESONANT CRYSTAL OSCILLATOR CIRCUIT 8.2.4 RC OSCILLATOR For timing insensitive applications the RC device option offers additional cost savings. The RC oscillator frequency is a function of the supply voltage, the resistor (Rext) values, capacitor (Cext) values, and the operating temperature. In addition to this, the oscillator frequency will vary from unit to unit due to normal process parameter variation. Furthermore, the difference in lead frame capacitance between package types also affects the oscillation frequency, especially for low Cext values. The user needs to take into account variation due to tolerance of the external R and C components. Figure 8-7 shows how an R/C combination is connected to the PIC16F8X. For Rext values below 4 k Ω, the oscillator operation may become unstable, or stop completely. For very high Rext values (e.g., 1 M Ω), the oscillator becomes sensitive to noise, humidity and leakage. Thus, we recommend keeping Rext between 5 k Ω and 100 kΩ. Although the oscillator will operate with no external capacitor (Cext = 0 pF), we recommend using values above 20 pF for noise and stability reasons. With little or no external capacitance, the oscillation frequency can vary dramatically due to changes in external capacitances, such as PCB trace capacitance or package lead frame capacitance. See the electrical specification section for RC frequency variation from part to part due to normal process variation. The variation is larger for larger R (since leakage current variation will affect RC frequency more for large R) and for smaller C (since variation of input capacitance has a greater affect on RC frequency). See the electrical specification section for variation of oscillator frequency due to VDD for given Rext/Cext values as well as frequency variation due to operating temperature. The oscillator frequency, divided by 4, is available on the OSC2/CLKOUT pin, and can be used for test purposes or to synchronize other logic (see Figure 3-2 for waveform). FIGURE 8-7: RC OSCILLATOR MODE 20 pF +5V 20 pF 10k 4.7k 10k 74AS04 XTAL 10k 74AS04 PIC16FXX CLKIN To Other Devices 330 k Ω 74AS04 74AS04 PIC16FXX CLKIN To Other Devices XTAL 330 k Ω 74AS04 0.1 µF Note: When the device oscillator is in RC mode, do not drive the OSC1 pin with an external clock or you may damage the device. OSC2/CLKOUT Cext Rext PIC16FXX OSC1 Fosc/4 Internal clock VDD VSS Recommended values: 5 k Ω ≤ Rext ≤ 100 kΩ Cext > 20pF |
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