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PIC16F570 Datasheet(PDF) 40 Page - Microchip Technology |
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PIC16F570 Datasheet(HTML) 40 Page - Microchip Technology |
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40 / 129 page  PIC16F570 DS40001684E-page 40 2013-2014 Microchip Technology Inc. TABLE 8-2: CAPACITOR SELECTION FOR CRYSTAL OSCILLATOR(2) 8.3.3 EXTERNAL CRYSTAL OSCILLATOR CIRCUIT Either a prepackaged oscillator or a simple oscillator circuit with TTL gates can be used as an external crystal oscillator circuit. 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 can be used: one with parallel resonance, or one with series resonance. Figure 8-3 shows implementation of 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 resistor provides the negative feedback for stability. The 10 k potentiometers bias the 74AS04 in the linear region. This circuit could be used for external oscillator designs. FIGURE 8-3: EXTERNAL PARALLEL RESONANT CRYSTAL OSCILLATOR CIRCUIT Figure 8-4 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 in a series resonant oscillator circuit. The 330 resistors provide the negative feedback to bias the inverters in their linear region. FIGURE 8-4: EXTERNAL SERIES RESONANT CRYSTAL OSCILLATOR CIRCUIT 8.3.4 EXTERNAL 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) and 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 will also affect the oscillation frequency, especially for low CEXT values. The user also needs to take into account variation due to tolerance of external R and C components used. Osc. Type Resonator Freq. Cap. Range C1 Cap. Range C2 LP 32 kHz(1) 15 pF 15 pF XT 200 kHz 1 MHz 4 MHz 47-68 pF 15 pF 15 pF 47-68 pF 15 pF 15 pF HS 20 MHz 15-47 pF 15-47 pF Note 1: For VDD > 4.5V, C1 = C2 30 pF is recommended. 2: These values are for design guidance only. Rs may be required to avoid overdriving crystals with low drive level specification. Since each crystal has its own characteristics, the user should consult the crystal manufacturer for appropriate values of external components. 20 pF +5V 20 pF 10k 4.7k 10k 74AS04 XTAL 10k 74AS04 CLKIN To Other Devices PIC® Device 330 74AS04 74AS04 PIC® Device CLKIN To Other Devices XTAL 330 74AS04 0.1 mF |
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