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PIC17C752-08/CL Datasheet(PDF) 17 Page - Microchip Technology |
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PIC17C752-08/CL Datasheet(HTML) 17 Page - Microchip Technology |
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17 / 328 page  © 1998 Microchip Technology Inc. DS30289A-page 17 PIC17C7XX 4.1.3 EXTERNAL CLOCK OSCILLATOR In the EC oscillator mode, the OSC1 input can be driven by CMOS drivers. In this mode, the OSC1/CLKIN pin is hi-impedance and the OSC2/CLKOUT pin is the CLKOUT output (4 TOSC). FIGURE 4-4: EXTERNAL CLOCK INPUT OPERATION (EC OSC CONFIGURATION) Clock from ext. system OSC1 OSC2 PIC17CXXX CLKOUT (FOSC/4) 4.1.4 EXTERNAL CRYSTAL OSCILLATOR CIRCUIT Either a prepackaged oscillator can be used or a simple oscillator circuit with TTL gates can be built. Prepack- aged 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 can be used: one with series resonance, or one with parallel resonance. Figure 4-5 shows implementation of a parallel resonant oscillator circuit. The circuit is designed to use the fun- damental frequency of the crystal. The 74AS04 inverter performs the 180-degree phase shift that a par- allel oscillator requires. The 4.7 k Ω resistor provides the negative feedback for stability. The 10 k Ω potenti- ometer biases the 74AS04 in the linear region. This could be used for external oscillator designs. FIGURE 4-5: EXTERNAL PARALLEL RESONANT CRYSTAL OSCILLATOR CIRCUIT Figure 4-6 shows a series resonant oscillator circuit. This circuit is also designed to use the fundamental fre- quency of the crystal. The inverter performs a 180-degree phase shift in a series resonant oscillator circuit. The 330 Ω resistors provide the negative feed- back to bias the inverters in their linear region. FIGURE 4-6: EXTERNAL SERIES RESONANT CRYSTAL OSCILLATOR CIRCUIT 20 pF +5V 20 pF 10 k Ω 4.7 k Ω 10 k Ω 74AS04 XTAL 10k Ω 74AS04 PIC17CXXX OSC1 To Other Devices 330 Ω 74AS04 74AS04 PIC17CXXX OSC1 To Other Devices XTAL 330 Ω 74AS04 0.1 µF |
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